Variability of Midtropospheric Moisture and Its Effect on Cloud-Top Height Distribution during TOGA COARE*

Abstract

The tropical western Pacific warm pool is often generalized to be a region of heavy precipitation. This concept is useful in constructing simplified models of the tropical circulation. However, the warm pool region is often punctuated by periods of little rain. Such drought periods may last up to 10 days over an area of at least 6 3 105 km2. Other common features of the drought periods include an extremely dry midtroposphere, few deep clouds typically associated with mesoscale convective systems, and a substantial amount of clouds that are too tall to be categorized as trade cumuli but too short to fall into the category of deep convective clouds. Midtropospheric moisture varies substantially (60% in relative humidity ,4gk g 21 in water vapor mixing ratio) between rainy and drought periods. The frequency distributions of humidity exhibit bimodal structures at certain levels above the freezing level. In either rainy or drought periods, or in a long period including both, the time-mean humidity above the boundary layer deviates substantially from the most frequent profile of humidity, defined as the relative humidity corresponding to the maximum frequency distribution at each level. Mean soundings, therefore, do not accurately represent the overall vertical structure of moisture in the warm pool. Calculations of a simple parcel model demonstrate that the warm pool atmosphere above the boundary layer can be dry enough to discourage the growth of deep convective clouds by depleting parcel buoyancy through entrainment. These results were drawn from an analysis of soundings collected during the Intensive Observing Period (1 November 1992‐28 February 1993) of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment.