Moistening, Convection, and the Diurnal Cycle during the Pre-onset Periods in DYNAMO

The Dynamics of the Madden–Julian oscillation (MJO, DYNAMO) field campaign was conducted in the tropical Indian Ocean from October 2011–March 2012 to sample the coupling between tropospheric circulation and convection during MJO initiation. Two MJOs were sampled by the comprehensive DYNAMO observational network, which included 3-6 hourly upper-air soundings and suite of cloud and precipitation radars. Prior field studies have shown that the period immediately preceding the deep-convective phase of the MJO (the pre-onset period) is characterized by shallow and congestus convection, which moistens the low–mid-troposphere to counteract the drying by the large-scale circulation (similar to a trade-cumulus regime). A new insight into such periods arising from DYNAMO observations is that this convective moistening exhibits a marked diurnal cycle. This diurnal cycle is manifest in fluctuations in cloud depth, and variations in sea surface temperature and surface latent heat flux of up to 3°C and 150 W m-2, respectively. A question that emerges from this facet of the pre-onset period is whether the diurnal cycle, associated afternoon SST warming, and mesoscale organization of convection in open cells, enhances convective moistening in its tug-of-war with the drying by the large-scale circulation.