AN ANALYSIS OF ERRORS IN SPECIAL SENSOR MICROWAVE IMAGER EVAPORATION ESTIMATES OVER THE GLOBAL OCEANS

Monthly averaged evaporation over the global oceans is estimated from 1 year of special sensor microwave imager (SSM/I) data using the metho d proposed by Liu (1984). Intercomparisons involving SSM/I and in situ data are made over a wide range of oceanic conditions during August 1 987 and February 1988 to determine the source of errors in the evapora tion estimates. The most significant spatially coherent evaporation er rors are found to come from estimates of near-surface specific humidit y q. Systematic discrepancies of over 2 g kg-1 are found in the tropic s, as well as in the middle and high latitudes. The q errors are parti tioned into contributions from (1) the parameterization of q in terms of the columnar water vapor W (i.e., the Liu q:W relationship) and (2) the retrieval algorithm for W. It is found that the spatial pattern o f discrepancies due to the q:W relationship can be interpreted readily in terms of dynamical and physical processes that maintain the vertic al profile of atmospheric water vapor content. The effects of W retrie val errors are found to be smaller over most of the global oceans and due primarily to the implicitly assumed vertical structures of tempera ture and specific humidity on which the physically based SSM/I retriev als of W are based. To improve estimates of q, it will thus be necessa ry to have supplementary information on both the temperature and water vapor profiles that is able to distinguish the most important dynamic al and physical regimes over the global oceans.