Annual Report 1999-2000

Scientific Research

Monsoon And Tropical Climate (MONTCLIM)

The Monsoon and Tropical Climate Programme is directed towards undertaking studies on monsoon climate variability/change, modeling atmospheric processes and technology development for atmospheric science research. In order to study the effect of weather and climate in Tropics, efforts are being made to improve parameterisation of land-ocean-atmospheric processes in the Atmospheric General Circulation Models (AGCM).

Land-Atmosphere:

A major multi-institutional field experiment for studying land surface processes (LASPEX) has been successfully conducted over Sabarmati River Basin in Gujarat. The experiment was conducted at five locations, comprising intensive observational programme (IOP) of 5 days in a month during January 1997 to March 1998. One minute averaged data during IOP and ten minute averaged data on regular basis are being archived at the Indian Institute of Tropical Meteorology (IITM), Pune. Part of the surface micro-meteorological data and upper air data have been quality checked and supplied to user scientists for analysis. A workshop has been organised to discuss the initial results obtained from LASPEX.

The preliminary analysis indicated that, (1) in April there was considerable lag in the temperature maximum with respect to solar radiation as compared to July. Also, in summer (April), during early morning hours whenever winds were slightly higher and air temperature minimum at all levels were raised. This phenomenon has been attributed to more upward soil heat flux. (2) The heat flux evaluated by profile method and eddy correlation methods was comparable. Soil heat flux during the same period was observed to be of only 10% of net radiation. (3) A sharp increase in soil temperature, up to 100C, was noticed within two days of the crop removal from the field. (4) The albedo of Sun Hemp crop was observed as 0.25 as compared to 0.175 for bare soil. (5) The drag coefficient was found to vary (0.001 to 0.014) with stability as well as the direction of wind, and (6) The sensible heat flux evaluated by NDVI (Normalised Differential Vegetation Index) and profile methods agreed satisfactorily.

Also, the work related to five research projects utilising this experimental data for better understanding of the boundary layer characteristics of Indian monsoon region and parameterisation of land-surface processes, is in progress. In order to take up new initiatives, a background technical paper on `Forest fire and transboundary haze pollution’ was prepared.

Ocean-atmosphere:

The analysis of expandable bathy thermograph (XBT) data sets collected under Indian TOGA (Tropical Ocean Global Atmosphere) programme indicated considerable year to year variability in coastal upwelling, western boundary current and Indian monsoon current (IMC) in the Central Bay of Bengal. For the first time, the influence of advection on warm pool development and its maintenance was studied. The development of warm pool exhibited spatial variation and is maintained by air-sea fluxes during monsoon period. The waters from Arabian Sea, enter the Bay of Bengal through the IMC during southwest monsoon at sub-surface depths. However, location of IMC and its strength show year to year variability. The observed annual ranges of Sea-Surface Temperature (SST) showed significant influence of ENSO (El Nino Southern Oscillation) events. In the southern Andaman Sea, the cyclone heat potential exhibited significant variability. For example, winter 98 showed exorbitantly higher value, indicating the influence of ENSO event. The Western boundary current appears to be the Western arm of the large-scale anti-cyclonic gyre in the Bay of Bengal. Presence of large scale cyclonic gyre is evident in the Central Bay of Bengal during southwest monsoon season. During the fast moving severe cyclone of June 1996, a 1.6oC fall in sea surface temperature followed by intense upwelling close to the storm centre was noticed.

Monsoon rainfall is positively correlated with October sea surface temperatures (SSTs) of previous year in the southeast region of Sri Lanka and Southern Oscillation Index. Harmonic analysis of the monthly mean rainfall showed that the annual wave has its largest amplitude in the northern Bay of Bengal, where the amplitude exceeds 250 mm/month, and the lowest amplitude are found in the western Indian Ocean. The air sea interaction process over the tropical Indian Ocean region has been studied using satellite derived information. The results indicated that evaporation rates are higher over the Arabian Sea & Bay of Bengal during the low rainfall year (1987) suggesting little or no influence of this parameter on the ensuing monsoon activity over the Indian subcontinent. However, the evaporation rate over the south Indian Ocean and the low level cross equatorial moisture flux seem to play a major role. A 3D-Model to simulate wind induced circulation features in the Bay of Bengal has been developed. The model was able to reproduce many of the observed features of wind driven circulation in the Bay of Bengal.

Surface flux measurements in the Ocean-Atmosphere boundary layer during INDOEX first field phase experiment (February-March 1998) were undertaken. A 4-meter micro meteorological tower was installed on the ship and sensors mounted at 3-levels. Both fast and slow sensors were used. The data collected are being analysed.

In order to study the air-sea interaction and monsoon variability in the Bay of Bengal during monsoon season using ships and met-ocean buoys, a multi-institutional pilot experiment was successfully executed on board ORV Sagar Kanya during October to November 1998. For the first time, continuous time series ocean-atmospheric data, extending from about 1000 M depth and about 15 Km above the sea surface, along with reliable surface flux data representing clear sky and rainy conditions have been obtained. A workshop has been organised to discuss the initial results obtained from the Pilot experiment. Twelve research papers were discussed, based on the data analysis from BOBMEX-Pilot study.