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 sp atial 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 sys tem 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 approx imately 12 hours. The system moved with a velocity of about 12 m/s tow ard 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 o btained from aircraft penetrations show features similar to tropical M CSs reported elsewhere (such as an area of strong to moderate convecti on surrounded by a broad region of stratiform precipitation, radar ech o bright band in the stratiform region, ''onion'' type sounding behind the convective region). The vertical structures of the EMEX 9 cloud c luster also contain two types of imbedded convection: an upright verti cal structure and a pronounced rearward slope (approximately 17-degree s), having a vertical extent of 14.5 km and above and a horizontal sca le of about 40 km. The cloud base and cloud top altitude in the strati form 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 t he stratiform region show a significant attenuation of shortwave flux through the cloud (the estimated transmission is 14% at cloud base). T he upward and downward solar flux profiles are almost parallel to each other in the atmosphere inside and below the cloud base, suggesting v ery little solar heating in these regions. The upward and downward inf rared radiation fluxes measured in the tropical MCS also show little i nfrared heating above and below the cloud cluster.