Microphysical characteristics of evolving mesoscale convective systems observed during active phases of the MJO over the Indian Ocean

The Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign over the Indian Ocean in 2011-2012 documented the structure and evolution of the cloud population in order to better understand coupling with the large-scale environment. The deployment of the National Center for Atmospheric Research dual-polarimetric S- and Ka-band Doppler radar (S-PolKa) as part of DYNAMO provided a rare opportunity to obtain an extensive set of statistics on the microphysical properties of precipitating oceanic tropical convection. Knowledge of the vertical distribution of hydrometeors and how that vertical distribution evolves is not only crucial for understanding the physical properties of MJO convection, but also for describing the feedback through latent and radiative heating and moistening of the surrounding environment. Models of the MJO must ultimately represent or parameterize these microphysical characteristics accurately.

Peak rainfall observed by the radar coincided with the presence of mesoscale convective systems (MCSs) and increased relative frequency of stratiform rain areas. When echo was observed by the radar, the percentage of graupel, relative to all hydrometeor species in the three-dimensional echo, remained relatively consistent regardless of whether or not MCSs were present. On the other hand, relative frequencies of large melting aggregate snowflakes peaked during each of the three active phases, corresponding to the increased presence of stratiform echo during those times. While the vertical distributions of these ice species appeared generally similar in organized systems during the active MJO periods over the Indian Ocean, less widespread stratiform echo with weaker melting layers and shallower echo occurred during December. This active period was characterized by deep, strong westerlies and MCSs that fit the leading convective/trailing stratiform conceptual model, in contrast to non-squall MCSs in October and November when the low-level westerlies were not as strong. These results highlight the need to understand the nature of the mesoscale cloud systems. The subtle changes in microphysical characteristics in these cloud systems are likely critical to modeling the interaction with the environment in differing large-scale scenarios to improve prediction of the MJO and its far-reaching impacts on weather and climate. This study will help guide modeling toward accurate representations of MCSs in the MJO.