ALBERT

Description: [1]  An unusually large, explosive convective cloud system was observed over the equatorial Indian Ocean on 28 November 2011 during the DYNAMO [Dynamics of the Madden-Julian Oscillation (MJO)] field campaign. The rapid development and expansion of extremely cold cloud tops (minimum −96 °C) gave the impression of an “explosion”. The significance of this large mesoscale convective system (MCS) is its size and rapid development during the initiation of a strong MJO event. Observations from the DYNAMO sounding array show that the large MCS developed within a well-defined synoptic-scale cyclonic circulation associated with an equatorial low-pressure system with characteristics of a mixed Rossby-gravity wave that dominated the flow in the DYNAMO array. Prior to the development of the MCS, the equatorial flow was characterized by strong vertical wind shear with low-level westerlies and upper-level easterlies. A region of decreased wind shear and enhanced upper-level divergence emerged over the DYNAMO array concurrently with the passage of the westward-moving mixed Rossby-gravity wave-related low-pressure system and convective activity during the MJO initiation. The in situ sounding observations suggest that widespread deep convection upstream of the large MCS may have contributed to the reduction of the upper-level easterlies through vertical momentum transport and convective outflow. Both the reduction in vertical wind shear and enhanced low-level convergence induced by the equatorial low-pressure system created a favorable environment for the rapid development of the large MCS. This study examines the development of the MCS and the associated synoptic-scale equatorial low-pressure system within the large-scale MJO circulation using high temporal resolution (3-hourly) in situ sounding observations from DYNAMO, which provide new insights into the interaction between convection and environmental flow during MJO initiation over the equatorial Indian Ocean.