A Numerical Study of Cloud Clusters and a Meso-α-Scale Low Associated with a Meiyu Front

Abstract

In order to understand cloud clusters and a meso-α-scale low, which were observed on the China Continent on 29 June 1998, numerical experiments are performed with a model which intends to resolve mesoscale organized convection, the effects of cumulus convection being incorporated as the subgrid-scale. The horizontal grid size is taken to be about 5 km for the fine grid area of the triply-nested grid model. The initial time for the numerical experiments is 00 UTC 29 June. Global analysis data (GANAL) of JMA is used as an initial condition, but two numerical experiments are performed, without and with slight reduction of low-level relative humidity of GANAL data.
Results from numerical experiments indicate that in both cases, convective clouds are formed in a banded form, and a front that can be called a Meiyu front is also produced. On this front, a meso-α-scale low is also formed. The positions of the mesoscale lows predicted at 24 hours after the initial time in both cases, are almost the same. These are roughly in agreement with the position from GANAL data.
Significant differences are found in the evolution stage for a period of 24 hours. In the original case, strong convective activity creates a mesoscale low at an earlier time (12 hours), and it is probably somewhat too intense compared with that observed. On the contrary, in the less humid case, three mesoscale convective systems and vortices (or closed isobars) are found at 12 hours. The easternmost vortex develops, and it becomes a mesoscale low at 18 hours.
Discussions in this paper are made with an emphasis on the relation among latent instability, mesoscale convective systems, vortices, a mesoscale low and a Meiyu front. One of the significant features of the present case is that convective activity takes a banded form in a relatively large area of latent instability, and a Meiyu front is produced and intensified for the given initial conditions. As expected, latent instability becomes weaker, owing to convective activities. Eventually a latently stable area appears first around and just to the north of the convective area, and it expands northward. It is important to remark that the latently stable area to the north of the Meiyu front in the later stages is created by convective activities (not by synoptic-scale motions). In an area to the south of the front, latent instability becomes weaker gradually. However, continual rainfalls are maintained in the trailing portion of the mesoscale low till 00 UTC 30 June. This is in agreement with the observations.
The relative importance of the effects of subgrid-scale cumulus convection and the grid-resolved condensation is also discussed. The importance of the latter increases with time; it contributes particularly to the development of the mesoscale low.