Gross moist stability in tropical systems

The concept of "Gross moist stability" (GMS) was introduced by Neelin and Held (1987) in the effort to relate the tropical vertical motion to the lower boundary conditions. This concept was extended to non-steady situations by Raymond (QJRMS-press) who recast it in terms of moist entropy. GMS is thought to be positive in the tropical atmosphere with small values corresponding to environments more sensitive to entropy forcing. Though not ruled out, negative values of GMS are thought to be rare as they can lead to self-amplifying convective systems.

In this work, We present results from direct estimation of the GMS performed on ten case-studies observed during the intensive operation period of TOGA-COARE. The reversible equivalent potential temperature is used to represent moist entropy in our calculations. This quantity was composed from in-situ measurements made by the NOAA WP3-42 and WP3-43 aircraft as well as balloon soundings made near the studied systems. The kinematic characteristics of the systems were synthesized using Dual Doppler analysis of data collected by the X-band radars mounted on the tail of the WP3's.

Our results indicate that for deep convective systems with cloud tops above 10 km, GMS is positive while negative values are obtained for systems with cloud tops less than 8 km.