Evolution of precipitation and convective echo top heights observed by TRMM radar over the Indian Ocean during DYNAMO

Radar data from the Tropical Rainfall Measuring Mission show the evolution of echo tops of convective elements over the Indian Ocean and Maritime Continent during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign of 2011–2012. Echo top heights exhibited a bimodal distribution wherein cumulonimbi of moderate height constituted a “congestus mode” while vertically extensive cumulonimbus made up a “deep mode.” An intraseasonal time scale dominated variability in these modes from October to January over much of the Indian Ocean. Over the Maritime Continent, there was no clear intraseasonal signal in convective echo top heights. Where the intraseasonal oscillation was detected, radar echoes evolved from being dominated by the congestus mode to being characterized by more deep mode convection on time scales of less than 1week. The areal coverage of congestus echoes began to increase 2–8days prior to the rise in area of deep echoes. These satellite-derived results confirm that the time scale for convective deepening seen at individual DYNAMO observational sites is consistent with that of convection on the large scale over the Indian Ocean. Intraseasonal variability of zonal wind, temperature, and humidity as depicted by reanalysis is also consistent with that derived from rawinsonde observations during DYNAMO. Thus, the gradual buildup of convection as depicted by recent versions of the “discharge-recharge” hypothesis does not accurately describe evolutions of convection prior to MJO events observed during DYNAMO, although cloudmoistening processes may still be relevant on time scales of 1week or less.