ESTIMATES OF SURFACE HUMIDITY AND LATENT-HEAT FLUXES OVER OCEANS FROMSSM I DATA/

Monthly averages of daily latent heat fluxes over the oceans for Febru ary and August 1988 are estimated using a stability-dependent bulk sch eme. Daily fluxes are computed from daily SSM/I (Special Sensor Microw ave/Imager) wind speeds and EOF-retrieved SSM/I surface humidity, Nati onal Meteorological Center sea surface temperatures, and the European Centre for Medium-Range Weather Forecasts analyzed 2-m temperatures. D aily surface specific humidity (Q) is estimated from SSM/I precipitabl e water of total (W) and a 500-m bottom layer (W-B) using an EOF (empi rical orthogonal function) method. This method has six W-based categor ies of EOFs (independent of geographical locations) and is developed u sing 23 177 FGGE IIb humidity soundings over the global oceans. For 12 00 FGGE IIb humidity soundings, the accuracy of EOF-retrieved Q is 0.7 5 g kg(-1) for the case without errors in W and W-B and increases to 1 .16 g kg(-1) for the case with errors in W and W-B. Compared to 342 co llocated radiosonde observations, the EOF-retrieved SSM/I Q has an acc uracy of 1.7 g kg(-1). The method improves upon the humidity retrieval of Liu and is competitive with that of Schulz et al. The SSM/I surfac e humidity and latent heat fluxes of these two months agree reasonably well with those of COADS (Comprehensive Ocean-Atmosphere Data Set). C ompared to the COADS, the sea-air humidity difference of SSM/I has a p ositive bias of approximately 1-3 g kg(-1) (an overestimation of flux) over the wintertime trade wind belts and wintertime extratropical oce ans. In the summertime extratropical Pacific and summertime eastern eq uatorial Pacific Ocean, it has a negative bias of about 1-2 g kg(-1) ( an underestimation of flux). The results further suggest that the two monthly flux estimates, computed from daily and monthly mean data, do not differ significantly over the oceans.