The oceanic mesoscale convective system and associated mesovortex observed 12 December 1992 during TOGA-COARE

This study documents the precipitation and kinematic structure of a mature/mature-to-decaying, westward propagating, near-equatorial oceanic mesoscale convective system (MCS) observed by airborne Doppler radars during TOGA-COARE, the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. This system occurred on 12 December 1992 during the convectively active phase of an intraseasonal oscillation, and was followed, at its dissipating stage, by a redevelopment of convection that led to the MCS observed on 13 December 1992. These two successive MCSs were associated with a two-day atmospheric disturbance. Radar-deduced airflows at two time periods reveal many similarities with other tropical oceanic cloud clusters. In particular, a marked rear inflow jet associated with a midlevel cyclonic mesovortex was observed within the rear of the stratiform region. The mesovortex was better defined both in size and depth during the mature-to-decaying stage and was located more inside the system within a region of convergence, due to the westward propagation of the rear inflow. Countergradient transports of momentum normal to the mean orientation of the system at midlevels, and downgradient transports at lower and higher levels helped to intensify this rear inflow. These transports were mostly accomplished by eddy structures. Cyclonic vorticity was concentrated at the rear of the MCS and peaked at the centre of the observed closed wind circulation. As previously observed, stretching of the pre-existing vertical vorticity was the dominant dynamical mechanism which helped to amplify the mesovortex at mid-to-upper levels, while tilting of the horizontal vorticity into the vertical was a lesser mechanism and had an opposite effect. In most respects, advective processes were negatively correlated with the stretching and tilting mechanisms, but not sufficiently to be balanced. The resulting tendency was an increase of cyclonic vorticity at low-to-mid levels and a net decrease above.