Analysis of Observation Characteristics of Heavy Rainfall at Beijing Capital International Airport in July 2023

The evolution and observation characteristics of the heavy rainfall that occurred at Beijing Capital International Airport (hereinafter referred to as Capital Airport) on 31 July 2023 were examined by using multi-source data including manual observation and automatic observation from Capital Airport, conventional and intensive surface and upper-air MICAPS data from China Meteorological Administration, FY-4A TBB (black body temperature) products, products from Beijing-Tianjin-Hebei wind profile radar network, images from the SA dual-polarization radar at Beijing Observatory. The results show that Capital Airport experienced heavy rainfall from 11:00 BT to 13:00 BT and from 15:00 BT to 17:00 BT, respectively. The heavy rainfall occurred on the east side of the weakened typhoon system with moist and energy by eastward jets in the mid- and low-levels. The sounding chart shows that the convective available potential energy (CAPE) has increased, indicating a higher instability. The lower atmosphere had high absolute humidity and saturation. The lifting condensation level and free convection level were both relatively low, while the height of the 0 ℃ layer was high, which was favorable for the development of convective precipitation. The changes in the location of the boundary layer and surface meso-β scale convergence lines played an important role in maintaining and enhancing the precipitation. The TBB data indicate that two mesoscale convective systems (MCS) successively affected Capital Airport. MCS-A had a longer duration and lower TBB values, bringing persistent precipitation to Capital Airport. In contrast, MCS-B developed rapidly. Although its TBB values were not as low as those of MCS-A, it caused the strongest hourly precipitation at Capital Airport. The dual-polarization radar images show that the precipitation at Capital Airport was characterized by low-centroid-echo and warm cloud precipitation, with stronger echo intensity in the afternoon than that in the morning. In the morning, the convective clouds exhibited both ice-phase and liquid-phase microphysical processes, and the growth of raindrops related to the effects of ice crystals and collision-coalescence. In contrast, the short-duration heavy rainfall in the afternoon was purely warm cloud precipitation, characterized only by the collision-coalescence growth of raindrops, with larger raindrop diameters and higher raindrop particle concentrations.