| A heavy rainstorm, which took place between May 22 and May 23,2014 in the central Guangdong Province of the South China warm region, was numerically simulated in this study using the ARW-WRF model. Three "echo-trainings" over rainstorm center were analyzed, and both the simulation and the observation showed that this extraordinary rainstorm was composed of three stages. In the first stage, a substantial warming appeared and led to the formation of a stable layer at the 900hPa level, which was the primary cause for gravity waves be triggered to form lineal convection with convective cells forming simultaneously but relatively independently and moving toward northeast through the rain center. In the second stage, with the formation of cold outflow on low layers, new convective cells were continuously initiated in the southwest and northwest of the rain area and then gradually moved to merge into the northeast rain area. In the third stage, the direction of the mid-layer-flow in the troposphere above the rainarea changed from southwest to west or northwest, while multiple rain bands moved southeastward passing through the strongest precipitation center.The composite analysis method was used to analyze the evolution process and structural features of the convective cells on the convective line during the first stage of three echo-trainings. This analysis identified three stages:(1) Stage of activation-the equivalent potential temperature surfaces as lower layers start to bulge and form warm cells and weak vertical convective cloud towers which are subject to the impact of low-level warm moist updrafts in the rainfall sector. (2) Stage of development-the warm cells continue to bulge and form warm air columns and the convective cloud towers develop upwards becoming stronger as they rise. (3) Stage of maturity-the warm air columns start to connect with the stable layer in the upper air. The convective cloud tower will bend and tilt westward with each increase in height, and the convection cell is characterized by a "crescent-shaped echo" above the 700hPa plane. The thermal structural characteristics of the convective cells show up as the internal temperature of cells are higher than the ambient temperature. The dynamic structural field is manifested as intensive vertical upward movement. The large-value centers of the northerly and westerly winds in the middle layer correspond to the warm moist center in the cells and the relatively cold center south of the warm air column.Further analysis shows that the formation of the "crescent-shaped" convective cell is associated with horizontal vorticity. With the upward extension of the internal warm air column of the convective cell, the horizontal vorticity in the center and west of the warm cell experiences stronger cyclonic and anticyclonic shear transformation over time. This not only causes the original suborbicular cell echo form shape to develop into a crescent-like shape, it also makes a convection line consisting of cells to develop to the northwest. What’s more, the dynamic structural feature of the cells is also related to the horizontal vorticity. |
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