A Study of Cloud Clusters Associated with a Baiu Front by Use of a Mesoscale-Convection-Resolving Model

[摘要] Numerical experiments are performed to simulate and understand cloud clusters associated with a Baiu front observed over Kyushu and the East China Sea on 16 July 1993. The clusters treated in this study are those that are not associated with any synoptic-scale low. A numerical model used is a mesoscale-convection-resolving model (MCRM) that resolves mesoscale organized convection by a grid, and treats cumulus convection as the subgrid-scale (Yamasaki 2001). The primary objective of this study is to investigate to what degree the MCRM can describe cloud clusters, and mesoscale organized convection that constitutes the clusters. In this study the grid size is taken to be 5/36 degrees (about 15 km) in the finest grid area of the triply-nested grid model. A global objective analysis data of JMA is used as the initial condition (00UTC, 16 July). A numerical experiment is performed by use of the MCRM referred above. For comparison, an old version of the MCRM (Yamasaki 1986) is also used, and an additional experiment is made for the case without parameterization of cumulus convection. The rainfall distributions at 12 hours after the initial time are compared with AMeDAS data, and with those from Peng and Tsuboki (1997) in which four cloud parameterization schemes are used. It is emphasized that the most important factor to prediction and understanding of the cloud cluster and rainfall over Kyushu at 12 hours is the eastward movement of a latently unstable area that exists at the initial time. A comparison of the results from the MCRM with those from the case without cumulusscale parameterization shows that the effects of cumulus convection are not essential to the eastward movement of the unstable area and rainfall over Kyushu at 12 hours, but time evolution (behavior) of mesoscale organized convection and cloud clusters are quite different, depending on the inclusion of the cumulus-scale effects. The performance of the model as to how cloud clusters and mesoscale organized convection behave realistically under the given initial condition is discussed, based on the studies in the past and physical considerations.

[学科分类] 大气科学