Fine Structure of an Active Monsoon Trough Observed in the Western North Pacific

Abstract

The internal structure and evolution of a monsoon trough (MT) and associated mesoscale convective systems (MCSs) in the western North Pacific were investigated, based mainly on radiosonde and a Doppler radar observations in Palau. The MT was observed on June 15-16, 2013, with the pre-existing disturbance of Typhoon Leepi (2013) being embedded in it. The large-scale circulation around the MT featured a pattern representing an active MT. Deep convection developed ahead and at the leading edge of the downward-sloping monsoonal flow, where intense low-level convergence was observed. Stratiform precipitation broadened rearward over the MT axis. A deep and wide layer of warm and moist air over the MT axis was undercut by a layer of cold air sloping downward from the trailing stratiform region to the leading convective region. An intense low-pressure zone formed in the interface between the warm layer above and cold layer below, with the westerly monsoonal and easterly trade flows being enhanced on its west and east sides, respectively, from the low to middle troposphere. The results suggest that a strengthening of the large-scale cyclonic circulation in response to the internal processes of the MCSs triggered by the MT is important for typhoon genesis.