MAINTENANCE OF THE FREE-TROPOSPHERIC TROPICAL WATER-VAPOR DISTRIBUTION .2. SIMULATION BY LARGE-SCALE ADVECTION

Analyzed wind fields are used to perform a simple advection of moistur e by the large-scale circulation in three dimensions at 2.5 degrees re solution. The unresolved moisture sink Q(2) due to convection is negle cted, except in regions of strong ascent where it is used to enforce a 90% relative humidity ceiling, as determined from sounding and geosta tionary satellite observations. The result is a simulation of water va por that agrees quantitatively with satellite (Special Sensor Microwav e Water Vapor) and sounding observations over the tropical oceans, in both arid and moist regions, to within 10% relative humidity or better from 700 to 300 mb inclusively. Horizontal transport into arid region s from convective regions is accomplished by large coherent structures . Implications of the results for the role of convection in maintainin g the observed humidity distribution, and for the interpretation of ob served correlations between cloud cover and vapor, are discussed.