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The Evaporation Rates Over The Tropical Indian Ocean have been computed using the bulk-aerodynamic formulae from the unpublished drinker data set and India Daily Weather Reports.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The mean annual evaporation estimates for the tropical Indian Ocean has been computed following Bunker (1976) using the unpublished pinker data set, as follows: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)
P
a = air density
C
e = exchange coefficient for water vapor
Q
s = specific humidity at sea surface temperature (g/Kg)
Q
a = 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.
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