Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall

In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.

Temperature sounding of the atmosphere over the Indian region using satellite data

Capability has been developed at the INSAT Meteorological Data Utilisation Centre (MDUC). New Delhi for making sounding retrievals using data from the U.S. Polar Orbiting Satellites; The International TOYS Processing Package (ITPP) developed by the University of Wisconsin, USA, was used for asking temperature soundings software for front-end processing and input of 1000 hPa analysis data Into the retrieval algorithm was developed at MDUC. In the physical retrieval method regression estimates generated listing stratospheric level HIRS channels and MSU channels were used as Initial guess. For the surface, two options were used, (i) climatological guess, and (ii) 1000 hPa analysis. The paper discusses temperature retrievals over the Indian region made on 13 selected dates from different seasons in 1989-91. Results of comparison of satellite retrievals with colocated radiosonde data are presented. There is good agreement between the two from 700 hPa to 150 hPa levels, with RMSE with 3 C. The error is higher at 850 hPa and near the surface, when climatologic IS used as surface guess, but IS within 3°-4° C when the 1000 hPa analysis is used.

Investigation of features of May, 2001 tropical cyclone over the Arabian Sea through IRS-P4 and other satellite data

In this paper, utility of satellite derived atmospheric motion vectors and geophysical parameters is brought out to discern appropriate signals for improving short-range forecasts in respect of development/dissipation of tropical cyclones over the Indian region. Results of a particular case study of May, 2001 cyclone, which formed in the Arabian Sea are reported. Analysis of wind field with input of modified cloud motion vectors and water vapour wind vectors is performed utilizing Optimum Interpolation (OI) technique at 850 and 200 hPa for finding dynamical changes such as vorticity, convergence and divergence for the complete life period of this cyclone. Simultaneously, variations in geophysical parameters obtained from IRS-P4 and TRMM satellites in ascending and descending nodes are compared with dynamical variations for discerning some positive signals to improve short range forecasts over the Indian region. The enhancement of cyclonic vorticity at 200 hPa over larger area surrounding center of cyclone was observed from 26 to 28 May 2001 which gave a positive signal for dissipation of storm.

Estimation of divergent wind from OLR data for use in objective analysis over the Indian region

A scheme is formulated for the use of OLR data in the estimation of vertical velocity; divergence and then the divergent part of the wind over Indian region. In this scheme, ascending motion over cloudy region is estimated from an empirical relation between the cloud top temperature and descending motion over cloud-free region is estimated from the thermodynamic energy equation and both are blended. From this blended vertical velocity field, aivergence, velocity potential and divergent winds at all standard levels from 4 to 8 July 1979 at 00 UTC are computed. These fields are compared with satellite cloud pictures, rainfall etc and they are found to be realistic in depicting the synoptic conditions. Total wind is computed as the sum of the estimated divergent component and rotational component computed from observed wind field. For assessment of the scheme, this total wind field at 850 hPa is used as initial. guess field in univariate optimum interpolation scheme and analyses were made for the period 4 to 8 July 1979. Results show that scheme is able to produce realistic analyses which included divergent part of the wind.

Precipitation chemistry over the Indian region

The present study is based on the precipitation chemistry data from the Background Air Pollution Monitoring Network (BAPMoN) in the Indian region, for the period 1976-87. Sampling is made on an event basis and the pH and electrical conductivity of the samples are determined from filtered samples immediately after cessation of rain. The chemical analysis is performed on monthly mixed samples. No trend is found in the pH of rainwater from background areas except at Allahabad, Pune and Visakhapatnam which suffer from sizable anthropogenic influences. The pH seems to be related more to NO3 ions compared to SO4-2 ions. A natural buffer appearing in the form of alkaline soil-derived species seems adequateat most places (except Mohanbari), in keeping a check on progressive acidification despite steady increase in concentration of nitrates. The ion balance cannot be achieved without including the presence of HCO3, which when done explains the observed electrical conductivity of rainwater. The interaction of marine aerosols with acid aerosols has also been studied for the marine regions in the Indian areas and reveals a substantial removal of chloride from sea-salt. Inland sources of NaCl have also been identified from the BAPMoN data.

Tropical tropospheric chemistry and climate change

A systematic programme of monitoring surface ozone and its precursor gases CH4, CO and NOx (NO + NO2) at some selected sites in the Indian region was started under ISRO's geosphere biosphere programme in 1991. Measurements have been made at Ahmedabad an urban polluted site, Gadanki a rural relatively clean site, Gurusikhar a high altitude site representative of the free troposphere and Trivandrum a coastal (relatively clean) site influenced by marine air. The data has been used to study different features of troposphere chemistry in the tropics. Some of the results from this programme relevant to the climate change problem are presented in this paper.

Application of Sasaki's numerical variational technique to the analysis of height and wind fields over Indian region

An objective analysis method based on Sasaki's numerical variational analysis technique has been taken up for the analysis of geopotential height and wind over the Indian region. The univariate optimum interpolation (UOI) method is used to generate the initial or input fields. These fields are then adjusted by the variational method. A study of this method over Indian and adjoining region for 850, 700, 500, 300 and 200 hPa levels is made from 4 to 8 July 1979 and the analyses obtained using this method are compared with the FGGE analyses.

Comparison of total ozone retrieved from NOAA satellite and measured from Dobson spectrophotometer over Indian region

An attempt has been made to compare the total ozone retrieved from HIRS channel-9 of NOAA-12 satellite using ITPP software at the facility of lMDPS, New Delhi with that of conventional Dobson spectrophotometer over Indian network stations. The satellite-retrieved total ozone agrees within an accuracy of +-8% with that of Dobson-measured total ozone except during the passage of a weather system over the Indian region. It is seen that whenever a western disturbance is passing over north India and neighbourhood, the difference between the satellite-retrieved and Dobson-measured total ozone becomes more than +- 8% (or +-20 DU).