川西高原夏季夜雨变化特征

The Characteristics of Summer Nocturnal Rainfall in the Western Sichuan Plateau

摘要: 川西高原位于青藏高原东缘，地形起伏强烈，基于1961~2023年夏季逐日降水资料，按照北京时间20:00至次日08:00统计夜雨量、夜雨日数和夜雨强度，采用趋势分析、距平百分率分析、旋转经验正交函数分解(Rotated Empirical Orthogonal Function decomposition，以下简称REOF分解)、Morlet小波分析和集合经验模态分解(Ensemble Empirical Mode Decomposition，以下简称EEMD分解)等方法，系统分析川西高原夏季夜雨的空间分布、年代际变化和多尺度时间演变特征。(1) 川西高原夏季夜雨量空间差异显著，总体呈现南多北少、东多西少、河谷多于高原的特征，区域夜雨量约为140~530 mm；夜雨日数约为29~65 d；夜雨强度约为4~13 mm/d。(2) 1961~2023年川西高原平均夜雨量呈弱增趋势，线性倾向率为1.954 mm/10 a；夜雨日数呈减少趋势，倾向率为−0.905 d/10 a；夜雨强度呈增强趋势，倾向率为0.165 mm/(d∙10 a)，说明近60余年夜雨变化主要体现为日数减少而单次降水强度增强。(3) REOF分析显示，川西高原夏季夜雨存在明显的南北差异和区域局地性。分区结果表明，I区对应RPC2，主要代表川西高原北部高原干区；II区对应RPC1，主要代表川西南山地湿区。(4) 小波和EEMD结果表明，分区尺度上，I区以30年左右年代际振荡最突出，同时存在4~6年短周期信号；II区除16~20年的年代际波动外，30年以上低频振荡更明显。

Abstract: The western Sichuan Plateau is located on the eastern margin of the Qinghai-Xizang Plateau, characterized by significant topographical undulations. Based on summer daily precipitation records from 1961 to 2023, nocturnal precipitation was calculated for the period from 20:00 Beijing Time to 08:00 the next day. Trend analysis, anomaly percentage analysis, rotated empirical orthogonal function (REOF), Morlet wavelet analysis and ensemble empirical mode decomposition (EEMD) were used to examine the spatial distribution, interdecadal variation and multi-scale temporal evolution of summer nocturnal rainfall over the western Sichuan Plateau. (1) The spatial differences in the summer nocturnal rainfall amount on the Western Sichuan Plateau are significant. Overall, it shows a pattern of higher in the south than in the north, higher in the east than in the west, and higher in river valleys than on plateaus. The regional nocturnal rainfall amount is approximately 140~530 mm; the number of nocturnal rainfall days is about 29~65 days; and the nocturnal rainfall intensity is around 4~13 mm/d. The frequency of nocturnal rain ranges from about 29 to 65 days, and the intensity ranges from about 4 to 13 mm/d. (2) During 1961~2023, the mean nocturnal precipitation amount shows a weak increasing trend of 1.954 mm/10 a, the frequency decreases at −0.905 d/10 a, while the intensity increases at 0.165 mm/(d∙10 a). It is indicated that the changes in nocturnal rainfall over the past six decades are mainly characterized by a decrease in the number of rainy days and an increase in the intensity of single precipitation events. (3) REOF analyses indicate clear regional differences. The regional division is defined as follows: Region I corresponds to RPC2 and represents the northern dry-cold plateau area, whereas Region II corresponds to RPC1 and represents the southern warm-humid mountainous area. (4) Results from wavelet analysis and EEMD indicate that on the subregional scale, Region I is dominated by interdecadal oscillations of approximately 30 years, accompanied by short-period signals of 4~6 years. In Region II, besides interdecadal fluctuations of 16~20 years, low-frequency oscillations longer than 30 years are more prominent.