Analysis of Extreme Short-term Heavy Rainfall Characteristics during the Meiyu Period in Jiangsu Province

Using the method of percentile, the threshold of the extreme short-heavy rainfall (ESHR) in Meiyu Period in Jiangsu province is defined and the climate characteristics of ESHR are analyzed. Results show that ESHR rainfall amount and frequency in the south are higher than that in the north in Meiyu period in Jiangsu. But the intensity of ESHR in the north is larger than that in the south. The ESHR contributes 8.5% precipitation to the total precipitation amount in Meiyu period. Considering diurnal variation, the period from 14:00-20:00 BT is the main occurrence period of ESHR while the ESHR events seldom happen during 08:00-14:00 BT.

Based on the above research, environmental conditions, sounding physical quantities and the radar features of ESHR events in Meiyu period in Jiangsu province are studied. Results indicate that the major water vapor sources come from the channel from the Indian Ocean through the Bay of Bengal to the southwest or the southeast coast of China, and another source comes from the confluent of the southwest airflow from the Indian Ocean and the southeast airflow from the west Pacific. When an ESHR event happens, the total precipitable water vapor (PWV) is above 50 mm, basically between the range of 50-65 mm and the location of rainfall corresponds well to the high-value area of PWV. By using the atmospheric sounding data, the statistical results of several environmental parameters (0℃ layer height, -20℃ layer height, CAPE value, K index, SI index and vertical wind shear) manifest that 0℃ layer height (0H), -20℃ layer height (-20H) and K index (KI) are the best indicating parameters for ESHR with the corresponding range of 5-6KM, 8-9KM and 35-50. The CAPE value is a better indicator, usually higher than 1500J/kg when an ESHR event happens. While due to the large differences in statistical data, SI index and VWS (vertical wind shear) have weak representability compared with the other parameters for the occurrence of ESHR.

Radar features based on Jiangsu historical WSR-98D radar data are also investigated. In the research, 11 ESHR events are selected by the percentile method according to the data from 2005 to 2013. Among the 11 events, mixed echo takes up six and convective echo occupies five. As far as propagation mode, 64% of events are ascribed to “train effect” or “back-building” or both together. 18% of events propagate in the form of squall lines and another 18% events move slowly. On the whole, most ESHR events occur accompanied by multi-cell storms, vertical structures, and high mass centers, and can be classified as continental precipitation. Then, through typical cases study, the corresponding relationship between max basic reflectivity (Max BR), vertically integrated liquid (VIL), echo tops (ET) and precipitation are investigated. Results demonstrate that there is a better relationship between ET and precipitation during the period of most intense precipitation. Besides, 4DVar wind field and the vertical cross section of the radial velocity before ESHR occurs are also analyzed. It can be seen that there exists a deep convergence when ESHR happens and mesocyclones can be detected in radial velocity field.

Some meaningful conclusions are further discussed. For the occurrence of ESHR events, PWV is an important condition. There is a larger PWV value generally above 50 mm when an ESHR event happens. Weak VWS is not a necessary condition for the occurrence of ESHR since sometimes the VWS is intense when ESHR happens. High CAPE values play a vital role in affording unstable energy to ESHR. The CAPE value in a certain range has a good indication for ESHR, but that doesn’t mean that the higher the value is, the more the precipitation. By analyzing radar data, it can be seen that most echoes in ESHR events propagate in the form of “train effect” and “back-building” with vertical structures and high mass centers. Mesocyclones are found in some cases and the possible roles for ESHR are discussed. When environmental conditions are conducive to ESHR, the existence of mesocyclones maybe increases precipitation. On the one hand, the position of most mesocyclones is overlapped or partially overlapped with the rising air flow, which leads to strong vertical helicity and extends the life cycle of the system. On the other hand, the interaction between mesocyclones and VWS leads to intense updrafts which induce severe precipitation.