The Arabian Sea Monsoon Experiment (ARMEX), which is the second observational experiment under the Indian Climate Research Programme (ICRP), aims at gaining more insight into the two major facets of the monsoon that are linked to convection over the Arabian Sea viz. the monsoon onset processes and the intense rainfall events over the west coast of the Indian peninsula. During the summer monsoon, copious rainfall occurs over the west coast of the Indian peninsula. On occasions, the rainfall is exceptionally heavy, exceeding 20cm per day at some stations. These intense rainfall events are attributed to meso-scale systems which are generally embedded in synoptic or larger scale convective systems over the Arabian Sea. It is believed that these intense rainfall events are often associated with offshore troughs. One of the aims of ARMEX is to elucidate the nature of the systems responsible for these intense rainfall events.
Prior to the summer monsoon, the highest sea surface temperature (SST) in the world oceans is found in the north Indian Ocean. Meteorologists have long believed that this warm pool, called the Indian Ocean Warm Pool, including the smaller region of high SST in the Lakshadweep Sea, is crucial for the onset of the summer monsoon over Kerala early in June. Recent research suggests that a suite of processes in the ocean and the atmosphere may be responsible for this coupled air-sea phenomenon. These processes include local air-sea fluxes and processes in the Arabian Sea that are triggered by events in the Bay of Bengal. Understanding the evolution of the Arabian Sea warm pool and associated dome in sea level called the Lakshadweep High, its maintenance in the pre-monsoon phase and collapse with the onset of the monsoon is the other major objective of ARMEX.
This document summarizes the scientific background of, and experimental strategy for attaining these objectives. ARMEX, a multi-institutional field experiment deploying state-of-the-art technologies, will provide crucial data sets for analysis and modeling of processes that govern the major facets of the Arabian Sea monsoon system.
In the Indian Climate Research Programme (ICRP) Science Plan (DST 1996), the outstanding problems in meteorology and oceanography that need to be addressed in the Indian context have been identified. The objectives of ICRP identified in the science plan are as follows:
1) Understanding the physical processes responsible for variability on the sub-seasonal, seasonal, interannual, and decadal time-scales of the monsoon, the oceans (specifically the Indian seas and the equatorial Indian ocean) and the coupled atmosphere-ocean-land system
2) Study of the space time variation of the monsoons from sub-seasonal, interannual to decadal scales for assessing the feasibility for climate prediction and development of methods for prediction
3) Study of change in climate and its variability ( on centennial and longer time scales) generated by natural and anthropogenic factors
4) Investigation of the links between climate variability and critical resources such as agriculture productivity, and for realistic assessment of the impact of climate change.
The ICRP aims at gaining new insights into the climate system and its objectives can be achieved only with well focussed programmes which study not only the individual components but also the interactions / feedbacks between the different components of climate. The programmes for attaining the objectives have been described in the ICRP implementation plan (DST 1998). The first two objectives aiming at understanding the variability of the monsoon, the oceans and the coupled system will be addressed under a major programme of the ICRP viz. MONVAR. The other programmes are the study of past climate and climate change (PCCC) and climate and agriculture (CLIMAG).
The MONVAR programme envisages a multi-pronged approach involving
monitoring of critical elements of the climate system with met-ocean buoy networks, satellites and special field campaigns as well as process studies. The Bay of Bengal Monsoon Experiment (BOBMEX), the first field experiment under the ICRP, has been very successfully implemented in July-August 1999. The capability within the country of using state of the art equipment for observing the coupled ocean-atmosphere system with adequate accuracy has been proved with BOBMEX. The BOBMEX observations clearly brought out the special and unique features of the atmosphere and the Bay of Bengal during the Indian summer monsoon (Bhat et al. 2001, Bhat 2001). BOBMEX has given the scientists the confidence necessary for carrying out such observational experiments in difficult monsoon conditions. Apart from the scientific contribution, BOBMEX also started close interactions and productive co-operation among different institutions, organizations and universities engaged in monsoon and ocean research in the country.
The second field experiment proposed in the ICRP Implementation Plan (DST 1998) is the Arabian Sea Monsoon Experiment (ARMEX). Most of the rainfall over the Indian region during the monsoon occurs in association with convective systems over the Bay of Bengal and the Arabian Sea, which propagate or extend onto the region. The onset of the monsoon over Kerala, which heralds the commencement of the summer monsoon, is associated with intense convection over the southeastern Arabian Sea. During the summer monsoon, copious rainfall occurs over the west coast of the peninsula. However, within the season, spells with heavy rainfall alternate with spells with little or no rainfall. On occasions the rainfall is exceptionally heavy, exceeding 20cm per day at some stations. These intense rainfall events are associated with convective systems/bands of mesoscale generally embedded in larger scale systems over the Arabian Sea. It is believed that these intense rainfall events are often associated with offshore troughs (Rao 1976). ARMEX addresses the two major facets of the monsoon which are linked to convection over the Arabian Sea viz. the monsoon onset processes and the intense rainfall events over the West Coast. One objective of ARMEX is to understand the coupled ocean atmosphere and land processes involved in the genesis and intensification of the systems responsible for the intense rainfall events on the west coast.
Prior to the summer monsoon, the highest sea surface temperature (SST) in the world oceans is found in the north Indian Ocean. Meteorologists have long believed that this warm pool, called the Indian Ocean Warm Pool, including the smaller region of high SST in the Lakshadweep Sea, is crucial for the onset of the summer monsoon over Kerala. Understanding the evolution of the Arabian Sea warm pool and the associated dome in sea level called the Lakshadweep High, its maintenance in the pre-monsoon phase and collapse with the onset of the monsoon is the second major objective of ARMEX.
Understanding the monsoon onset processes and the link to the dynamics and thermodynamics of the southeastern Arabian Sea is of importance for prediction of the onset of the monsoon over Kerala. Over the last hundred years, the date of monsoon onset over Kerala has varied from 11 May in 1918 to 18 June in 1972. The onset of the monsoon over Kerala is closely related to the developments in the southeastern Arabian Sea. The predictive value of the SST in this region has been well established (Kung and Shariff, 1982). The prediction of the timing of the monsoon onset is critical for the management of crops, water, and power. Understanding the nature of the systems responsible for the intense rainfall events over the west coast is a prerequisite to their prediction. The prediction of such events is also extremely important, because they can lead to extensive flooding and damage.
For addressing these problems, detailed observations of several features of the atmosphere, the ocean and their coupling over the Arabian Sea are required. Beginning with the International Indian Ocean Expedition (IIOE) in the 1960s, several observational programmes have been conducted in the Arabian Sea. These include the summer monsoon experiments (ISMEX-73, MONSOON-77, MONEX-79). However, these experiments did not provide much information on the Arabian Sea warm pool. The Lakshadweep SST high and the associated oceanic high and low, during genesis, are too small in spatial extent and too rapidly evolving to be resolved by the data from the previous field programmes. They were discovered only after near-synoptic satellite-based observations became available (Bruce et al., 1994; Shankar and Shetye, 1997; Shenoi et al., 1999). The data from satellite-based sensors reveal only surface information. The evolution of SST and heat content is fully three-dimensional. There are no observations that reveal the three-dimensional structure of the Indian Ocean warm pool or the Lakshadweep High. Further, the processes and hypotheses proposed so far are based on numerical models, but detailed in situ observations to test the hypotheses are lacking.
Detailed observations of the summer monsoon onset processes were carried out during MONEX-79. In the last two decades, major advances have occurred in our understanding of the coupling of tropical convection with the oceans. This has come about with new observations with advanced instrumentation in the major international programmes over the Pacific-TOGA and TOGA-COARE on the one hand and theoretical studies of ocean-atmosphere coupling on the other. Detailed observations of the atmosphere, the ocean and their coupling with state of art instruments is bound to give new insights into the Arabian Sea warm pool and monsoon onset processes. An enhanced understanding of the dynamics and thermodynamics of the Arabian Sea warm pool will also contribute towards deeper insight into the other important parts of the Indian ocean warm pool such as the Bay of Bengal. Some observations of the mesoscale convective systems (which were embedded in the larger scale systems) resulting in intense rainfall on the West Coast were also made during MONEX-79. More detailed observations of the role of Arabian sea SST and surface fluxes in the genesis and maintenance of such systems and of the large-scale convection over the region are needed.
A variety of platforms are to be used to collect oceanic and meteorological observations of relevance to ARMEX. For such observational experiments and process studies, a multi institutional effort (such as the one successfully used for BOBMEX) is a must. Therefore ARMEX is conceived as a national programme involving several organisations, each contributing in its own area of expertise and interest.
This document comprising the ARMEX science plan is organized as follows. The background with a short summary of earlier field campaigns in the Arabian Sea is described in section 2. The present understanding of the problems addressed is given in section 3. Outstanding key issues are discussed in section 4. In section 5 the broad scientific objectives of ARMEX are listed. The suggested observational programme is given in section 6. In section 7, the expected contribution of ARMEX data to modelling studies is briefly pointed out. A tentative budget is given in section 8.
