Scale Interactions Involved in the Initiation, Structure, and Evolution of the 15 December 1992 MCS Observed during TOGA COARE. Part II: Mesoscale and Convective-Scale Processes

Abstract

This paper, the second of a series, documents the precipitation, and kinematic and thermodynamic structure of a tropical mesoscale convective system observed by instrumented aircraft on 15 December 1992 during TOGA COARE. Radar-derived precipitation fields indicate that the studied system consists of two subsystems, S1 and S2, characterized by distinct internal dynamics and morphological structures. The retrieved kinematic and thermodynamic structures are compared in detail with the synoptic-scale characteristics described in Part I of this paper, so as to evaluate the scale interactions involved in the internal organization of this convective system. It is shown essentially that the synoptic-scale circulation governs the mesoscale and convective-scale motions and, therefore, determines the internal organization of the selected system. In particular, this study highlights the major role played by the synoptic-scale vertical wind shear in the internal structure of this tropical mesoscale system. Momentum flux calculations show that upward transport of horizontal momentum on the mesoscale is large and mostly carried out at a scale of motion larger than the mesoscale (i.e., by the mean component of the total momentum flux). The westerly rear inflow exhibits characteristics consistent with the density current theory. However, specific mesoscale and convective-scale processes linked to the presence of precipitation modulate this dominant synoptic-scale forcing. A mesoscale interaction between the two distinct subsystems S1 and S2 is identified. The apparition of a shallow density current resulting from the downward spreading of air at the ground within S1 is suspected to be the triggering mechanism for S2.