Thermodynamic and Circulation Characteristics, of Winter Monsoon Tropical Mesoscale Convection

Abstract

During the December 1978 field phase of the International Winter Monsoon Experiment (Winter MONEX), a regular diurnal cycle of deep convective activity occurred over the South China Sea immediately to the north of Borneo. The convection was initiated during the late night hours by convergence of the low-level northeast monsoon flow with the land breeze along Borneo's north coast. The precipitation feature that developed was characterized during its mature stage by a nearly continuous stratiform cloud, referred to here as a mesoscale anvil, of ∼ 100–500 km dimension extending from the mid-troposphere (∼5 km) to near the tropopause (∼ 17 km). Using rawinsonde data from the sounding network, thermodynamic and Circulation characteristics of the convection are documented. Mesoscale upward motion is found to within the anvils, with a MCSOMC downdraft below producing a warm, dry region over a lame area just above the boundary layer. Air flow relative to the anvil on both its leading and trailing edges is inward net its base (in the mid-troposphere), and outward near its top. Mesoscale downdraft air flows out to the rear of the system. A significant cold anomaly atop the mesoscale anvils is observed with maximum amplitude ∼6°C at the tropopause or in the lower stratosphere. The propensity for mesoscale anvil development over the Indonesian region and many other areas of the tropics may be partially accounted for by the deep layer of conditional instability that is observed prior to their development. This conditional instability extends not only from the surface to the midtroposphere, as is common over most of the tropics, but also, when ice processes are considered, all the way to the upper troposphere (∼ 10 km). A cooperative instability mechanism to account for the rapid development of the mesoscale anvils in the winter MONEX region is suggested by the above observations.