Equatorial Indian Ocean evaporation estimates from operational meteorological satellites and some inferences in the context of monsoon onset and activity

The estimation of evaporation from the sea surface has not yet been achieved adequately by remote sensing techniques, in general. However, for approximate averaged estimates over moderate space and time scales over a specific tropical region, e.g., weekly values over the Indian Ocean as needed in monsoon moisture diagnosis, it may be possible to extrapolate satellite wind and humidity data to the ocean surface and then use bulk aerodynamic parameterization for estimating evaporation. In the present investigation, GOES low-level cloud winds and TIROS-N moisture profiles over the Indian Ocean are extrapolated to the ocean surface. The planetary boundary layer (PBL) wind shear is obtained over different subregions and periods during the monsoon season, by reference to objectively analyzed fields. These shear values are applied to GOES satellite winds to obtain sea surface winds. The humidity extrapolation was based on an exponential fit for water vapor density and a vertical distribution of relative humidity approximately proportional to atmospheric pressure. The exchange coefficient is varied slightly, depending upon wind speed and boundary-layer stability inferred approximately from TIROS-N sea surface temperatures and temperature profiles. The evaporation estimate as based on these satellite parameters is assessed by comparison with ships' surface observations. Sensible heat exchange is also estimated and assessed, and some inferences based on these estimates are presented, in relation to monsoon onset and activity.