暴雨灾害 (Apr 2020)
Comparison of convective cloud characteristics during two torrential rain events in Shenyang based on Himawari-8 satellite data
Abstract
Using NCEP FNL reanalysis data, GNSS retrieved water vapor, wind profile radar products and the national intelligent grid real-life live fusion analysis data, we have conducted a comparative analysis of the convective cloud characteristics during two torrential rain events occurred in Shenyang on 14 July 2017 (event Ⅰ) and 7 August 2018 (event Ⅱ), with focus on the trigger and maintaining mechanism of convective cloud and their difference in affecting precipitation. The results are as follows. (1) The two events are local abrupt heavy rain and regional extreme rainstorm events, respectively. Both torrential rain events that occurred in Shenyang urban areas were induced by two mesoscale convective cloud clusters, whose merging made precipitation continue. During eventⅡ, the merging of cloud clusters occurred on the rear of their moving direction, with back propagating feature. The merged cloud clusters moved along its long axis and affected Shenyang, leading to a long precipitation time. (2) Before and at the initial stage of precipitation, both the convective cloud top and the water vapor layer top rise rapidly and the former exceed the latter quickly during eventⅠ, while during eventⅡ the brightness temperature decreased slowly. Before the short-term heavy precipitation in the two events, the simultaneous and rapid decrease of brightness temperatures to -60 ℃ at the infrared and water vapor channels is used as a reference indicator for the forecast of short-term heavy precipitation in advance. The top of convective cloud that caused 10-minute precipitation being greater than 10 mm concentrated in the areas with infrared brightness temperature below -55 ℃ and the brightness temperature difference between -5 ℃ and 0 ℃. (3) Shenyang is located in the rear of the northeast cold vortex and the north edge of subtropical high during the two events. During eventⅠ, the radiosonde curves show the shape of "X", and CAPE reaches up to 2 584 J·kg-1, which correspond with the formation of deep convective cloud, with precipitation intensity weakening due to the raindrop evaporation caused by the dry layer below the cloud bottom. So, high intensity precipitation is only produced during the consolidation of convective cloud clusters. During eventⅡ, the wet layer from the cloud bottom to the ground is obvious, which made the raindrops fall to the ground without much evaporation. TBB of cloud clusters that producing the same level precipitation is higher than that during the eventⅠ. (4) There is an obvious convergence in the surface wind field before the severe precipitation occurred. When the atmospheric precipitable water vapor increases by 8 mm within 2 hours, severe precipitation was observed at the stations. The jump of local water vapor content may be due to the increase of the southerly component of the low-level southwest airflow or the northerly cold air invading into the warm and wet air.
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