2. Previous field campaigns in the Arabian Sea relevant to ARMEX
To unravel the mysteries of the monsoon developing over the Indian Ocean, a multinational effort was made in the form of International Indian Ocean Expedition (IIOE) during the period 1963-65 during which aircraft soundings (drop-sondes) were launched for the first time. Colon (1964) studied air-sea interactions over the Arabian sea and discovered the existence of the lower tropospheric inversions over the west Arabian Sea. Pisharoty (1965) focussed on the importance of the evaporative process over the Arabian Sea in contributing to the moisture flux across the West Coast of India. Raman (1965) established the existence of southern hemispheric equatorial trough. Joseph and Raman (1966) stressed the role of the low level westerly jet stream over south India. Findlater (1969) emphasised the cross-equatorial flow in relation to the monsoon over India. Miller and Keshvamurty (1968) established the existence of a major rain bearing transient circulation system in the form of a mid tropospheric cyclone.
After the IIOE, the Indo-Soviet Monsoon Experiment (ISMEX-73) was conducted during the summer of 1973 by U.S.S.R. and India. Six research vessels (four from the U.S.S.R. and two from India) obtained meteorological and oceanographic measurements over the Arabian Sea, the equatorial region and southern Indian Ocean. Important insights into the onset of monsoon, active and break monsoon periods and oceanographic phenomenon were obtained. Using ISMEX-73 data Ghosh, Pant and Dewan (1978) demonstrated that the net zonal fluxes of water vapour from surface to 400 hPa across the west coast of India shows a large increase from weak to active monsoon while no significant change is found in the cross-equatorial moisture flux in the two periods. This demonstrated that a large amount of moisture is picked up from the Arabian Sea by evaporation during active monsoon. Similar conclusions were drawn earlier by Pisharoty (1965).
The next field experiment Monsoon-77, was organized from 15 May to 19 August 1977 to collect surface and upper air observations over the vast oceanic areas surrounding the Indian sub-continent for unraveling the peculiarities of monsoon circulation such as onset conditions, depression formation, etc. It was executed as a forerunner of the experiment on sub-regional scale known as Monsoon Experiment (MONEX) which was a sub-programme of First Global GARP Experiment (FGGE). It was jointly conducted by India and USSR for an intensive study of different scales of monsoon disturbances and for the development of numerical simulation of the general atmospheric and ocean circulation in the monsoon area.
The routine observational programme over India was augmented during MONEX by i) arranging additional upper air observations at Car Nicobar in the Bay of Bengal and Amini Devi in the Arabian Sea ii) establishing three upper air observatories at Gopalpur, Balasore and Digha for taking upper air observations in July- August 1977, and iii) by arranging increased radio-sonde flights from 16 existing upper air observational stations.
All the coastal radar stations at Mumbai, Chennai, Vizag, Paradip and Calcutta took frequent observations during Monsoon-77 and radiometer ascents were arranged from 2-3 stations in the country during this period. M-100 Rocket soundings were conducted from Thumba and Sriharikota every Wednesday. Observations from the oceanic areas were maximised by collecting all the ship observations, efforts were made to collect meteorological data from meteorological agencies of the countries bordering Indian ocean.
MONEX-79 provided a more comprehensive data set from a large area around India where surface and upper air networks were augmented to meet the requirements. Two special observing periods were organised from May 1 to June 30, 1979 and from January 1 to February 28, 1979. Upper air network was either augmented or newly established at 9 observatories for Radiowind/ Radiosonde observation in the basic network. In addition, upper air observations were also established at 8 more stations for the purpose of MONEX-79, 3 surface observatories were also established. Efforts were made to collect observations from commercial ships. Advantage was also taken of the observing network of FGGE where about 20 ships participated. Upper air observations were organised from these ships. In addition 3 additional ships were deployed in the Arabian Sea to meet MONEX-79 requirements. Hydrographic observations on board the Indian research vessel RV Gaveshani were made in a selected 4 squares in the east central Arabian Sea in relation to different phases (pre-monsoon, onset and post-onset) of monsoon over the study area. Stabilized platforms were provided to the ships for making wind observations. The results of these studies more or less supported the earlier findings on the air-sea interactions and inversions over the west and central Arabian Sea. Monex-79 provided enough data to establish the structure of the onset vortex (Krishnamurti et al, 1981).
3. Present Understanding
Fig.1: Variation of the average number of convective days per month with SST over the tropical Indian Ocean. The vertical bar denotes one standard deviation. (adapted from Waliser et al., 1993).
India is surrounded by oceans, and convection over these oceans plays a major role in the monsoon rainfall over India. An important aspect of the deep convection over the tropical oceans is its relationship with SST. Studies of the variation of convection over the oceans with SST based on different measures of deep convection all suggest a similar and highly non-linear relationship (Gadgil et al 1984, Graham and Barnett 1987, Waliser et al. 1993). In particular, it is observed that convection is very sensitive to variations in SST around 28&de g;C; a 1°C change about this value will make the atmosphere move from an almost no-convection state to near peak convection state (Fig 1). Actually, deep convection is an atmospheric phenomenon, an outcome of the destabilizing tendency of the surface and mixed layer air on the one hand and the stabilizing/inhibiting influence of the stably stratified atmosphere above on the other. SST is important because the properties of the surface and mixed layer air are closely linked to the SST, due to continuous interactions of the upper layer of the ocean and the atmosphere immediately above (Bhat et al, 1996). When the atmosphere is free of clouds, the ocean surface receives net heat, SST increases, increasing the energy of the surface air as well. When deep convection occurs, the clouds block solar radiation and latent heat flux increases due to increased winds. Hence the ocean cools. This can lead to reduced convection. Thus, whereas atmospheric convection depends upon the SST, the SST changes in response to atmospheric convection and the system is coupled. Therefore, understanding the seasonal evolution and the intraseasonal variation of Arabian Sea SST and air sea coupling is of critical importance in the studies related to monsoon onset over Kerala and maintenance of the West Coast monsoon.
3.1 The Arabian Sea Warm Pool
The region of the world oceans with SST greater than 28°C is called the warm pool. A significant fraction of this warm pool lies in the tropical Indian Ocean covering the equatorial Indian Ocean, the Bay of Bengal, and the Arabian Sea (Fig. 2a). This warm pool shows seasonality both in spatial extent and intensity with peak values in excess of 30°C occurring in the southeastern Arabian Sea (Joseph, 1990a, Vinayachandran and Shetye, 1991, Shenoi et al., 1999; Rao and Sivakumar, 1999). There is considerable interannual variation in the SST of the warm pool (Fig. 2b).
Fig. 2a: Seasonal cycle of SST from Reynolds's weekly climatology (1982-1993). Only SST greater than 28oC is shown. Contour interval is 0.5oC.
Fig. 2 (b)
The Arabian Sea warm pool, associated with this SST maximum, progressively builds up during the pre-summer monsoon season and appears to play an important role in the onset process of the summer monsoon. The Arabian Sea warm pool collapses dramatically with the onset of the summer monsoon (Fig.3).
The SST of the warm pool in the Arabian Sea has been recognized to play an important role in the onset of the monsoon and the onset vortex. It is worth investiating the extent to which the variability of the warm pool determines the nature of the monsoon over the Arabian Sea on both intraseasonal and interannual timescales.
Figure 3. The variation of SST during 1998 and 1999 measured by DOD's moored buoy in the south east Arabian Sea.
Genesis and Maintenance
The sea level, as seen in satellite altimetry, shows an annual cycle in the southeastern Arabian Sea (Fig. 4a). The interannual variation of the anomalies of the sea surface height and the SST during 1994-97 is shown in Fig. 4b. Anticyclonic geostrophic flow around a high in the sea level in the Lakshadweep Sea during January--May is followed by cyclonic flow around a low in sea level during June--October. The high, called the Lakshadweep high, was discovered by Bruce et al. (1994). The Lakshadweep high and low expand and shift progressively westward with time and extend across the southern Arabian Sea a
Fig.4a: Seasonal cycle of sea surface height anomaly from TOPEX/Poseidon. Climatology based on the 1993-1997 period is shown with a contour interval of 5 cm.
few months after genesis. Numerical and analytical models show that the high and low are due to Rossby waves radiated by coastal Kelvin waves propagating poleward along the Indian west coast (McCreary et al., 1993; Shankar and Shetye, 1997). It has been suggested that these coastal Kelvin waves are, in turn, forced primarily by winds blowing along the east coast of India. According to this suggestion, the Lakshadweep high owes its existence primarily to downwelling Kelvin waves forced along the east coast of India by the collapse of the summer monsoon (Shankar, 1998; Shankar et al., 2001).
