Maintenance of the free-tropospheric tropical water vapor distribution. Part II: Simulation by large-scale advection

Abstract

Analyzed wind fields are used to perform a simple advection of moisture by the large-scale circulation in three dimensions at 2.5° resolution. The unresolved moisture sink Q2 due to convection is neglected, except in regions of strong ascent where it is used to enforce a 90% relative humidity ceiling, as determined from sounding and geostationary satellite observations. The result is a simulation of water vapor that agrees quantitatively with satellite (Special Sensor Microwave 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 regions from convective regions is accomplished by large coherent structures. Implications of the results for the role of convection in maintaining the observed humidity distribution, and for the interpretation of observed correlations between cloud cover and vapor, are discussed.