Radiative influences on the ITCZ circulation in DYNAMO

During the 2011 special observing period of the Dynamics of the Madden-Julian Oscillation (DYNAMO; MJO) field experiment, two sounding arrays with 3-hourly observations were established over the central Indian Ocean, one north and one south of the equator, referred to here as the NSA and SSA, respectively. During the first half of October when convection was suppressed over the NSA but prominent over the SSA, the circulation over the sounding arrays could be characterized as a local Hadley cell. Strong rising motion was present within the ITCZ extending across the SSA with compensating subsidence over the NSA. A prominent diurnal pulsing of this cell was observed, impacting conditions on both sides of the equator, with the cell running strongest in the early morning hours (05-08 LT) and notably weakening later in the day (17-20LT). Vertical motion and apparent heating Q₁ within the ITCZ exhibited a diurnal evolution from early morning bottom-heavy profiles to weaker daytime top-heavy profiles, indicating a progression from convective to stratiform precipitation. Within the ITCZ, observations indicate that the diurnal cycle of overturning circulation is consistent with a rapid adjustment towards weak temperature gradient balance. Furthermore, both horizontal radiative heating gradients and direct cloud radiative forcing exert an important influence on the diurnal variations of ITCZ convection.