Abstract
To understand the mechanism of hierarchical organization of the tropical super cloud cluster and its eastward propagation, we performed numerical experiments using a 2D cumulus-scale resolving model (Yamasaki, 1984). In the experiments, synoptic-scale convection similar to super cloud cluster (SCC) developed with a reasonable eastward phase velocity of 3-6 ms-1, and a realistic hierarchy of convection. The hierarchy includes large-scale cloud cluster (LCC) with a scale of 1, 000-1, 500 km, cloud cluster (CC) with a scale of O(100 km), mesoscale convection (MC) with a scale of a few to several tens of kilometer, and individual cumulus convection. The eastward propagation of SCC occurs favorably under the condition that the environmental surface flow is easterly. The surface easterly, with its superposition with convectively induced surface flows, enhances evaporation from the sea surface and moistens the lower-level atmosphere more effectively in the eastern side of the convection than the western side, which assures successive formation of new clouds to the east. This result supports the mechanism of WISHE suggested by Emanuel (1987), and Neelin et al. (1987). However, we also found that WISHE operates only as a preconditioning of the cloud formation. The formation is in practice contributed by mesoscale (O(10)-O(100) km) gravity waves that are induced by inner convection of SCC (that is, IVIC, CC and LCC) and propagate at phase velocities of 10-20 ms-1. The gravity waves play the role as follows. Under the condition of WISHE (the existence of the environmental surface flow), the waves propagate eastward from the existing convections and their upward motions initiate low-level clouds in some distance to the east where the air is moist enough. The clouds grow into taller convections aided by the subsequent passages of upward motions of gravity waves. The growing convections in turn induce gravity waves one after another to the east, and likewise contribute to generate the convections to the east. In this manner each of MC, CC, and LCC organizes itself into CC, LCC, and SCC, respectively, with the help of the mesoscale gravity waves. Additionally, equally important in maintaining the hierarchy is the interaction between the evaporatively driven cold pool and the environmental low-level warm flows, whose importance is now familiar in various mesoscale convective systems. The interaction is found to be essential to the maintenance of MC and CC, in particular at their formative and developing stages. It is also shown that MC, which seems to have received little attention in the previous studies of SCC, is one of significant classes of hierarchy in the organization of SCC. Furthermore, discussion is given concerning whether or not the SCC and the induced larger-scale disturbance can interact cooperatively. The development and maintenance of the eastward-propagating large-scale disturbance interacted with SCC can be explained by gravity (Kelvin)-wave CISK.
