Diurnal Cycle of ITCZ Convection during DYNAMO

During the special observing period (SOP) of the DYNAMO/CINDY/AMIE field campaign, conducted over the Indian Ocean (IO) from October to November 2011, two sounding networks, one north and one south of the equator, took 4-8 soundings/day. This dataset with 3-h time resolution provides a unique opportunity to investigate the diurnal cycle of Intertropical Convergence Zone (ITCZ) convection which was present within the southern sounding array (SSA) for extended periods during the SOP. For example, during the first half of October 2011 characterized as the preonset stage of the MJO over the IO, the ITCZ was located between 3°S and 8°S. During this period TRMM 3B42 3-h rainfall averaged over the SSA exhibited a prominent diurnal cycle with a late night/early morning maximum and an early evening minimum. The associated diurnal rainfall range of 4.8 mm was ~50% of the daily mean (10.1 mm). The diurnal pulsing of the ITCZ during this period impacts conditions on the opposite side of the equator (i.e., over the Northern Sounding Array or NSA). As daytime convection within the ITCZ weakens over the SSA, there is a concurrent reduction of subsidence over the NSA within the subsiding branch of the cross-equatorial Hadley cell. This large-scale diurnal modulation may allow daytime shallow convection and associated moistening to deepen over the NSA leading to favorable conditions for the ensuing active phase of the MJO.