Microphysical characteristics of MJO convection over the Indian Ocean during DYNAMO

Abstract

The microphysical characteristics of precipitating convection occurring in various stages of the Madden-Julian Oscillation (MJO) over the Indian Ocean are determined from data obtained from the National Center for Atmospheric Research dual-polarimetric Doppler S-band radar, S-PolKa, deployed as part of the Dynamics of the MJO (DYNAMO) field experiment. Active MJO events with increased rainfall occurred in October, November, and December 2011. During each of these active MJO phases, in addition to enhanced rainfall, convection became deeper and ice-phase microphysics played a greater role. S-PolKa consistently showed nonoriented small ice particles dominating the radar echoes at altitudes of 9–10 km, dry aggregates concentrated between 7 and 9 km, and wet aggregates and graupel near the melting level (~5 km). Graupel occurred mainly in actively convective towers, while the wet aggregates occurred almost exclusively in the stratiform regions of mesoscale convective systems (MCSs). During each of the three multiweek MJO active phases, the maximum rainfall occurred in short bursts lasting a few days. Each multiday rainy period began with deepening convective elements and a concurrent increase in occurrence of dry aggregates, which maximized just prior to organization into MCSs. The peak rainfall occurrence coincided with the maximum coverage of the radar domain by MCSs, reflecting large stratiform regions that exhibited the most frequent occurrence of wet aggregates. During the December active MJO phase, however, the MCSs were shallower and had a slightly lower tendency for wet aggregates in the stratiform regions and, therefore, generally weaker brightbands.