Moisture flux divergence over
the tropical Indian ocean using insat Ib data M. R. Ramesh Kumar and L.
V. Gangadhara Rao Physical oceanography Division National Institute of Oceanography Dona Paula, Goa-403004, India. Abstract The Evaporation Rates Over The Tropical Indian Ocean have been computed using the bulk-aerodynamic formulae from the unpublished drinker data set and India Daily Weather Reports. The satellite derived precipitation from the Indian national satellite (INSAT-IB) VHRR (very High Resolution Radiometer) sensor operating in the range 10.5 to 12.5 um has been used for computing the precipitation over the tropical Indian Ocean. The moisture flux divergence has been computed as the difference between the evaporation and precipitation. The study shows that the Bay of Bengal region is highly conductive for large-scale convergence. As a case study the moisture flux divergence over the Arabian Sea for the summer monsoon months (June to September) of 1987 have been computed and the values being -0.02 x1010 2.55 x 1010, 0.70 x 1010 and 0.44 x 1010 tons/day respectively and compared with the previous estimates for other years. roduction A detailed knowledge of the moisture flux divergence over the tropical Indian Ocean is essential for identifying the source of the moisture for the summer monsoon rainfall which contributes to about 80 to 90% of the mean annual rainfall for the various meteorological subdivisions over India. A major difficulty in assessing the relative role of the contributions from the Arabian Sea and the Southern Indian Ocean has been the paucity of the evaporation estimates and precipitation measurements over the oceanic regions. Several investigators (Howland and Sikdar, 1983: Murakami et al., 1984: Cadet and Greco, 1987: Sadhuram and Ramesh Kumar, 1988)have looked into the role of the moisture flux divergence over Arabian Sea and Indian Ocean with limited amount of available data over specific regions or specified seasons. In the present study we look into the annual mean moisture flux distribution over the tropical Indian Ocean. In addition, we present the net flux divergence values for the as, for the monsoon months of June to September 1987. Data and Methodology The mean annual evaporation estimates for the tropical Indian Ocean has been computed following Bunker (1976) using the unpublished pinker data set, as follows: E = Pa Ce
(Qs-Qa)U Where E = evaporation rate (in mm) Pa = air density Ce = exchange coefficient for water vapor Qs = specific humidity at sea surface temperature (g/Kg) Qa = specific humidity at the air temperature (g/Kg) U = wind speed (ms-1) The data needed for computing the evaporation estimates for the AS in 1987 have been extracted from the Indian Daily Weather Reports (IDWR), and the data pertaining to 1987 precipitation over the Arabian Meteorological division of India Meteorological department. From the Conservation of Water (Rasmusson, 1972), we have (¶w/¶t)+ÑQw = E - P
where, W is the total column water mass,ÑQw the divergence of the moisture flux throughout the column, E is the rate of evaporation, and P the rate of precipitation. (¶w/¶t) = 0 for the long term annual mean. at The annual mean precipitation values used in the present study are extracted from the INSAT-IB derived precipitation values for the year 1987 over the tropical Indian Ocean using the Very High Resolution Radiometer (VHRR) sensor working in the range 10.5-12.5 um. We assume that there will not be large inter annual variations in the oceanic precipitation for the study area. The major reason for using the satellite derived precipitation is the non-availability of oceanic precipitation values for the tropical Indian ocean. Rao et al. (1981): Vinayachandran and Ramesh Kumar (1989) have validated the satellite derived precipitation values with the observed island station data, and have found that they differ by about 50% and 25% respectively. Results and Discussions Since the bulk of the moisture exists in the lower part of the atmosphere, we can interpret Fig. 1 in terms of the column divergence/convergence Figure 1 presents the moisture flux divergence/convergence/at the sea surface over the tropical Indian Ocean. The most interesting feature of the above figure is the region of large scale convergence (dashed area) in the eastern Indian Ocean, especially Bay of Bengal area. This indicates that this region is highly conductive for large-scale connective activity like the formation of monsoon depressions and tropical cyclones round the year. From this it can be seen that the evaporation exceeds precipitation over all the tropical Indian Ocean except the eastern half of it. Thus it further emphasizes the role of the Arabian Sea and the cross-equatorial moisture flux than the moisture from the Bay of Bengal region for the summer monsoon rainfall over the Indian subcontinent. Table 1 gives a comparison of the water vapor flux values for the AS during several contrasting monsoon seasons. An analysis of Table 1 shows that the moisture flux over the AS exhibits large inter seasonal and inter annual variations. The values for 1975 and 1979 were obtained using the upper air data, where as for the rest of the years it is obtained as the difference between the E-P estimates. Hence the large variability in the moisture flux divergence values can be attributed to the precipitation fluctuations or the errors in the precipitation estimation. Conclusion
The authors are grateful to the Director, N.I.O. and Dr. J.S. Sastry, Deputy Director and Head, Physical Oceanography Division for their keen interest and encouragement. References
Table-1 A comparison of water vapour flux values
over the Arabian Sea during contrasting monsoon years. Unit: 1010 tons/day
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