The coastal current along the Indian east coast (the East India Coastal Current), associated with this downwelling Kelvin wave, is equatorward during October-December. This current in association with the westward Winter Monsoon Current south of Sri Lanka and the poleward West India Coastal Current, transports low-salinity water from the northern Bay of Bengal to the southeastern Arabian Sea. The bending of the Winter Monsoon Current around the Lakshadweep high forces a significant fraction of this low-salinity water to be trapped in the Lakshadweep Sea (Fig.5).
Fig.5: Horizontal distribution surface salinity in February, based on the monthly mean climatology (Levitus et al., 1994). The low salinity off southwest India is a result of inflow from the Bay of Bengal during November-January. Contour interval is 0.5psu.
Thus, before the start of the summer, the Lakshadweep Sea is characterized by downwelling and a low-salinity cap about 60 m deep. This highly stratified and insulated layer plays an important role in the warm pool dynamics. A clear evidence for this is seen in the interannual variation of the anomalies of SST and SSHA depicted in Fig. 4b, it is seen that highs in SSH anomaly during December precede the peak SST in March-April. Although the latent heat flux is low over the entire Arabian Sea warm pool because of weak winds (Rao and Sivakumar, 1999), the SST is higher by about 0.5°C in the Lakshadweep High because here the oceanic surface layer can warm quickly (Shenoi et al., 1999). This hypothesis is supported by satellite altimetry (Tapley et al., 1994), climatological surface and subsurface temperatures, salinity, and air-sea fluxes (Rao and Sivakumar, 1999), and numerical simulations with reduced-gravity models (Han and McCreary, 2001) and oceanic general circulation models (Vinayachandran and Yamagata, 1998).
The summer monsoon experiments (ISMEX-73, MONSOON-77 & MONEX-79) conducted over the Arabian Sea have revealed a dramatic drop in SST over the warm pool in association with the onset vortex and/or the onset of the summer monsoon (Rao, 1990). This is also revealed from the moored buoy data in the Arabian Sea (Fig. 3). It is observed from Fig. 3 that SST decreases dramatically by 2-4°C following the onset of monsoon, which is large for an ocean having mixed layer depth of about 60m. The subsequent cooling in this region is relatively less rapid during the rest of the course of the monsoon.
3.2 Monsoon Pre-Onset and Onset Processes
It is observed from Fig. 3 that SSTs are above the convection threshold from mid-March onwards over the Arabian Sea. A region of high SST such as the Indian Ocean warm pool can cause large-scale moisture convergence, which in turn can lead to the building up of an active equatorial trough there, with its associated deep convective clouds, heating of the tropospheric column above, lowering of surface pressure, strengthening of lower tropospheric winds and the associated high net heat loss cools the ocean temperature. But this does not happen for more than a month over the southeastern Arabian Sea till the dramatic monsoon onset in June. What prevents the triggering of convection over the Arabian Sea with such high SSTs is not understood.
Soundings taken during IIOE and MONEX-79 showed the presence of a low level inversion (at around 1 km height) in the atmosphere before the onset of monsoon over the Arabian Sea west of 65°E (Colon 1964). Narayanan and Rao (1981 and 1989) using TIROS-N (NOAA)-TOVS data have determined the extent of the temperature inversions in the lower troposphere over the Arabian Sea. The shift of the inversion (convection) regions from west (east) to east (west) was seen during different phases of the monsoon. Strong inversions can suppress the development of convective clouds. Such strong inversions were observed over the southeastern Arabian Sea during March (Fig. 6). However the nature of the inversion during April - May is not known.
A surge in convection occurs over south-east Arabian sea about six weeks prior to monsoon onset over Kerala (Joseph and Pillai,1988). The nature of the impact of this convection on the SST needs to be elucidated. The recovery time-scale from this impact may be one of the factors determining the date of the monsoon onset. It is necessary to understand why this convective surge does not culminate in monsoon onset.
Monsoon onset over Kerala
Intense convective activity over the southeast Arabian Sea is a necessary condition for the onset of the summer monsoon over Kerala. Pearce and Mohanty (1984) have documented the rapid growth of convection and the integrated moisture in the atmosphere over the entire Indian Ocean warm pool during a two to three week period prior to monsoon onset over Kerala.
An active convection area begins over southeast Arabian Sea at least 15 days before the onset and the area and intensity of convection there steadily grows and reaches a peak at the time of the onset (Joseph, et al. 1994). Ananthakrishnan et al's (1968) analysis for a 68 year period showed that, in many of these years, active convection over the southeastern Arabian Sea is manifested as trough of low pressure, low pressure area, depression or tropical cyclone. The onset over Kerala is associated with the formation of an onset vortex in several years. Detailed observations of the onset vortex of 1979 were made during MONEX-79 (Krishnamurti et al, 1981). The onset vortex is usually located close to the warm Lakshadweep sea (Seetharamayya and Master, 1984, Kershaw 1985). The development of the Arabian sea part of the Indian Ocean warm pool seems to be necessary for monsoon onset over Kerala, irrespective of whether an onset vortex forms or not; monsoon onset gets delayed when this portion has negative temperature anomalies (Joseph et al 1994).
Figure 6. Vertical variation of potential temperature (Theta) with height near the equator (thin line) and in the Arabian Sea (thick line). These profiles were obtained by IISc in March 2001 during the IRSP4 validation cruise on ORV Sagar Kanya. In the Arabian Sea two prominent inversions (sudden increase in theta with height) are seen. Such strong inversions prevent cloud vertical development. The profile seen near the equator is conducive for the development of deep convective clouds.
One measure of the strength of air-sea coupling is the variation of sea air temperature difference DT (=SST-Ta where Ta is air temperature at 10 m height) and specific humidity difference Dq (= qs-qa, where qs and qa are respectively saturation specific humidity at SST and specific humidity of air at 10 m height) with SST. Some available data for over the Indian Ocean and Bay of Bengal suggests that air-sea coupling over the tropical Indian Ocean and tropical West Pacific are different. For example, Fig. 7 shows the variation of DT and Dq with SST for the data collected during the Intensive Field Phase of INDOEX in January-March 1999 and during BOBMEX in the North Bay of Bengal in July-August 1999. Also plotted are the variations suggested by Waliser and Graham (1993) based on their analysis of data for the equatorial West Pacific (which is used in many studies in the absence of data for the Indian Ocean), namely,
DT = 1.5 for SST < 29°C (1)
= SST-27.5 for SST > 29°C
Dq = qs(SST) - 0.8 qs(Ta). (2)
The value 0.8 in equation (2) implies a relative humidity of 80% for the air at 10 m height. The variation of DT over tropical Indian Ocean is seen to be significantly different from that over the West Pacific in Fig.7. Values of DT are generally much smaller over the Indian Ocean. The difference in DT values between the two regions is larger than possible measurement errors. Similar trends are seen in the Arabian Sea buoy data also (Fig. 7c). On the other hand, the variation of Dq with SST during INDOEX-99 is comparable to that over West Pacific. It so happens that warmer air temperature at a given SST and lower relative humidity (around 75% compared to 80% assumed in equation (2)) combine to give a Dq variation with SST over the Indian Ocean similar to that over the West Pacific during INDOEX. However, during the monsoon period, there is a large difference.
3.4 Intense rainfall events on the West Coast
West coast of the peninsula receives copious rainfall during the summer monsoon with the average monsoon rainfall over parts of coastal Karnataka, Konkan and Goa being more than 250 cms. The rainfall is even higher on the Sahyadri (Western Ghats), the highest of over 700 cms occuring at Agumbe ( Rao 1976 Tables 10.1,2.). In fact, the hill ranges are closest to the coast between 14° and 15°N and the seasonal rainfall is maximum around 14°N. Within the monsoon season, the rainfall fluctuates between spells with very heavy rainfall (exceeding 20 cms per day at some stations on several occasions) and spells with little or no rainfall. One of the aims of ARMEX is to elucidate the nature of the systems responsible for the events with exceptionally heavy rainfall.
The high rainfall over the West Coast and the heavier rainfall over the Western Ghats is generally attributed to the forced ascent over the orography of the Western Ghats. However, it is the experience of the forecasters that on several occasions when the rainfall is heavy at the coastal stations, several stations on the Ghats do not get such heavy rains (Mukherjee et.al,1978). Such a distribution of rainfall probably arises from a combination of factors including synoptic and mesoscale systems as well as the orography. It has been suggested that the systems associated with coastal rainfall are offshore troughs and midtropospheric cyclones (MTC).
During the summer monsoon season, a quasi-stationary trough, in which sometimes northward moving vortices are embedded, persists off the coast of southwest India (Rao, 1976). Nearly half the number of active to vigorous monsoon situations in Konkan and three quarters of such occasions in coastal Karnataka are associated with troughs off the west coast (Rao 1976). Jayaram (1965) found that for the case of the offshore trough during 3-6 July 1962, the highest rainfall occurred to the south of the apex of the trough and this belt moved slowly northward with the trough. Srinivasan et. al. (1972) have described the upper and lower level circulation and rainfall associated with the offshore trough during 11-16 July 1969. The trough was clearly identified because of the presence of easterly surface winds along the coast. In this case, the heaviest rainfall occurred to the north of the trough. The satellite imagery for this period clearly showed organized convection over the Arabian Sea and the Bay of Bengal extending from around 13°N to 18°N and a thinner cloud band off the coast of Kerala. It is not clear whether the offshore trough results from the Arabian Sea convection or is a factor promoting the offshore convection.
