Preliminary estimation of horizontal fluxes of cloud liquid water in relation to subtropical moisture budget studies employing ISCCP, SSMI, and GEOS-1/DAS data sets

Special Sensor Microwave/Imager (SSM/I) retrievals of cloud liquid water, International Satellite Cloud Climatology Project (ISCCP) cloud estimates, and winds from the Goddard EOS (GEOS‐1/DAS) assimilation are employed to evaluate vertically integrated cloud liquid water (CLW) transport for 1992. First, GEOS‐1/DAS multiyear data are used to confirm an earlier finding of a paradoxical net moisture sink over the Arabian‐Iraqi desert [Alpert and Shay‐El, 1993]. The negative vertically integrated moisture flux divergence over this region is balanced mainly by the negative incremental analysis updates (IAU) of moisture. Moisture fluxes reveal strong convection but without precipitation in a shallow Hadley‐type cell. Vertical profiles indicate that the moisture removal process is associated with middle and high clouds and probably with CLW flux divergence. The CLW fluxes are estimated explicitly and globally from ISCCP and SSM/I by using linear regression methods. Areas of significant CLW divergence are found over the eastern coasts of both the United States and Asia, in the vicinity of the Gulf Stream and Kuroshio currents, as earlier conjectured by Peixoto [1973]. In both the Arabian‐Iraqi desert and over the Sahara, divergence of a vertically integrated CLW flux opposes the convergence of a vertically integrated horizontal moisture flux, thus explaining at least partially the paradoxical net sink and source in these regions. However, the magnitude of the annual CLW flux estimates as calculated here is, in general, too small to play any significant role in the vertically integrated water budget, except perhaps along coastal regions and over dry subtropical deserts where precipitation minus evaporation is relatively small.