Diurnal cycle of deep convection in the super cluster embedded in the MJO

Tian et al. (2006) investigated the impact of the MJO on the diurnal cycle of the tropical deep convection cloud amount (DCC) using the ISCCP D1 cloud product. Their analysis indicated that the diurnal cycle of the DCC is enhanced (reduced) during the convectively active (inactive) phase of the MJO. They also showed that the diurnal phase of DCC is not so significantly affected by the MJO. Their study mainly investigated the area-weighted mean “MJO diurnal cycle”, so that they did not identify the type of convective organization, i.e., the convectively coupled wave and isolated deep convection.

Nakazawa (1988) showed the MJO includes the distinct super clusters which propagate eastward with the phase speed of 10–15 m/s. When we observe the MJO by outgoing long wave radiation (OLR) or equivalent blackbody temperature (Tbb), MJO is often characterized by super clusters which appear to be a kind of convectively coupled Kelvin wave. The diurnal variation of deep convection associated with convectively coupled wave has not been studied minutely.

In this study we investigate the diurnal cycle of deep convection in the super clusters using the global IR (Tbb) data provided by CPC/NOAA. The spatial and temporal resolution of the data is 0.5°× 0.5° and 1 hour, respectively. The period between the autumn of 2000 and the spring of 2005 was analyzed. We selected the MJO events that took place in boreal winter using the MJO-wave-filtered NOAA daily OLR. On the longitude–time diagram of the global IR without the MJO-wave filter, we chose 30 super clusters (the phase speed of 5–13 m/s) embedded in the MJO events, and made the composite of the super clusters (CSC) at each 3 hour of local standard time. We made CSC for the four selected regions, i.e., the Indian ocean (IO) (60°E–95°E), ocean of the Maritime continent (MCO) (100°E–150°E), lands of the Maritime continent (MCL) (100°E–150°E), and the western Pacific (WP) (150°E–180°). The main findings are as follows: 1) In the IO region, deep convection in the CSC is enhanced from the morning to the afternoon. 2) On the other hand, deep convection in the CSC is enhanced from midnight to early morning in the WP region. 3) In the MCO and MCL regions, the diurnal cycle of deep convection in the CSC is clearly affected by the land/sea distributions. 4) The structure of CSC is also different between the IO and the WP regions, even though both are oceanic.