Moistening processes before the convection onset of Madden-Julian Oscillation events during the CINDY2011/DYNAMO period

Initiation processes of the MJO events that occurred during the CINDY2011/DYNAMO IOP are investigated in terms of moistening processes prior to the convection onset. The moisture build up is known as major precursor of MJO convectiion onset. It varies among cases, related to tropical and extratropical disturbances through local processes and multi-scale interactions, as well as large-scale dynamical forcing. In this study, MJO episodes that initiated in mid-October and mid-November 2011 (hereafter, MJO1 and MJO2, respectively) are analyzed using global nonhydrostatic model (Nonhydrostatic Icosahedral Atmospheric Model, NICAM) simulation, objective analysis (ERA-Interim), and satellite (NOAA daily interpolated OLR) data. Convective onset is defined by the initial date of the lowering of OLR over the equatorial western Indian Ocean with intra-seasonal time scale (20-80-day period). The onset date was 15 October and 16 November 2011 for MJO1 and MJO2, respectively. Activation of convection in the Southern hemisphere and equatorial easterly anomalies in the lower to middle troposphere preceded the MJO convection onset in both cases. The moistening took around 10 days in MJO1 with contribution from mid-tropospheric zonal moisture advection associated with wave trains that originated from mid-latitudes, and contributions from equatorial waves in the lower troposphere. In the MJO2 case, acceleration of deep easterly anomalies on the Equator contributed to rapid moisture build up in several days. Once MJO-scale equatorial upward motion was established, vertical advection primarily contributed to the moistening that balanced with moisture sink by condensation. These results were similar in ERA-Interim and in simulation data, but with more significant effects of diabatic processes in the lower troposphere in the nonhydrostatic model.