A comprehensive observational study of a midtropospheric cyclone during 2-10 July 1963 was presented by Miller and Keshavamurthy (1968). The MTC was associated with mesoscale bands. A weak trough off the coast north of Bombay was observed. This trough was attributed to the intensification of the MTC; it was not present during the early stages of its lifetime. Significant fluctuations in wind-speed were observed with speeds weaker before and after the MTC and strongest during its life-time with wind convergence along the coast. Rainfall was heavy during the MTC lifetime with some stations recording over 30 cms in one day. Miller and Keshavamurthy (1968) also attributed two other cases of heavy rainfall on the west coast to MTC and pointed out that in most cases of MTC development over the west coast of India, the coastal stations receive more rainfall initially than the inland stations on the western slopes of the western ghats. Carr (1977) found that during 1967-76 MTCs occur one to four times per year and are more common in the first half of the monsoon season.
Intense convection over the Arabian Sea resulting in heavy rainfall over the west coast also occurs in the onset phase of the monsoon and the revival of the monsoon by northward propagations (Sikka and Gadgil 1980). The nature of the mesoscale convection embedded in the larger scale convection in the onset phase of 1979 over the eastern Arabian Sea was elucidated using the wealth of aircraft data collected during MONEX-79. Benson and Rao (1987) showed that several convective bands were embedded in the synoptic scale cloud cluster over the Arabian Sea on 20 June 1979. They suggested that the bands formed and decayed as a result of the complex interactions between the low-level westerly flow, and the upper-level tropical easterly jet-stream and the mesoscale convective features. It was also pointed out that there were significant differences between the convective bands over the tropical Atlantic region and those over the Arabian Sea. The major difference was the high vertical shear that led to the shearing of the anvil cloud westward over the high energy westerly flow. Rao and Hor (1991) studied the observed characteristics of bands on 24 June 1979 and the momentum and kinetic energy exchange between a band and the larger scale. They found that vertical shear and buoyancy were responsible for the changes in momentum flux and that there was a net transfer of kinetic energy from the large scale to the band-scale motion.
Important radar studies of the bands off the coast of Bombay as they approached the coast were made by Narayan (1970). The observations were for a rainsquall which occurred about a week after the onset at Bombay. These studies pointed out the major characteristics of the bands. The tops of convection were found to be between 6 and 8 km. The propagation speed of the bands was high over the sea but decreased on approaching the coast. Heavy rains near the coast were associated with the bands.
Offshore troughs have on occasion, vortices embedded in them. George (1956) was the first to highlight the importance and formation of offshore vortices and linked them to very heavy rainfall pockets off Karnataka. Offshore vortices are mesoscale in character, with linear dimensions of the order of 100 km and their presence is detected by weak easterly winds at coastal stations. Notwithstanding their small dimension, they are effective in bringing about a spell of very heavy rain in their vicinity. Their normal duration is of the order of 1 to 5 days. The dynamics of these vortices has not been examined in much detail so far. Mukherjee (1980) studied the structure of an offshore vortex from research aircraft dropwindsonde data of 20 June 1979. However, this offshore vortex was probably not an isolated system but embedded in the synoptic-scale clouding noted by Benson and Rao (1987). Rao and Hor (1991) suggeted that many of offshore vortices studied by Mukherjee et al. (1978) could have been associated with bands.
Theories about mechanisms
Miller and Keshavamurthy (1968) suggested that the formation of an MTC is linked to the movement of a disturbance from the Bay of Bengal. Ramage (1966) suggested that the heat low over northwest India plays an important role in the formation of an MTC by transpoting cyclonic vorticity to the region. Mak (1975) attributes the genesis to the baroclinic instability of a basic state with meridional as well as zonal shear. Numerical simulations by Krishnamurti and Hawkins (1970) and Carr (1977) suggest that latent heat plays an important role in the intensification and maintenance of the MTC.
The problem of orographically forced rainfall was first investigated as dynamical lifting of neutrally stratified moist layer in the westerlies by Sarkar (1966,67). Grossman and Duran (1984) studied the effect of the orography on the low level flow, using the data of MONEX on 24 June 1979 and concluded that the Western Ghats are capable of producing deep convection well offshore (50-200kms) by gently lifting potentially unstable air as it approaches the coast. However this model did not incorporate some important physics such as effect of wind shear, air-sea interactions and latent heat. Smith and Lin (1983) showed that latent heat induces upward motion in and downstream of the heating area and strengthens the orographic lifting of potentially unstable air. The latent heating in this model was prescribed on the basis of the observed rainfall distribution and not determined by the dynamics. Ogura and Yoshizaki (1988) simulated the orographic convective precipitation near the coast by using a model that resolves individual convective cells and incorporates the interaction of the low level flow with the orography. After an analysis of six cases with and without vertical shear in the wind and including or omitting heat and moisture fluxes from the ocean, they concluded that including two factors viz (i) heat and moisture fluxes from the ocean and (ii) wind shear was essential for realistic simulation of the rainfall distribution. Raodcap and Rao (1993), identified the most unstable mode in the mesoscale using the observed vertical profiles of moisture, temperature and wind (from MONEX 79) as base states in a linear model. The latent heating was estimated through a cloud model. The cloud growth in the model was found to depend on conditionally unstable stratification. The orientation of the simulated bands were found to be similar to the observed.
Thus the theoretical studies suggest that it is important to measure (i) fluxes of heat and moisture from the Arabian sea to the atmosphere (ii) vertical shear of the wind, and (iii) the stability of the atmosphere, i.e., vertical profiles of temperature and humidity.
Statistics of intense rainfall events
Soman and Krishnakumar's (1990) analysis of the daily rainfall over the Indian region showed that the mean rainfall on a rain day (i.e. with a measurable amount of rain i.e. 0.1 mms or more) over the west coast is around 3 cms/day. They also showed that on an average 50% of the seasonal rainfall over the west coast is contributed by days with more than about 6 cms/day. IMD uses the terms heavy for rainfall between 6.5 and 12.4 cms/day and very heavy for rainfall of 12.5 cms/day and higher. Intense rainfall events on the west coast which cause considerable damage are associated with exceptionally heavy rainfall. In order to identify the most favourable locations and timing of the intense rainfall events on the west coast, Gadgil and Francis (2001) analysed daily rainfall data at 42 meteorological observatories provided by India Meteorological Department for varying periods during 1901 - 89. Analysis of the variation of the probability of rainfall greater than specified thresholds of 15, 20, 25, 30 cms/day at one or more stations on the west coast in each pentad during the summer monsoon showed that for thresholds of 25 and 30 cms/day the probability is always less than 20% and 10% respectively. Since rainfall higher than 15 or 20 cms/day is not so rare, these thresholds may be considered to be reasonable. Although such rainfall occurs on only 2-3% of the days, on an average it accounts for about 20-30% of the seasonal total rainfall (Gadgil and Francis 2001). The classic case of MTC studied by Miller and Keshavamurthy (1968) as well as the mesoscale convection during MONEX-79 are associated with intense rainfall more than 20 cms a day at some stations on the west coast. The offshore trough studied by Srinivasan et al (1972) was associated with rainfall of 19 cms at some stations.
A list of the stations, the rainfall distribution and the sample size is given in Table 1. The variation of the expected number of days with rainfall greater than 20 cms and 15 cms per day per season with latitude is shown in Fig. 8 in which the locations of the stations from which data were available is also shown.
It is seen that the expected number of days is highest between about 15° and 16° N. There is a second peak at 19° N (Mumbai). The expected number of days for a 15 cms/day threshold is almost thrice that for a 20 cms/day threshold. The latitudinal variation is similar for the two thresholds. The information in Table 1 and Fig 8 can be utilized in choosing the coastal region for intensive observations.
This latitudinal variation of the frequency of intense rainfall events could be related to the distribution of SST. Hydrographic observations along the west coast of India during June-August 1987, showed upwelling off the coast which decreased in intensity from the south to the north (Shetye et al., 1990). The upwelling caused a reduction in SST of about 2.5° C; off Trivandrum the SST near the coast was 26oC comapred to 28.5°C about 100 km offshore.
The upwelling was not visible north of 15°N (i.e. off Goa) and the SST increased towards the coast in the offshore region north of Ratnagiri. An isothermal layer of thickness 50m with a temperature of 28°C was observed north of Ratnagiri.
