A Satellite Study of Tropical Moist Convection and Environmental Variability: A Moisture and Thermal Budget Analysis

Abstract

he thermodynamic variability associated with moist convection over tropical oceans is analyzed by making use of a variety of satellite sensors including radars, an infrared and microwave sounder unit, and a microwave radiometer and scatterometer aboard different platforms. Satellite measurements of atmospheric parameters including air temperature, water vapor, cumulus cloud cover, and surface wind are composited with respect to the temporal lead or lag from Tropical Rainfall Measuring Mission (TRMM)-detected convection to obtain statistically continuous time series on hourly to daily time scales. The Atmospheric Infrared Sounder (AIRS)-observed temperature and humidity profiles, representing cloud-cleared sounding, are combined with semitheoretical estimates of in-cloud temperature and humidity to construct the large-scale mean field. Those measurements are ingested to the moisture and thermal budget equations integrated vertically over each layer separated by cloud base. This strategy makes it possible to evaluate the free-tropospheric (FT) convergence of moisture and dry static energy and their vertical flux at cloud base from satellite observations alone. The main findings include the following: 1) vertical moisture transport at cloud base is the dominant source of FT moistening prior to isolated cumulus development while overwhelmed by horizontal moisture convergence for highly organized systems; 2) FT diabatic heating is largely offset on an instantaneous basis; and 3) FT moistening by convective eddies amounts to a half of the total cloud-base moisture flux in the background state, while large-scale mean updrafts modulate the variability of cloud-base flux when highly organized systems develop. The known correlation between congestus clouds and FT moisture before deep convection may be accounted for by large-scale mean moisture updraft rather than congestus eddy moistening.