| The role of large-scale circulation anomalies in the convective onset of the Madden-Julian Oscillation (MJO) over the Indian Ocean during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign, conducted Oct. 2011--Feb. 2012, is explained using radar and rawinsonde observations, reanalysis, and regional model simulations. Convective onset was characterized by two episodic and rapid increases in the vertical growth of the cumuliform cloud population over the Indian Ocean: First, the areal coverage of moderately deep (~5 km) convection increased; about 1 week later, the areal coverage of deep (up to the tropopause) convection increased rapidly. Deep tropospheric wavenumber 1 anomalies in zonal wind and vertical velocity circumnavigated the tropics repeatedly during DYNAMO. MJO convective onset occurred when the upward branch of this wavenumber 1 circulation arrived over the Indian Ocean because a reduction in large-scale subsidence cooled the troposphere and steepened the lapse rate below 500 hPa. This made the environment more conducive to development of moderately deep convection. The moderately deep convection moistened the environment during week-long transition periods by transporting moisture vertically from the boundary layer to the free troposphere and detraining it into the clear-air environment, particularly between 650--850 mb. Regional cloud-permitting model simulations of convection during MJO onsets reproduced the distinct transition periods. The modeling results confirmed that rapid cooling of the environment enhanced the areal coverage of, and thus total vertical transport of water within, moderately deep convection at the beginning of transition periods. Evaporation of cloud condensate via entrainment or dissipation of clouds was directly responsible for environmental moistening. Cooling of the climatologically stable layer between 700--850 mb was particularly important because it allowed a greater number of cumulus elements growing out of the boundary layer to completely penetrate the 700--850 mb stable layer and grow upward to the next stable layer near 500 mb. The results are descriptive of MJO convective events observed during DYNAMO, but not necessarily all MJO events. As such, they should not be generalized as the single operative method for MJO convective onset. |
