An MJO-like Gravity Wave and Superclusters Simulated in a Two-Dimensional Cumulus-Scale-Resolving Model under a Warm Pool Condition.

Abstract

To further our understanding of the large-scale convection organization in the tropics, including super cloud clusters and the Madden-Julian oscillation (MJO), a 120-day long integration is performed using a 2D (zonal-height) cumulus-scale-resolving model. The periodic horizontal domain spans 40, 000 km and mimics the equatorial circumference. A zonal inhomogeneity of the sea surface temperature (SST) is taken into account by allowing the SST to vary between 302 K over the central area of 5, 000 km width and lower value outside with its minimum at 299 K. Such a computational setup provides an analogue of the tropical western Pacific warm pool. Due to computational limitations, we employ a cloud-resolving horizontal grid of 1 km only in the warm-pool area of 10, 000 km scale, and a stretched coarser grid across the rest of the domain. The initial surface flow is taken to be an easterly at 5 m s-1 which makes zonal asymmetry allowing the Wind-Induced Surface Heat Exchange (WISHE) set in. The numerical simulation features a large-scale gravity wave of zonal wavenumber 1, referred to as the MJO-like wave, that propagates eastward with the phase velocity in the range of 8 to 15 m s-1 and travels across the computational domain in 40 to 60 days. The overall evolution of convection exhibits two organization patterns: i) an eastward-propagating convection (EPC), and ii) a quasi-stationary convection (QSC). EPC is identified as an envelope of westward-moving cloud clusters of O (100 km) scale which develop successively to the east of the existing clusters in accord with the propagation of the upward motion of the MJO-like wave. QSC has a horizontal scale in excess of several thousands of kilometers and it is associated with the existence of the warm pool. Propagation of the MJO-like wave modulates QSC quasi-periodically. The convection in the active regime of QSC has an extent of O (1, 000 km), propagates west-to-east, and involves a hierarchy of organization similar to that in Oouchi (1999). It is identified as a super cloud cluster. It is argued that QSC can provide a basic framework for explaining the slow phase velocity (3 to 10 m s-1) of the super cloud cluster. The governing mechanism in the active phase of the super cloud cluster and MJO-like wave can be viewed as wave-CISK, with WISHE playing an important role in driving the eastward movement of the super cloud cluster. Similarities and differences in the wave-CISK mechanism between this study and previous investigations using parameterized convection are discussed.