Abstract:

Based on the hourly precipitation data of May to September at 38 meteorological stations in Xizang from 1981 to 2020, the spatial and temporal distribution, variation trend, mutation and period of daytime precipitation(P_d) and nighttime precipitation(P_n) during the growing season of plants in Xizang in recent 40 years were analyzed using the methods of the climatic propensity rate, Pearson coefficient, linear regression method, and five mutation tests including the Mann-Kendall, Cramer, etc. Results showed that: (1) the annual day and night precipitation in Xizang varied greatly in different regions. During the P1 period(1981−2010), the average P_d in the growing season in Xizang ranged from 21.3 to 240.5mm, generally characterized by a decreasing distribution pattern from the northeast to the southwest, and the P_n ranged from 36.3 to 430.7mm, showing a distribution pattern decreasing from the northeastern part of Lhasa and the southern part of Nagchu city to the surroundings. The P_n was 1.7% to 436.0% more than P_d for all stations except the Lhünzê station, which was 6.8% less than P_d. Among them, that were the most obvious in the middle reaches of the Yarlung Zangbo river, with an excess of 1.0−4.4 times. Compared with the P1 period, the spatial distribution of P_d and P_n was basically unchanged in the P2 period (1991−2020), except for a southward shift of the 200mm isoline of P_d and a slight expansion of the 400mm isoline of P_n. The highest and lowest values of P_d and P_n were more of 1.8%−2.4% and 8.3%−8.9% respectively, especially the lowest values. In addition, the P_d and P_n during the growing season were mainly affected by longitude. (2) In the past 40 years, both the P_d and P_n decreased in most of Lyingchi city, Lhorong and Nyalam stations during the growing season, and increased in most other areas, but the trends were not significant. Moreover, the trend of P_n was greater than that of P_d at 63.2% stations. The P_d and P_n co-increasing type accounted for 63.2%, the P_d and P_n co-decreasing type accounted for 13.1%, the case of P_d increasing and P_n decreasing type accounted for 7.9%, and the P_d decreasing and P_n increasing type accounted for 15.8%.The dominant factor influencing the climatic tendency rates of P_d and P_n was the altitude, which increases with elevation, and the rate by 0.52mm·10y⁻¹·100m⁻¹ and 0.67mm·10y⁻¹·100m⁻¹, respectively. (3) On the decadal scale, the P_d and P_n in the growing season at most stations in Xizang were lower in the 1980s, higher in the southeast and lower in the northwest in the 1990s, lower in the east and west and higher in the north in the 2000s, but the opposite in the 2010s, which the lower in the southeast and higher in the northwest. (4) In the last 40 years, the P_d and P_n averaged at 38 stations in Xizang during the growing season increased by a rate of 3.4mm·10y⁻¹ and 6.9mm·10y⁻¹, respectively. However, the increase trend of P_d was significantly reduced in the growing season during the last 30 years, and P_n showed a decreasing trend. The P_d and P_n had a significant 3-year period and mutated in 2004 and 2005.

Key words:

 Growing season, Daytime and nighttime precipitation, Change trend, Inter-decadal change, Climate mutation, Period