Characteristics of Spatial and Temporal Distribution of Hail Duration in China during 1961-2015 and Its Possible Influence Factors

Based on 55-year (1961 to 2015) ground-based hail and atmospheric sounding observations, the climatological characteristics of hail duration in mainland China, including hail durations at individual stations, average single hail duration, regional average single hail duration, hourly hail duration, and whole hail duration, along with convective parameters, are analyzed. The results show that: (1) The hail duration is highly positively correlated with altitude and the correlation coefficient is up to 0.99. The largest hail duration of about 250 min for the period 1961-2015 appeared in the Tibetan Plateau, followed by that in the mountainous areas in the central and northeastern Inner Mongolia (about 150 min). (2) Overall, the average single hail duration increased by 1 minute from 1961 to 2015, and the trend is statistically significant at the 95% confidence level. (3) Regionally average single hail duration shows a downward trend in NWC (Northwest China), NCP (North China Plain), and SEC (Southeast China), but an upward trend in NWC and TP (the Tibetan Plateau) during 1961-1980. There are positive relationships between regional average single hail duration trend in NWC and the trends of T_min (daily minimum temperature) and DTR (daily temperature range), while the hail duration trend in NCP is mainly correlated with the DTR trend. The hail duration trend in SEC is well correlated with all convective parameters. The upward trends of regional average hail duration both are positively correlated with the trends of T_max (daily maximum temperature) and T_min in NWC and TP during 1970-2015 and 1961-2015. (4) In terms of diurnal variability, the hail duration of cases that occurred during afternoon and night is longer than the duration of cases that occurred in the morning. The hail duration peaks appear at 1700 LT (local time) and 1800 LT. The analysis of relationships of hail duration and CAPE (Convective Available Potential Energy) , Totals-totals index based on long-term sounding data shows that CAPE and Totals-totals index at 2000 BJT (Beijing time) may be two factors that influence the diurnal cycle of hail duration in mainland China.