Cloud Parameter Characteristics of Three Strengthening Convective Systems During Downhill Processes in Beijing

Based on FY-4A geostationary satellite multi-channel data, mesoscale convective systems (MCSs) during three downhill processes of convection strengthening (1 July 2021, 12 June 2022 and 4 August 2022) in Beijing are identified by temperature threshold method. Precipitation data from Beijing-Tianjin-Hebei automatic weather stations, along with data from CMA-BJ mesoscale numerical model, are also utilized to analyze cloud characteristics of MCSs in formation and mature stages. Results indicate that the area of the convective cloud increases slowly, but the brightness temperature gradient is high, the brightness temperature decreases rapidly, the lowest temperature is below -65 ℃ and the maximum temperature variation rate in the area is -40 ℃·(15 min)-1 in the formation stage of MCSs. The maximum bright temperature gradient area corresponds to the inflow and water vapor convergence area, and it is located on the side of the cloud movement direction. In the mature stage, the area of the convective cloud increases rapidly. The number of short-time heavy precipitation stations is the largest, and the rainfall intensity is higher when the area is the largest. The brightness temperature is maintained at a relatively low value, but the area and amplitude of the temperature variation zone, as well as the brightness temperature gradient in the mature stage are smaller than that in the formation stage. The brightness temperature difference between water vapor brightness temperature and infrared brightness temperature can represent convective cloud development intensity, showing characteristics of slow fluctuation increase-fast increase-stable maintenance over time which indicate stages of MCSs. In the view of kinematic characteristics of the cloud cluster, it can be seen that strongly convergent and strong upward motion are located in the inflow region, the cloud top height increases rapidly, the infrared brightness and water vapor brightness temperature decreases rapidly, and the brightness temperature is close to 0 ℃ in the initial stage. In the mature stage, the cloud top height, infrared brightness and water vapor brightness temperature stay a relatively stable state, strong updraft and downdraft coexist, there is obvious outflow within the upper air, and the convective area increases to the maximum. The downdraft in the cloud body creates a strong outflow ahead of the low-level movement, leading to the extreme wind on the ground. The convergence of environmental wind and outflow strengthens the updraft movement in front of the MCS, which is conducive to the formation of new cells and the strengthening of MCS. Above results can reveal the development stage of MCSs and provide reference for determining whether MCSs in Beijing are strengthening or weakening during the downhill processes, as well as the potential area of severe weather on the surface.