Precipitation is closely related to the occurrence and development of clouds. The study for the characteristics of convective cloud is of great significance to the precipitation monitoring and prediction, as well as researches of precipitation mechanism. A torrential rain struck Qingyang on15 July 2022, which is located in semi-arid region of Northwest China, causing the daily rainfall and hourly rainfall at several stations to exceed the historical extreme values. Based on products of the advanced geostationary radiation imager (AGRI) from FY-4A and FY-4B geostationary satellite, fusion products of micro-wave humidity sounder (MWHS) and micro-wave temperature sounder (MWTS) from FY-3D polar orbit satellite, the cloudsytem evolution, macro and micro characteristics of cloud, and the atmospheric environmental conditions during this torrential rainstorm event were analyzed. The results are as follows: (1) The rainstorm cloud top types formed from supercooled water clouds, mixed clouds, opaque ice clouds and multi-layer clouds. The cloud top types of heavy precipitation are mainly opaque ice clouds, and the height of cloud top is more than 14 km. The rainstorm cloud system is deep and mainly composed of small ice particles, and accompanied by strong updraft. (2) In this rainstorm event, there exist a process of convective cloud formation, merging and strengthening, and the continuous influence of convective clouds leds to the occurrence of extremely heavy rain. Precipitation was closely related to the black body temperature (TBB) of cloud top and its variation. Low TBB corresponded to heavy precipitation, and the TBB drops rapidly before the heavy precipitation. (3) Before the occurrence of heavy precipitation, atmospheric stratification is shallow convective instability. The strong humidification in the middle and lower layers is the main reason for the development of convective instability. The difference of absolute humidity in the lower layers leads to the difference of instable conditions and precipitation intensity.
Based on the refined numerical prediction products of the European Center of Medium-range Weather Forecast (ECMWF), precipitation guidance products from China Meteorological Administration (TP_CMA) and precipitation observation data at 340 meteorological stations of Gansu Province, the objective divisions of precipitation at 340 meteorological stations of Gansu Province from April to September during 2017-2019 were done by using spatial cluster and Tyson polygon (SCTP) approach. On this basis, the physical quantity factors related to precipitation were selected and used to build prediction model by using random forest (RF) algorithm, and the correction experiment of short-term quantitative precipitation objective forecast in Gansu Province was carried out, the forecast effect was verified. The results are as follows: (1) There were 7, 6, 14, 13, 14 and 11 precipitation regions in sequence from April to September in Gansu Province. (2) In terms of rain probability forecast, the forecast ability of SCTP-RF products in flood season (from June to August) in Gansu Province was better than that of TP_CMA guidance products and ECMWF model products, and the prediction accuracy improved by 6.1% and 4.2%, respectively. In space, SCTP-RF products had a certain ability to correct rain probability forecast at all stations of Gansu Province, and the prediction accuracy at most stations improved by 5%, especially in the east of Yellow River in Gansu (Hedong area). (3) For graded precipitation forecast, the forecast ability of SCTP-RF products to moderate rain and heavy rain was superior to TP_CMA guidance products and ECMWF model products, and the correction effect at most stations was better, especially in the middle part of Hedong and southeastern Gansu. However, the correction ability to light rain and rainstorm forecast was unstable during the heavy rainfall processes, especially to light rain in southeastern Gansu.
