Quasi-stationary and eastward-propagating convection and an MJO-like gravity wave simulated over a warm water pool in a 2D CRM experiment

To obtain a better understanding of the mechanism of hierarchical convective organization including the supercluster over the tropical warm-pool area and its interaction with the MJO, we performed a 120-day time integration using a 2D, wide-domain(40,000km), cloud-resolving model(Yamasaki,1984). The central 10,000km area is covered with a 1km-mesh grid where cumulus convection and its mesoscale organized form can be explicitly resolved. We prescribed a relatively high sea surface temperature(302K), approximately in the 1km-mesh area mimicking the warm water pool in the western Pacific. Basic surface flow was assumed to be easterly at 5m/s which makes WISHE possible.

Time integration resulted in quasi-stationary convection(QSC) residing over the warm-pool region, and eastward-propagating convection(EPC). The QSC induced a planetary-scale vertical circulation which was persistent during the course of the integration. Superposed on this circulation was an MJO-like gravity wave that propagated at a faster phase velocity of 15m/s to the west of the QSC, and at a slower phase velocity of 3-5m/s to the east of QSC. It completed its domain-long propagation in 30-60days. The vertical structure of the wave in the slow regime indicates an unstable gravity wave which suggests a wave-CISK mechanism in which the EPC plays a significant role in the wave maintenance.

A vertical profile of the domain-averaged horizontal velocity showed that easterly shear(westerly in the lower levels) is enhanced and maintained during the time integration. A momentum budget analysis indicated that the upgradient momentum transport due to convection acts to maintain the vertical shear against internal dissipation. This suggests that the momentum transport may play an important role in the maintenance of the planetary-scale gravity wave if the vertical shear is very important to convective activity.