The outgoing longwave radiation (OLR) corresponding to few cases of days with rainfall higher than 20 cms at one or more stations on the west coast is shown in Fig. 9. The OLR corresponding to events associated with (i) organized convection in a tropical convergence zone (TCZ) during onset and revival phases (ii) a mid-tropospheric cyclone (MTC) (iii) MTC with offshore convection and (iv) offshore convection is shown in the figure. It is found that for the period 1979-89 (for which the rainfall as well as OLR data are available) there were 37 days with
intense rainfall. Of these, the OLR pattern was indicative of 23 being associated with TCZ, 4 with MTC and 10 with offshore convection. Of the latter category, in two cases, the offshore convection appeared to be associated with an isolated vortex. The probability of occurrence of at least one day or two days with rainfall higher than 20 cms in different weeks is given in Table 2. It is seen that the highest probability is from about mid-June to mid-July. Shyamala's (2001) analysis of the events with rainfall exceeding 20 cms/day at Colaba and Santacruz during 1985-2000 showed that out of the 11 events that occurred, 4 were during the onset phase during 9 - 17 June, 6 in active phase towards end of June or between 10-15 August and one during the withdrawal phase in September. Analysis of intense rainfall events on the west coast during (1991-2000) showed that the highest probability of occurrence is during 11 July to 5 August and the second highest during 6-20 June (Thapliyal 2001).
These results are consistent with Table 2. The probability for longer periods of 2, 4, 6 weeks starting on different dates is also given in Table 2. This table will be useful in choosing an appropriate period for the field study, depending on the length of the period of availability of ships, etc.
Comparison of the data on existence of the offshore trough in the weather charts from IMD with occurrence of the intense rainfall events suggested a weak relationship. Only on some of the days with intense rainfall, the trough was reported. On the other hand, such an offshore trough was seen even when the rainfall was not very high. This is illustrated for the years of 1986-88 in Fig. 10, in which the variation of the maximum rainfall on each day in the summer monsoon is shown.
4. Key issues
The studies based on the data from previous field experiments have given some information about the major problems to be addressed in ARMEX viz
the intense rainfall events on the west coast
the monsoon onset processes and the Arabian sea warm pool.
Most of the information about Arabian sea systems responsible for intense rainfall events has come from studies of MONEX-79 data. These data have also provided insight into the structure of the monsoon onset vortex. Another important feature is the build up of moisture in the atmosphere prior to the onset (Joshi and Desai 1985, Pearce and Mohanty, 1984). We have gained further insights using these and other available data sources and through numerical model studies of the rainfall over the west coast and western ghats and the formation of southeast Arabian Sea warm pool. Mohanty et. al. (1983, 90, 93, 94, 96, 2000) have studied the relationship of the variability of the summer monsoon to the air-sea fluxes and heat budget of the Arabian sea. However, many key issues are yet to be addressed and understood regarding the monsoon related processes over the Arabian Sea.
a. Intense rainfall events on the west coast
Generally, high rainfall on the west coast rainfall has been attributed to orography. However, it has been clear for some years that while orography is important, other factors such as synoptic conditions (vertical thermal stability, wind shear, etc.) and evaporation from the ocean not too far from the coast, are also equally important. This is mainly based on theoretical/numerical model studies for which MONEX-79 data provided the basis. We need detailed observations to confirm this. Further, the offshore trough has been considered to play an important role in the intense rainfall events on the west coast. However, its nature, spatial extent, association with synoptic scale settings have not been established because of the lack of detailed observations after MONEX-79. Also, whether the convection precedes or lags offshore trough is not known. Another entity/phenomenon often talked about but needs confirmation about its independent existence, structure and dynamics is the offshore vortex.
Intense rainfall events on the west coast have been associated with mid-tropospheric cyclones, the organized convection of the tropical convergence zone during onset phase or revival with northward propagation and offshore vortices/offshore troughs. It is suggested that the travelling systems from the Bay of Bengal help in the genesis of MTC. It may be noted that about 10% of the intense rainfall events are associated with MTC. What about the other cases, which account for nearly 90% of the intense convective events? Does intensification of convection over the Bay of Bengal and monsoon trough zone provide the large scale conditions favourable for growth of systems off the west coast? Thus, there are major issues concerning the precise nature of the offshore trough, type and nature of mesoscale systems that form during intense rainfall events, their links to Bay of Bengal convection, role of local orography and ocean conditions that needs to be clearly documented and understood.
Study of the genesis and evolution of the offshore trough requires measurements of the surface pressure and wind field at high resolution along the coast and within about 100 kms from the coast, at least eight times a day. Such measurements can be made by an enhanced network of coastal stations, buoys and ships from coast guard, etc. It appears that the SST variation near and off the west coast plays an important role in determining the favourable location of convection. The role of the SST field and the nature of the coupling between convection and SST needs to be understood. There has been considerable discussion on systems characterized by more rainfall over the coast than orography. It is necessary therefore, to get a detailed spatial distribution of the rainfall events. The role of the air-sea fluxes, vertical stability of the atmosphere and the vertical windshear in genesis intensification and propagation of the convective systems needs to be investigated. The structure of the meso-scale convection, embedded in the larger scale needs to be elucidated. For this, detailed measurements are required from ships at strategic locations.
b. Formation of stable layer - role of remote forcing
Available observations clearly show that relatively low saline water occupies the top layer of the ocean in the south east Arabian Sea from December onwards. Model studies suggest that this water originates in the Bay of Bengal. The collapse of the summer monsoon in October and the onset of winter monsoon triggers a downwelling coastal Kelvin wave that propagates along the periphery of the Bay of Bengal (McCreary et al., 1993; Shankar and Shetye, 1997; Shankar, 1998; Shankar et al., 2001). This Kelvin wave, which forces an equatorward East India Coastal Current, bends around Sri Lanka to propagate poleward along the Indian west coast, forcing a poleward West India Coastal Current. These two coastal currents are connected by the Westward Monsoon Current south of Sri Lanka. The Kelvin wave along the Indian west coast radiates downwelling Rossby waves, which lead to the formation of the Lakshadweep high. What is the role of this downwelling in providing a breeding ground for the formation of the Lakshadweep Sea high and Arabian Sea warm pool in the following months? How important is remote forcing dynamics of the southeastern Arabian Sea for the thermodynamics of the region?
The existence of the Lakshadweep SST high also implies that a large fraction of the low-salinity water is trapped in the Lakshadweep Sea, not carried poleward along the west coast of India by the West India Coastal Current. This perhaps is a consequence of the Winter Monsoon Current bending around the high before flowing into the West India Coastal Current. What is the pathway of the Bay of Bengal water into the Arabian Sea? Does it flow around Sri Lanka, or does it flow through the gap between India and Sri Lanka via the Palk Straits and the Gulf of Mannar? The choice of pathway seems to determine how the low-salinity water is spread in the Arabian Sea (Han and McCreary, 2001). Is low-salinity water transported into the Arabian Sea by the winter monsoon current that flows across the southern Bay of Bengal during January-April? How important is the inflow from rivers along the west coast of India? What is the salt budget of the Lakshadweep Sea? The last of these questions encompasses the ones listed before and needs to be answered for clarifying the role of salinity in the mixed-layer dynamics, and hence, in the thermodynamics of air-sea coupling in the region covered by the Arabian Sea warm pool.
c. Low-salinity water and SST evolution
The low salinity water from the Bay increases the stratification of the near-surface layer. It has been demonstrated in the western Pacific that high stratification near the surface can lead to high SST owing to the trapping of the heat flux from the atmosphere in a shallow layer. The evidence for the low-salinity water in the southeastern Arabian Sea has hitherto been known only through climatological data sets. What is the magnitude of the stratification caused by the fresh water? How does the low-salinity water affect the mixed-layer depth and SST in the southeastern Arabian Sea?
The SST evolution is determined by the net energy balance. The available climatologies suggest that the skies over the Arabian Sea are clear prior to the onset of the summer monsoon, implying that the ocean receives a large amount of heat through solar radiation. The winds are weak and hence latent heat loss is small, causing large heat gain by the ocean (Rao and Sivakumar, 2000). How much heat does the ocean gain from the atmosphere? How much heat is absorbed within the mixed layer and how much penetrates into the thermocline? Is the local rainfall important for the salinity budget of the Lakshadweep Sea? What are the implications of the local processes to the Lakshadweep SST high and the Arabian Sea Warm Pool? In other words, how important are local oceanic processes and air-sea fluxes to the heat and salt budgets of the Arabian Sea Warm Pool?
The surface heat flux Q is given by,
Q = NSW - NLW - SH - LH (3)
where, NSW is net shortwave radiation, NLW is net longwave radiation, and SH and LH are sensible and latent heat fluxes. INDOEX data show that the incoming shortwave radiation is reduced by about 30 W/m2 due to high amounts of aerosols in the atmosphere over the Arabian Sea (Satheesh & Ramanathan, 2000). The amount of aerosol keeps building up from January till March end and then remain constant till the monsoon arrives. During March-April, the source of air in the lower troposphere over the southeast Arabian Sea changes from Indian sub-continent origin to that of African origin (Ramanathan et. al. 2001). This will also change the nature of aerosols (perhaps from high carbon content to lower carbon content). However, we have no data to understand how this affects the incoming solar radiation at the surface.
