Abstract
Three distinct mesoscale convective systems (MCSs), composing a tropical cloud cluster, observed on January 18-19, 1993 were investigated. This event was observed by two Doppler radars on Manus Island (2°S, 147°E), Papua New Guinea, during the TOGA-COARE TOP. The cloud cluster was formed within the radar range, and extended to a diameter of about 200km. The environmental wind was weak, but a strong vertical shear existed at the middle level (3-8km).
The first characteristic MCS moved faster than the environmental wind, and thus is referred to as a fast-moving convective system (FMCS). The orientation of this FMCS was nearly perpendicular to the direction of the dominant vertical shear. Behind the convective cell of the FMCS, an anvil echo extended to upper levels. The front inflow had two branching sub-flows in front of the convective cell with a strong downdraft. The lower level outflow from the downdraft also branched out to southward and northward flows. The branching inflow and outflow were associated with symmetrically-arranged strong convergence regions. These features were similar to a structure produced by the storm splitting, and is recognized as a squall line system with a three-dimensional structure. The second MCS was composed of individual convective cells, moving at a slower speed than the first system. This slow-moving convective system (SMCS) was associated with the south-easterly inflow at lower levels, and extended in a line parallel to the vertical shear direction from north to south. The last system was a stratiform-looking echo system marked by an obvious bright band at the melting level. This system brought more precipitation than the other MCSs. In the echo system, a vortex with a diameter of 20km was found at the middle layer (4-6km). In the middle layer, some vertical circulations were also found, and vertical circulations with a maximum velocity of 5ms-1 were found in the upper layer (>6km). These circulations seemed to cause the heavy precipitation.
