Characteristics of a Mesoscale Convective System Observed by Shipborne Polarimetric Radar in the Tropical Western Pacific

On 6 January 2015, a mesoscale convective system (MCS) was observed by the C-band polarimetric radar on board the research vessel Mirai in the tropical western Pacific. The MCS formed during the active phase of a Madden-Julian oscillation (MJO) in the western Pacific. It was linearly organized and oriented from the southeast to the northwest, which was approximately perpendicular to the low-level vertical wind shear and parallel to the upper-level vertical wind shear. The leading convection of the MCS developed in a convergence zone associated with a cold pool and descending rear-inflow jet. The trailing stratiform region of the MCS was incorporated with decaying cells from the convective line. Distinct polarimetric signatures provide insight into the microphysics of the MCS. Along strong reflectivites in the leading edge of the MCS, columns of highly positive differential reflectivity (ZDR) and specific differential phase (KDP) extended above the melting level, suggesting the presence of intense updrafts and lofting of liquid hydrometeors by the updrafts in the leading edge of the MCS. In addition, enhanced ZDR and depressed copolar correlation coefficient (CC) were found around the 0 °C level (~5 km altitude) in the convective region of the MCS, indicating the existence of mixed-phase hydrometeors in this region. Furthermore, decaying convection in the stratiform region of the MCS was characterized by high KDP above the freezing level. This strong signature in KDP suggests an enhanced growth of ice crystals in stratiform precipitation. The microphysical characteristics of the MCS revealed by the polarimetric radar on board the Mirai are consistent with those observed in other tropical convective systems.