Similarly, lack of accurate data on difference sea and air between the surface of the specific humidity differences (DT and Dq respectively) result in significant uncertainties in the estimation of sensible and latent heat fluxes. If we use climatological Dq data in a simple upper ocean heat balance calculation, then the predicted SST is 3-4°C higher than that observed over a period of 3 months (Fig. 11). If the surface air relative humidity (RH) is decreased by 5%, then, that gives a better agreement with the observations (Fig. 11). It is also likely that the aerosols significantly reduce (by perhaps 20-30 W/m2) the amount of solar radiation reaching the surface during April-May. However, observations on the role of aerosols in attenuating the radiative heat fluxes over the southeast Arabian Sea is not available during April-May.
Thus, some important measurements need to be carried out before we can assess the relative contributions of the individual components of surface heat flux over the Arabian Sea warm pool to understand its build up and maintenance.
d. Air sea coupling at high SSTs
It is observed from Fig. 3 that SSTs above 30°C prevailed for about 45 days before a cyclone cooled the sea in June during the year 1998. The published SST versus convection relationships (e.g., Graham and Barnett 1987, Arking and Ziskin 1994, Waliser et al., 1993), show that on the global scale, there are few cases where monthly mean SSTs exceed 30°C. We know very little about the nature of air-sea interaction over oceans which are very warm and without convection. Recent observational and modelling work emphasises the role of water vapour convection feedback in the existence of very high SST regions over the tropical warm pool (Tompkins 2001). However, we know nothing about the variation of Dq with SST [or relations similar to equations (1) and (2) derived for the West Pacific over the southeast Arabian Sea during April and prior to monsoon onset when the SSTs are high. This information is crucial because the surface sensible and latent heat fluxes directly depend on DT and Dq respectively.
e. Pre-Onset atmospheric conditions
As stated earlier, number of observations at SSTs above 30°C are relatively few even on global scale. The main reason is that such high SSTs occur under clear sky conditions, that, however, are not sustained for long as convection eventually develops and reduces SST (Tompkins 2001). The mechanism responsible for keeping the deep convection from developing over the Arabian Sea even at high SSTs for prolonged periods is not clear. It is likely that either a strong atmospheric inversion prevails inhibiting convection or sufficient moisture to cause convective instability is not present in the surface layer air. Thus, there is more than one mechanism that can prevent the deep convection from triggering. Only observations can tell which mechanism works in the south east Arabian Sea. It is important to investigate the role of advection (of dry warm continental air from Africa and Arabia) and subsidence over the Arabian Sea in the maintenance of the temperature inversions. In situ and satellite soundings, cloud motion vector data, NCEP analysis etc will help in studying this process.
f. Intense convection over south-east Arabian sea and monsoon onset:
The suppressed state of convection is replaced by an active phase of deep convection with the monsoon onset. How the atmospheric conditions that prevent convection from developing even at high SSTs change to those conducive for convection is an important issue. The build up of moisture prior to the monsoon onset perhaps contains key information. It appears that, the air in the lower troposphere which was of continental origin is replaced by air of marine origin. This can be due to a change in the atmospheric circulation. What are the synoptic scale changes that brought out this? How important is convection over Bay of Bengal in the monsoon onset process over the Arabian Sea?
g. Soil-moisture flux and hydrology of the Western Ghats
It is expected that the flux of moisture from soil on the windward side of the western ghats plays an important role in evolution of convective systems. Hence there is a need to get quantitative description of this flux. There is also a need to construct models of hydrology of the region that can simulate the flux. Such empirical and model studies provide important information on runoff associated with rainfall events along the west coast. The runoff is an important variable that influences salinity and stability of the coastal ocean. The stability in turn influences SST variability.
Thus, there are several scientific issues related to the south east Arabian Sea warm pool build up and monsoon onset over Kerala which are scientifically important but not resolved. In particular, it looks like, this ocean provides a unique place in the world to study the coastal wave dynamics in transporting and trapping fresh water, air sea coupling at high values of SST, effect of land plumes from surrounding land masses on atmospheric stability, and aerosols possibly influencing the SST evolution.
5 Approach and Objectives
The issues involved are multidisciplinary and no individual group or institute in the country can attempt/handle all these aspects on its own. A variety of platforms are to be used to collect oceanic and meteorological observations of relevance to the needs of ARMEX. Therefore ARMEX is conceived as an inter-institutional national programme involving several organizations each contributing in its area of expertise and interest. ARMEX will involve collecting data from research vessels, moored buoys deployed by DOD, instrumented moorings and satellites along with enhanced observations at the nearby coastal and island stations.
Since the intense rainfall events on the west coast occur during the summer monsoon, whereas the study of the Arabian Sea warm pool must extend from November until the monsoon onset in June, the ARMEX observational programme will be in two parts. The objectives are
Study of the Arabian Sea Convection associated with intense rainfall events on the west coast. This includes the study of
genesis, intensification and propagation of convective systems (of synoptic and meso-scale) over the eastern Arabian sea leading to very heavy rainfall events along the west coast
variation of air-sea fluxes, vertical stability and wind-shear in association with these systems
impact of the SST field including wind-induced upwelling on convective systems
spatio temporal variation of offshore trough and its relationship with the convection on the eastern Arabian sea.
Study of the evolution, maintenance and the collapse of the Arabian Sea warm pool and preonset and onset phases of the monsoon
the life cycle of the Indian Ocean Warm Pool and Lakshadweep SST high over the Arabian sea: genesis, maintenance, and collapse of the Lakshadweep SST high, and its spatio-temporal structure vis-a-vis the Indian Ocean Warm Pool.
nature and strength of inversion in the atmosphere over southeast Arabian sea during the growing, mature and collapsing phases of warm pool.
components of surface fluxes and air-sea interaction processes in the Lakshadweep Sea during the study period.
the role of the warm pool in the southeastern Arabian Sea in the process of onset of monsoon over Kerala.
6. Observational programme
A variety of platforms are to be used to collect oceanic and meteorological observations of relevance to the needs of ARMEX. The issues involved are multidisciplinary and no individual group or institute in the country can attempt/handle all these aspects on its own. Therefore ARMEX is conceived as an inter-institutional national programme involving several organizations each contributing in its area of expertise and interest. ARMEX will involve collecting data from research vessels, DOD buoys, instrumented moorings and satellites along with enhanced observations at the nearby coastal and island stations. ARMEX measurements will be supplemented with the data collected by tide gauges, drifting buoys, XBTs along merchant shipping lines, satellites such as INSAT, IRS-P4 and other operational satellites for the ARMEX year. The reanalysis and other data coming from various sources (such as NCMRWF, NCEP, ECMWF, NASA, etc.) needs to be archived for the ARMEX year. Apart from these observations, any other measurement(s) that comes within the broad scope of ICRP and ARMEX will be exploited.
The problems and issues to be addressed in ARMEX Part I and II are distinct and therefore separate observational strategies have been proposed for each. For each part, some observations are absolutely essential for meeting the objectives and we refer to them as essential in the following. Additional observations which will broaden the overall quality, applicability and outcome of ARMEX data set, are referred to as desirable. Also, certain resources such as IMD's network of stations, ORV Sagar Kanya, buoys and satellite data are essential for both components of ARMEX.
For ease of communication and to facilitate on-board processing of data, it is necessary to have modern modes of communication, such as FAX, e-mail and internet, on board ORV Sagar Kanya. It may be recalled that the availability of FAX on board was extremely useful during the Intense Observation Phase (IOP) of BOBMEX. The additional equipment bought for Part I of ARMEX will be used for PART II also.
Area of study
The area of study will encompass the Indian west coast from 8°N to 24°N in the north-south direction, and up to 200 km from the coast in the east-west direction as shown in figure 12a. The offshore trough is expected to be within 100 km from the coast, and a larger number of observations within this belt will be carried out using available resources including research ships, airforce, air-craft coast guard vessels, buoys and ships of opportunity.
Period of study
The observation period will span from the beginning of June to August end in 2002. All the ground stations, including the specially installed Automatic Weather Stations will collect data during this period. However, research ships may not be available for this entire period. Research ships are to be deployed during mid June to mid August period. Within this period, there will be a few intense observation periods (IOPs). The exact period and frequency of IOPs will be decided by the operational group of ARMEX.
