Evolution characteristics of vortex cloud clusters in an excessive torrential rainfall event

Evolution characteristics of vortex cloud clusters in an excessive torrential rainfall event occurred in Suining, Sichuan on 30 June 2013 are analyzed in detail by using MODIS satellite data, Doppler weather radar data and surface meteorological observations. The major results are as follows. (1) The vortex cloud clusters caused the torrential rainfall. Many cumulus clusters in the north side presented a forward-tilling structure, which shows that cloud clusters accompanied by torrential rainfall experienced a transition from weak cumulus to deep convective clouds firstly and to cloud felt lastly. (2) As for vortex cloud clusters that caused torrential rainfall, there are six spiral cloud bands at most from south to north around the eastern side of the vortex center. The thickness of collision-coalescence growth (from -10℃ to -1℃) greater than that of the condensation growth (from -1℃ to 3℃) in the southern spiral cloud band as opposed to those in the northern spiral cloud band, which suggests that the predominant microphysical process is collision-coalescence growth during the development of southern convective cloud clusters, and then the cloud droplet spectrum is broaden faster to make the formation of cloud droplets grow rapidly into raindrops while falling down. (3) The strongest spiral cloud bands locate in the southern area of vortex cloud clusters caused torrential rainfall from Ziyang to Suining. The eight convective cells are found in the southern spiral cloud band in the vertical cross section of radar reflectivity factor, and newborn cells moved continuously to Suining and strengthen in order from south to north, thus resulting in "train effect". During the mature stage of these cells, the collision-coalescence and condensation growth (thickness is about 6 km) are active below height of -10 ℃ level, and there is a thick ice growth layer (thickness is 5-8 km) above height of -10 ℃ level. When these cells are mature, both collision-coalescence growth and ice-phase are the predominant microphysical processes, and collision-coalescence growth zone and glaciations growth zone in the cloud propagate dramatically down to the lower levels from the development to the mature stage. These features are advantageous to the rapid formation of precipitation, and resulting in continuous severe precipitation in Suining from10:00 BT to 17:00 BT on 30 June 2013.