A comparative study on environmental conditions and doppler radar echo characteristics of two downburst events in Ningbo

Thunderstorms with wind gusts of magnitude 13 and 12 caused by downburst storms occurred in Ningbo on July 12 and 17, 2022 (referred to as the “7.12” event and “7.17” event, respectively). The “7.17” event caused significant casualties. A comparative analysis of environment conditions and Doppler radar echo characteristics of the two downburst events is carried out by using multi-source data from S-band Doppler radar, conventional sounding and surface observation data, 1-minute high-resolution automatic station observation data, and ERA5 reanalysis data. Results show that: (1) Environmental weather conditions differ significantly between the two events. The “7.12” event occurred under weak synoptic-scale forcing and weak vertical wind shears. Convergences related to sea breezes and thermal uplifts triggered the occurrence and development of strong convections. However, the “7.17” event occurred under unstable layers with “lower warm-wet and upper cold-dry” conditions, which was related to forward tilting troughs. It displayed more significant synoptic-scale forcing and low-level vertical wind shears on 17 July than on 12 July. (2) Both processes showed significant mid-level dry layers and high vertical lapse rates from 0 to 3 km. The intrusion of mid-level dry air was one of the main reasons for the generation of downburst storms. (3) During the “7.12” event, a series of pulsive storms, along with moving gust fronts, triggered multiple micro downbursts and extreme winds. However, during the “7.17” event, the outbreak of macro downbursts and extreme winds resulted from the merging and intensification of multiple storms. In both events, radar echo characteristics showed a decrease in reflectivity core and collapse of 45 dBz echo-top. The descent rate of the reflectivity core on 12 July was faster than that on 17 July.