Diurnal Variation Characteristic Of Precipitation During The Flood Season In Lhasa

Based on the hourly precipitation observation data from 2005 to 2017 and that data by 12 hours from 1969 to 2017,the diurnal cycle signal of hourly precipitation by monthly was discussed and the diurnal variation difference between Lhasa station and other representative stations on the plateau was compared using method of harmonic analysis which is on the basis of the analyzed daily variation characteristics of the precipitation that including the amount of rainfall,frequency and intensity of precipitation and so on in flood season(from May to September)at Lhasa station.What is more,the relationship between relevant factors and daily variation of precipitation at Lhasa station was further analyzed based on hourly temperature and 10-minute averaged wind speed,cloud cover by 6-hoursly,sounding data by 12-hoursly and the ERA-Interim reanalysis dataset.In the end,the typical cases of precipitation were diagnosed and analyzed by using MICAPES data.The main results are as follows:(1)The characteristic of diurnal variation of hourly precipitation shows a “unimodal” in the flood season at Lhasa station.Besides,there is difference in the diurnal variation of different types of precipitation in that the diurnal peak of short-duration precipitation occurs between afternoon and early midnight,while that of medium-duration and long-duration occurs after midnight.Light rain is easy to appear in both day and night,and moderate rain is mainly concentrated at night.During the recent 49 a(1969-2017)flood season,the daily precipitation in Lhasa increased significantly,and the precipitation intensity increased significantly,but there was no obvious trend in the number of precipitation days.(2)The diurnal variation of precipitation in Lhasa is the diurnal cycle,among which the daily cycle signal is the strongest and the amplitude is the largest during the midsummer which is from July to August.Compared with other areas on the plateau,the diurnal circulation signal in Shigates in Lhasa station is the strongest and most stable.(3)The diurnal variation of precipitation was positively correlated with the daily variation of temperature,wind speed and total cloud cover,among which the daily peak time of temperature and wind speed anomalies was 2-hour earlier than the precipitation peak time.There was a good correlation between day and night rainfall and atmospheric precipitation,and the correlation coefficient between atmospheric precipitation and night rainfall was the highest at 20:00.From the 500 h Pa anomalous wind field,it can be seen that the anticyclone circulation is basically used to control the precipitation weather in the Tibetan plateau during the day,while the cyclonic circulation is mainly used to control the precipitation weather at night,and the most obvious diurnal variation of the circulation is located in the Shigates area of Lhasa.(4)Moreover,according to the diagnosis and analysis of individual weather cases,it was found that there were significant differences in the diurnal variation of meteorological elements in the upper air and on the ground among different cases.At 20:00 in the night rain days,there were obvious shear lines and high air humidity near Lhasa,as well as obvious diurnal variations in the surface temperature and cloud cover.There were no such characteristics in the daytime rain days and the non-rain days.