Modulation of Diurnal Cycle of Tropical Convective Clouds by the MJO

The infrared cloud amount from the International Satellite Cloud Climatology Project D1 cloud product and the Tropical Rainfall Measurement Mission 3B42 precipitation product are employed to study the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of tropical convective clouds. This analysis is based on the composite of 18 MJO events during seven boreal winter seasons from January 1998 to April 2005 and covers the Indo-Pacific warm pool and the Indian Ocean, where MJO convective activity is greatest. Our analysis demonstrates that the diurnal cycle of tropical deep convective cloud amount (DCC, with cloud tops above 180 hPa) over both land and water is enhanced during the convectively active phase of the MJO, while it is reduced during the convectively suppressed phase of the MJO. The MJO impact on the diurnal amplitude is weakest over the western Pacific and the islands of the Maritime Continents (around 20% of the mean diurnal amplitude) and largest over the eastern Indian Ocean (around 50% of the mean diurnal amplitude). However, the diurnal phase of DCC seems to be unaffected by the MJO. Similarly, the diurnal cycle of the tropical high clouds (with cloud tops above 440 hPa) over both land and water is also enhanced during the convectively active phase of the MJO, while it is reduced during the convectively suppressed phase of the MJO. In contrast, the diurnal cycle of the tropical low clouds (with cloud tops below 680 hPa) is reduced during the convectively active phase of the MJO, while it is enhanced during the convectively active phase of the MJO. The diurnal cycle of low clouds at different phases of the MJO is similar to the diurnal cycle of SST documented by the TOGA COARE.