Radar analysis of storms over equatorial Indian Ocean during DYNAMO experiment

The Madden-Julian Oscillation (MJO) represents dominant mode of tropical convection and has multiple remote impacts (e.g. monsoon active/break phase, tropical cyclone etc). Forecasting skill of MJO is limited due to the lack of representation of convection over ocean in the numerical models. Several scanning precipitation and vertically looking cloud radars provided an unprecedented view of three dimensional structures of convective entities during the DYNAMO experiment. In the present study, the Thunderstorm Identification, Tracking, Analysis and Nowcasting (TITAN) software is used to identify and track convective storms by ingesting 3-dimensional S-Pol radar reflectivity measurements. Three main periods of enhanced rainfall occurred in association with active MJO phases for October-December 2011 period. Large-scale organization of storms, their frequency of occurrences and spatial distribution, as well as their evolution, size and intensity are studied. The storms are categorized based on their horizontal dimensions. The time series of storm area and major axis length showed three extended periods of Mesoscale Convective System (MCS) occurrence constituting active MJO periods (15-27 October, 15-28 November, 15-27 December). The active period not only has more MCS but also has more SubMCS compared to suppressed period. The SubMCSs are present through-out MJO cycle. Each active MJO phase began with deepening convection and storm areas were greatest during period of enhanced rainfall. The occurrence of MCS show 2 days periodicities in October while it is at 4-6 days interval in November & December. The storm occupies very large areas during October-November as compared to storms in December. For a given storm, percentage of area containing regions of reflectivity > 40 dBZ decreases with increasing storm areas indicating that as the storm becomes wider, there is a possible reduction in the intensity of storms. Majority of storms were of short durations, i.e. predominantly less than 2 hours. The short-lived storms associated with shallow-isolated convection of SubMCSs and long-lived storms associated with organized convection of MCSs have occurred during suppressed and active MJO conditions respectively. The 30 dBZ and greater reflectivities are rarely exceed a few kilometers above melting level which highlights that the lightning was exceptionally rare during DYNAMO experiment. In order to examine the relationship between cloud development and mid tropospheric humidification, radar estimated divergence profiles and radiosonde observed relative humidity profiles are used. The events of moistening (convergence) were separated by periods during which a deep layer of anomalously dry air (divergence) was present.