Abstract:  Based on hourly observation data from 2005 to 2017 and three-hour observation data from 1969 to 2017 in Lhasa station, diurnal variation characteristics of precipitation during flood season (from May to September) and the long-term evolution characteristics of daytime and nighttime precipitation were revealed. The results are as follows: (1) The diurnal variations of hourly precipitation amount and frequency in Lhasa were characterized by a single peak distribution, both peak values occurred at 05:00 and the valleys occurred during 15:00-17:00, while the hourly precipitation intensity presented a double peak distribution, the peak value occurred at 17:00 and 00:00, and the valley value occurred during 13:00-15:00. (2) The diurnal variation phases of hourly precipitation amount and frequency of different grades precipitation were different. The diurnal variation of hourly precipitation amount and frequency of slight rain and light rain presented single peak  distribution, the peak value occurred at 05:00, and the time of peak hourly precipitation amount and frequency of medium rain and above were earlier than that of the former two grades precipitation. (3) The daytime and nighttime precipitation amount and intensity increased significantly, while precipitation days had no obvious trend in Lhasa during 1969-2017. The main reason of precipitation amount increase was precipitation intensity increase in Lhasa during the flood season.

Key words: diurnal variation, precipitation amount, precipitation frequency, precipitation intensity, daytime and nighttime precipitation

摘要： 利用拉萨2005—2017年逐小时降水观测资料和1969—2017年逐3 h降水观测资料，在分析该站汛期（5—9月）降水日变化特征的基础上，揭示该站昼夜降水的长期演变特征。结果表明：（1）拉萨小时降水量和降水频次日变化呈单峰型分布，两者峰值均出现在05:00（北京时，下同），谷值出现在15:00—17:00；小时降水强度日变化呈双峰型分布，峰值出现在17:00和00:00，谷值出现在13:00—15:00。（2）拉萨汛期不同等级降水的小时降水量和降水频次日变化位相不同，其中微雨和小雨的小时降水量和降水频次日变化为单峰型，且峰值均出现在05:00，而中雨及以上小时降水量和降水频次日变化峰值出现时间较微雨和小雨略有提前。（3）近49 a拉萨汛期昼夜降水量显著增多，降水强度显著增强，而降水日数无明显趋势，降水强度增强是拉萨汛期降水量增多的主要原因。

关键词:
日变化, 降水量, 降水频次, 降水强度, 昼夜降水