Over the past two decades, little advance has been made in prediction of tropical cyclone intensity while substantial improvements have been made in forecasting hurricane tracks. One reason is that we don't well understand the physical processes that govern tropical cyclone intensity. Recent studies have suggested that the Saharan Air Layer (SAL) may be yet another piece of the puzzle in advancing our understanding of tropical cyclone intensity change in the Atlantic basin. The SAL is an elevated mixed layer, forming as air moves across the vast Sahara Desert, in particular during boreal summer months. The SAL contains warm, dry air as well as a substantial amount of mineral dust, which can affect radiative heating and modify cloud processes. However, so far, this effect has not been well understood and explicitly considered in operational forecast models. The 2005 hurricane season saw the largest number (27) of named storms and the largest number (14) of hurricanes. While recent studies attempted to relate the enhanced tropical cyclone activity to the increase in the tropical sea surface temperature (SST) over the past three decades, little is known about how and to what degree the SAL can counter the local effect of the SST warming by preventing tropical cyclones from reaching their maximum potential intensity (MPI).
Several NASA satellites that have been launched in recent years provide a unique opportunity for advancing our understanding of the influence of the SAL on tropical cyclone intensity. In particular, the data from the A-Train satellites are nearly coincident in time. The Aqua satellite, launched with the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), and the microwave Humidity Sounder of Brazil (HSB) in 2002, provides vertical profiles of atmospheric temperature and humidity that can reveal the thermodynamic effect of the SAL. The Cloud-Aerosol Lidar and Infrared Pathfinder Spaceborne Observation (CALIPSO) and CloudSat are using radar and lidar to cut through the layered structure of clouds to see how water droplets and airborne particles or aerosols are distributed. The TRMM data provide a close view of the precipitation structure in the inner core of tropical cyclones.
Here we propose a three-year research plan, which will extend our ongoing research on tropical cyclones supported by the NASA EOS project (EOS/03-0000-0144). The goal of this proposed study is to investigate the impact of the SAL on tropical cyclone intensity by combining the data from the Aqua, CloudSat, CALIPSO and TRMM satellites. This study will consist of two main tasks: 1) observational analysis of the influence of the SAL on tropical cyclone intensity, and 2) cloud-resolving simulation of real tropical cyclone intensity change in the presence of the influence of the SAL.