Observations and plan
As intense rain producing systems are meso-scale in nature, a dense network of surface stations along the west coast is required.
a. IMD network. IMD will be one of the major resource provider for ARMEX with both surface and upper air observations. It is essential that all coastal stations from Trivendrum to Ahmedabad, inland stations located up to 200 km from the coast line, and island stations Amini, Minicoy and Lakshdweep will participate in the observation and collect surface met data at synoptic hours. It is also important that data from the stations along the east coast of India and island stations are also included in the station surface met data set. This is necessary for studying the connections between west coast rainfall events and convection over the Bay.
b. Automatic weather stations (AWSs). In order to bring out the meso-scale structure of the convective systems, about 10 new AWSs need to be installed for ARMEX at suitable locations. Important observations needed from the AWSs and other specially equipped ground stations are wind speed and direction, surface pressure, precipitation, air temperature and relative humidity every 5 minutes. One or two micrometeorological towers with complete surface flux measurements is required on land near the coast.
ORV Sagar Kanya. Surface met data needs to be continuously recorded from this ship. Details of the data to be collected from ORV Sagar Kanya are given in Table 3. Apart from the usual surface met data, all component of surface fluxes need to be directly measured from this ship.
NIOT buoys. Some buoys are already deployed in the study area and data to be collected from the buoys are given in Table 4. It is important to strengthen the network of observations with two additional deep water buoys, and one shallow water buoy at suitable locations.
Two ships from Indian Coast Guard that will take routine surface meteorological observations. Feasibility of installing and operating AWSs on Coast Guard vessels needs to be explored.
ONGC rigs/platforms and Atomic Energy wind towers. If the weather monitoring systems at these locations are made operational and data are collected during ARMEX, the spatial coverage of data is enhanced.
Merchant ships. Wherever possible, request may be made to merchant ships operating in the Arabian Sea to collect surface met data at regular time intervals along with ship position.
Feasibility of obtaining surface met data from one or more Navy ship(s) to be explored.
Upper air observations
IMD radiosonde stations. IMD stations at Mumbai, Mangalore, Trivandrum, Ahmedabad, Minicoy, Amini and Goa will be part of the essential data. During normal period, 2 ascents per day and during IOP 4 ascents per day are desirable.
ORV Sagar Kanya. High resolution Vaisala radiosondes with GPS upper winds. While 2 launches during normal period is required, 4-6 launches during IOPs are necessary.
Suitable location on west coast/second ship. In order to form a triangle/polygon of radiosonde stations that is useful for studying certain aspects of atmospheric circulation and intense convective events, it is important to have a second Vaisala system at one of these places.
Navy/Airforce. The spatial coverage, especially over the ocean and between Goa and Mumbai, is enhanced if Airforce and Navy can operate their high resolution radiosonde systems and launch radiosondes.
SST, surface salinity, skin temperature, CTD, shortwave absorption and ADCP data from ORV Sagar Kanya. The required equipment and their is status is given in Table 3.
SST, surface salinity, XBT and CTD observations from a second ship to obtain more data on upwelling and improve spatial coverage.
OLR data from INSAT. It is essential to have OLR at 1o x 1o resolution for the entire period (May-August) at 3 hourly intervals covering 50 o E to 120 o E and 25 o S to 35 o N. During IOPs, pixel level OLR data is required to study the mesoscale structure of the convective systems.
Visible imagery from INSAT. Pixel level, 3 hourly during IOPs. Other period, at 1o x 1o resolution.
Water vapor and sounding data from satellites
MSMR-IRS P4 & TRMM. Surface wind, precipitable water content, SST, and cloud liquid water content from May till August end.
Useful information/data from other satellites such as Meteosat, NOAA, etc.
Pilot Balloon Observations
Pilot balloon observations at Veraval, Cochin, Pune, Gadag and Bhuj by IMD, at Baroda, Belgaum/Karwar by Indian Air Force are desirable. During normal observations 2 ascents a day and 4 observations a day during IOP.
Radar observations and echo patterns from Mumbai, Goa, Ahmedabad and Bhuj. If Navy and Airforce have cloud observing radars, their data will be useful. Frequency of observation may be decided by the synoptic conditions and weather advice.
Aircraft can provide detailed structure of the offshore trough and vortex. If air craft from Navy or Air Force become available, then these will be flown during periods when offshore systems intensify.
Other observations: Observations of boundary layer height from sodars atmospheric aerosol and water vapour profiles from lidars available at different institutions; data from ARGO floats surface drifters, and; reanalysis.
ARMEX Part II
ARMEX Part II focuses on two aspects of the warm pool. First, there is the initial build-up phase, in which the current hypotheses indicate a major role for remote forcing from the Bay of Bengal. Second, the growing, mature and collapse stages in which local air-sea interaction and vertical stability of the atmosphere are important.
In order to address the physical processes adequately, both remote and local effects have to be captured by the observational programme. Given that resources are limited, a combination of cruises and moored buoys with attached sensors is proposed. The cruises should provide detailed information on the three-dimensional structure of the warm pool, and if the timing is right, should capture the effect of remote and local processes in the region. The moored buoys, on the other hand, will provide a time series showing the temporal structure of the warm pool. The following cruises and additional moored buoys are proposed for ARMEX.
To ensure uniformity in collection of the raw data, it is necessary that the same hardware (ship and instruments) and software are used for all the cruises or detailed intercomparison exercises are carried out in case of multiple platforms. It is also important to standardize the data format for various variables. Detailed analysis of the processed data could be done by the scientists later using techniques of their choice.
Area and period of study
Three ship surveys are proposed using ORV Sagar Kanya along the track shown in Fig. 12c. Measurements are to be made every 0.5 degree (about 50 km) along the track. To discern the diurnal cycle, time series measurements are proposed at two locations; for these measurements, the ship will remain stationary for two days. In all, about 70 CTD stations are proposed and the total cruise time is estimated to be about 30 days (for each cruise). What follows is the time schedule of the cruises and the measurements to be made on board the ship.
A. 1 November to 30 November 2002
This is the time when the Kelvin waves from the Bay of Bengal pass into the Arabian Sea and the East India Coastal Current, the Winter Monsoon Current south of Sri Lanka, and the West India Coastal Current form a continuous current transporting low-salinity water from the northern bay to the Arabian Sea. This cruise is expected to capture the arrival of low-salinity water into the southeastern Arabian Sea.
B. 15 March to 15 April 2003
This is the time when the Lakshadweep Sea SST High is in its mature phase and stands out in the southeastern Arabian Sea. This cruise is expected to provide detailed information on the role of the downwelling Rossby waves in maintaining a stably stratified top layer and its heat content. The nature of air-sea coupling, vertical stability of the atmosphere, aerosols and radiation are other important issues that will be monitored.
C. 15 May to 15 June 2003
The Indian Ocean Warm Pool and the Lakshadweep SST High are at their peak early in May, and the distinction between them is clear. Large-scale moisture convergence into the region takes place at this time, laying the ground for the onset of the summer monsoon. With monsoon onset, the SST in the warm pool falls sharply. This cruise is expected to provide information on the mature and decay phases of the warm pool, its coupling to the atmosphere in the presence of large-scale moisture convergence, and the interaction between ocean and atmosphere during the onset of the summer monsoon.
Observations required for the collapse stage of the Lakshadweep High and monsoon onset component of Part II are very similar to that of Part I. During this period, it is necessary to obtain additional data from near by and other weather monitoring stations already set up for Part I. Efforts should be made to document the changes in the synoptic circulation pattern and build up of humidity in the atmosphere just before the monsoon onset takes place. Also, if a second ship becomes available, then a cruise from mid-April to mid-May will be carried out in the Lakshadweep high area.
For ARMEX, part II it is necessary to augment and strengthen this network to obtain information not obtainable from ship surveys. All new buoys should be equipped with the following additional sensors, not available on the current NIOT buoys: Rainfall, radiation, humidity, subsurface temperature (profiles) and salinity, current meters or ADCPs. Existing buoys should also be upgraded if possible.
To obtain information on the currents that transport low-salinity water from the Bay to the Arabian Sea and to make measurements in the warm-pool region, five additional moored buoys are proposed; the moorings have to be completed a month before the first cruise, which is scheduled for 1 November, and are expected to function for one year.
It is proposed to have subsurface current meters installed at one location (DS8/AR5). This mooring should be equipped with several subsurface current meters/ADCP.
Current Meter Moorings
In order to understand the pathways of fresh water into the southeastern Arabian Sea, it is proposed to deploy current meter moorings at strategic locations to the east and south of Sri Lanka/India. These moorings will also help to document the salt balance/exchange in the region between the Arabian Seas and the Bay.
iii. ARGO floats
An additional source of in situ quasi-time-series measurements is likely to be available during ARMEX because of the ARGO programme. The Indian ARGO programme is expected to be launched during July 2001. For ARMEX, six floats are proposed to be launched in the region covered by the warm pool. These floats should buttress both - the spatial coverage of the ship surveys and the temporal coverage of the moored buoys.
iv. Other instrumentation
In addition, ARMEX can take advantage of the existing network of tide gauges at coastal and island stations, satellite-based measurements such as altimeter, SST, OCM (Ocean Colour Monitor), and winds from scatterometers.
