THE OCEANIC MESOSCALE CONVECTIVE SYSTEM OBSERVED WITH AIRBORNE DOPPLER RADARS ON 9 FEBRUARY 1993 DURING TOGA-COARE - STRUCTURE, EVOLUTION AND BUDGETS

Data, collected on 9 February 1993 during the Tropical Ocean and Globa l Atmosphere Programme-Coupled Ocean-Atmosphere Response Experiment (T OGA-COARE) with the Doppler radars on board the two National Oceanic a nd Atmospheric Administration WP-3D and the National Center for Atmosp heric Research Electra aircraft, are analysed to study the structure a nd evolution of an equatorial oceanic mesoscale convective system (MCS ). Observations lasted more than five hours, a period during which the cloud-cluster characteristics changed from convective to stratiform. This mesoscale system was approximately aligned along the tropospheric wind shear, but it was nearly perpendicular to the low-level shear. S eries of three-dimensional fields of reflectivity, air-velocity compon ents, retrieved pressure and temperature perturbations show similariti es with previously observed convective systems over the tropical ocean s. In particular, although the cloud top reached 18 km altitude, relat ively weak vertical motions and temperature perturbations resulted fro m a moderate convective instability. In the low levels, a cold front-t o-rear flow intensified and deepened as the overall structure evolved from convective to stratiform. The vertical profiles of mean horizonta l divergence evolved from low-level convergence in the cross-line dire ction for the earliest analyses to mid-level convergence from all dire ctions during the later stratiform phase. Vertical momentum fluxes wer e important and led to an increase in the cross-line wind shear and a decrease in the along-line one. Analysis of radiosounding data from th e nearest stations of the TOGA-COARE array shows that the period durin g which airborne Doppler observations were conducted was characterized by relatively strong large-scale ascent and 'apparent' sources of hea t, moisture and momentum. These 'diagnosed' values and the mean radar- derived ones agree fairly well. This shows that the early convective a nd late stratiform phases of the MCS are equally important for the 'ap parent' sources of heat and momentum.