The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA–COARE: Structure, evolution and budgets

Abstract

AbstractData, collected on 9 February 1993 during the Tropical Ocean and Global Atmosphere Programme—Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) with the Doppler radars on board the two National Oceanic and Atmospheric Administration WP‐3D and the National Center for Atmospheric Research Electra aircraft, are analysed to study the structure and 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.Series of three‐dimensional fields of reflectivity, air‐velocity components, retrieved pressure and temperature perturbations show similarities with previously observed convective systems over the tropical oceans. In particular, although the cloud top reached 18 km altitude, relatively weak vertical motions and temperature perturbations resulted from a moderate convective instability. In the low levels, a cold front‐to‐rear flow intensified and deepened as the overall structure evolved from convective to stratiform. The vertical profiles of mean horizontal divergence evolved from low‐level convergence in the cross‐line direction for the earliest analyses to mid‐level convergence from all directions during the later stratiform phase. Vertical momentum fluxes were 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 the nearest stations of the TOGA‐COARE array shows that the period during which airborne Doppler observations were conducted was characterized by relatively strong large‐scale ascent and ‘apparent’ sources of heat, moisture and momentum. These ‘diagnosed’ values and the mean radarderived ones agree fairly well. This shows that the early convective and late stratiform phases of the MCS are equally important for the ‘apparent’ sources of heat and momentum.