7. Modelling using ARMEX data
The data sets collected during ARMEX will have applications well beyond meeting the immediate objectives mentioned in this document. Here we list a few important ones. Mesoscale modeling is now being pursued as a priority area in India. However, this effort basically suffers from lack of initial data on sufficiently small spatial and temporal resolution for the Indian region. To meet the basic objectives of ARMEX part I, data from a dense network of stations along the west coast will be collected. This will be supplemented by satellite, radar, aircraft and other observations and data sources. Thus, data sets collected during ARMEX will have reasonably good spatial and temporal resolution, and will be relevant to the large-scale circulation features. Therefore, we expect ARMEX part I data to be highly valuable in the development and validation of mesoscale and other regional scale models for the Indian region.
A major focus of ARMEX part II is the genesis and evolution of the Lakshadweep High and the associated warm SST. The data will enable us to understand the origin and seasonal cycle of the larger scale warm pool in the Indian Ocean. The modeling of the warm pool is a challenging task. Worldwide, the simulation of warm ocean SSTs using models is at a nascent stage. There does not yet exist in India an ocean model that can simulate realistic SSTs. The existing models in the country, however, are capable of simulating the wind-forced dynamics of the warm pool. It is necessary to initiate steps towards building models, or adapting the existing models, to simulate realistic SST in the Indian ocean.
It is expected that the ARMEX observations will also provide insight into the air sea interaction over the other parts of the Indian Ocean warm pool including the Bay of Bengal. in early summer, and its role in establishing the circulation and rainfall associated with the Asian monsoon .The ARMEX data should also provide valuable guidance in the development of realistic models of the thermodynamics of the warm tropical oceans. because they will permit quantitative study of the role of large scale air-sea interaction in regional climate, including monsoons.
8. Tentative budget Part I
ARMEX will utilize available national resources. The funds required from DST for the ARMEX program is expected to be as follows. It is to be noted that the equipment acquired for Part I of ARMEX is required for Part II.
Other resources needed from different national agencies
Other resources needed for different national agencies. Budget for this is to be provided by the respective agencies. In addition, these agencies are also expected to contribute manpower at different levels during the operational phase.
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Appendix - A
Participating / interested institutes
India Meteorological Department (IMD), New Delhi
Indian Institute of Tropical Meteorology (IITM), Pune
National Centre for Medium Range Weather Forecasting (NCMRWF), New Delhi
National Centre for Antarctic and Oceanic Research, Goa
National Institute of Ocean Technology (NIOT), Chennai
National Institute of Oceanography (NIO), Goa
Indian Institute of Science, Bangalore
Naval Physical and Oceanography Laboratory (NPOL)
Indian Institute of Technology, New Delhi
Indian Air Force, IAF, New Delhi
Indian Navy, IN, New Delhi
Indian Coast Guard, IN, New Delhi
Space Application Centre (SAC), Ahmedabad
Space Physics Laboratory (SPL), Trivendrum
National Remote Sensing Agency (NRSA), Hyderabad
National Physical Laboratory, NPL, New Delhi
Bhabha Atomic Energy Centre, Tarapur
Andhra University, Vishakapatnam
Cochin University, Cochin
Jawaharlal Nehru Port Trust
Oil and Natural Gas (ONGC)
Other universities/institutions such as Banaras, Jadavpur, Mangalore, IIT Kharagpur, Goa University, Agriculture University at Dapoli and Mulde etc. where meteorological / atmospheric science groups are active in the related research.
ARMEX observational Programme
All the observations / data platforms mentioned under this programme will take routine as well as Intensive observation as per advise. Meteorological observatories not mentioned here, but with in 200 km of coastline will take routine meteorological observations.
Contribution of major national agencies
Daily all India surface, upper air, RS/RW, rainfall observations will be needed for detailed analysis and modeling studies during the entire period of the experiment. The data from observatories with in 200 Km of the western coastline would be crucial and any ship, aircraft, data collection platform, automatic weather station would go a long way in supplementing the effects towards the observational programme of ARMEX. The under-mentioned surface, RS/RW, pilot balloon, weather radar, and surface radiation observations, AWS/Self Recording Stations etc., in addition to their routine observation programme would participate in the Intensive Observational Programme (IOP) of ARMEX.
*Class- I: Nalia, Okha, Porbandar, Viraval, Rajkot, Ahmedabad, Baroda, Nashik, Dhanu, Santacruz, Bombay, Pune, Mahabaleswar, Sholapur, Ratnagiri, Panjim, Goa, Belgam, Gadag, Karwar, Hanovar, Agumbe, Chitradurg, Mangalore, Bangalore, Kozikot, Trivendrum, Minicoy, Amini
*Class- IIa: Deesa, Bhavnagar, Surat, Alibagh, Harnoi, Satara, Kolapur, Cannanore, Aleppey, Sangli
*Class- IIb: Kandla, Vallabh Vidyanagar, Jalgaon, Malegaon, Beed, Surendra Nagar, Osmanabagh, Baramati, Bijapur, Bellary, Hassan, Chickmaglore, Mysore, palghat, Kottayam, Punalur, Devangiri
Class- IIc: Vengrula
* The basic instrumental equipment of a second or third class observatory is a mercury Barometer, four thermometers (dry, wet, maximum and minimum) fixed inside the Stevenson Screen, rainguage with measure glass, wind vane and anemometer. In addition to these, first class observatories have open evaporimeter, sunshine recorder and autographic instruments for most of the weather elements. The meteorological elements observed are atmospheric pressure, dry bulb, wet bulb, maximum and minimum temperatures, humidity, amount of rainfall, direction and speed of wind, visibility, amount, form and direction of movement and height of base of clouds and also wave observations in case of coastal stations. Evaporation and hours of bright sunshine are recorder at first class observatories.
RS/RW observatories: Ahmedabad, Mumbai, Mangalore, Trivendrum, Minicoy, Amini, Goa
Pilot Balloon obseravtories: Veraval, Cochin, Pune, Gadag, Bhuj.
Weather Radar Observatories: Mumbai / Santacruz, Cohin, Goa
INSAT - observatories: OLR, Cloud top temperature, cloud pictures Vis&IR, water vapour winds, Pixel data for specific cases, SST, Temp. and humidity profiles from NOAA.
Radiation observations: Ahmedabad, okha, Anand, Mumbai, Pune, Goa, Mangalore, Amini, Minicoy and Trivendrum will record every 3 hours the total incoming and outgoing radiation. These observations would be helpful in the heat budget of the atmosphere.
State Raingauge observatories: Data of all state raingauge stations of Maharastra, Karnataka and Kerla along the coast and inland upto about 200 kms.
(Xerox copies of the traces of the daily autographic recording instruments/hourly extracted data would be made available to the Data Management Team along with the other data sets)
National Centre for Antarctic and Oceanic Research, Goa
To provide ORV Sagar Kanya with full complement of meteorological instruments
National Institute of Ocean Technology (NIOT)
4 Shallow water and 2 deepwater NIOT buoys will take meteorological observations. Additional 2 deepwater buoys, which are likely to become available by April 2002 (in map it was shown as DS3, DS4), would be positioned accordingly. Locations of new buoys to be within 100 - 150 Km off the coast
Indian Air Force (IAF)
Surface observatories : Jamnagar, Baroda, Nasik, Pune, Belgaum.
Pilot Balloon observations: Pune, Baroda, one PB observatory to be set up at Karwar in the Agriculture University subject to availability of accommodation, is failing which the observatory would be set up Belgaum.
Air borne observations: The Indian Air Force would provide a few aircraft based observations consisting of inflight winds and free air temperature. The aircraft after crossing the western Ghats will fly at 1km a.m.s.l. between 50 to 150 km. Thereafter the aircraft may climb 7 km and return to base. In addition to winds and free air temperature,
some cloud pictures of air borne weather radar aboard the aircraft would also be taken. A few broad flights would be planned in advance in the implementation plan. However, the exact details of the flights would be based upon the availability of the aircraft and the prevailing synoptic situation.
Manpower: The Indian Air Force may also provide 10 Met/Assts. to help in the observational programme.
Surface observatories: Cochin and Goa
RS/RW station: Cochin, Goa and Ohkha
Ship-borne observations (IN): One or two ships with on board GPS would be operated. It would take surface and GPS observations.
Coast Guard (Ships): The coast guard would operate 2 or 3 ships in the area and would take only surface meteorological observations. AWS will be set up with these moving platforms to get continuous surface observations. The ships may ply North-South or East-West sections off 100 - 150 Km from the coast.
National Centre for Medium Range Weather Forecasting (NCMRWF)
Quick - Sat : Surface wind would be provided by NCMRWF.
SSMI/TRIMM: Sea surface temperature (SST), surface winds, total precipitable water, rainfall analysis.
Reanalysis of surface and upper air data after assimilation of the additional observations taken during ARMEX.
Space Application Centre (SAC)
MSMR - IRSP4 data pertaining to surface wind, precipitable water content, sea surface temperature (SST), and cloud liquid water content would be provided by SAC, Ahmedabad.