From Hot Towers to TRMM: Joanne Simpson and Advances in Tropical Convection Research

Abstract

Joanne Simpson began contributing to advances in tropical convection about half a century ago. The hot tower hypothesis jointly put forth by Joanne Simpson and Herbert Riehl postulated that deep convective clouds populating the “equatorial trough zone” were responsible for transporting heat from the boundary layer to the upper troposphere. This hypothesis was the beginning of a 50-year quest to describe and understand near-equatorial deep convection. Tropical field experiments in the 1970s [Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) and the Monsoon Experiment (MONEX)] in which Joanne participated documented the mesoscale structure of the convective systems, in particular the deep, stratiform, dynamically active mesoscale clouds that are connected with the hot towers. In the 1980s these new data led to better understanding of how tropical mesoscale convective systems vertically transport heat and momentum. The role of the mesoscale stratiform circulation in this transport was quantified. Tropical field work in the 1990s [especially the Coupled Ocean-Atmosphere Response Experiment (COARE), in which Joanne again participated] showed the importance of a still larger scale of convective organization, the “supercluster.” This larger scale of organization has a middle-level inflow circulation that appears to be an important transporter of momentum. The mesoscale and supercluster scale of organization in tropical convective systems are associated with the stratiform components of the cloud systems. Joint analysis of satellite and radar data from COARE show a complex, possibly chaotic relationship between cloud-top temperature and the size of a stratiform precipitation area. The Tropical Rainfall Measuring Mission (TRMM) satellite, for which Joanne served as project scientist for nearly a decade, is now providing a global census of mesoscale and supercluster-scale organization of tropical convection. The TRMM dataset should therefore provide some closure to the question of the nature of deep convection in the equatorial trough zone.