THE RADIATIVE BUDGETS OF A TROPICAL MESOSCALE CONVECTIVE SYSTEM DURING THE EMEX-STEP-AMEX EXPERIMENT .1. OBSERVATIONS

This paper is the first of a series of two that aim to describe the spatial and temporal variation of the radiative heating associated with tropical mesoscale convective systems (MCSs). This paper describes the analysis of data collected in and around a tropical cloud cluster system studied as part of the Equatorial Mesoscale Experiment (EMEX). The data analysis indicates that the cluster originated off the northern coast of Australia along the midlevel monsoon trough and lasted approximately 12 hours. The system moved with a velocity of about 12 m/s toward the northeast and the low-level surface northwesterly flow at the vicinity of the premonsoon trough area seems continuously to feed the EMEX 9 cloud cluster with energetic warm, moist equatorial air. Data obtained from aircraft penetrations show features similar to tropical MCSs reported elsewhere (such as an area of strong to moderate convection surrounded by a broad region of stratiform precipitation, radar echo bright band in the stratiform region, ''onion'' type sounding behind the convective region). The vertical structures of the EMEX 9 cloud cluster also contain two types of imbedded convection: an upright vertical structure and a pronounced rearward slope (approximately 17-degrees), having a vertical extent of 14.5 km and above and a horizontal scale of about 40 km. The cloud base and cloud top altitude in the stratiform region are estimated to be of the order of 4.8 km and 15 to 16 km, respectively. The composite aircraft shortwave radiation data from the stratiform region show a significant attenuation of shortwave flux through the cloud (the estimated transmission is 14% at cloud base). The upward and downward solar flux profiles are almost parallel to each other in the atmosphere inside and below the cloud base, suggesting very little solar heating in these regions. The upward and downward infrared radiation fluxes measured in the tropical MCS also show little infrared heating above and below the cloud cluster.