Abstract:

Based on laser ceilometer observations from 2022 to 2023 at two stations, Xinyuan and Yining, in the Ili River Valley region, Xinjiang, China, this study statistically analyzed and compared cloud frequency (CF), number of cloud layers, and cloud base height occurrence frequency (CBHOF), incorporating topographic and climatic features. Precipitation clouds were further categorized using raindrop spectral data. The results indicated that the following: (1) Both stations exhibited significant commonalities in cloud macroscopic characteristics. The daily variation in CF at the two stations followed a single-peak pattern, with less cloud cover during the daytime and more cloud cover at night. This diurnal pattern was more pronounced in summer owing to the marked solar radiation variability. The annual average CF remained approximately 35% at both sites, With high-level CF consistently low throughout the year. Increased midlevel cloud cover and reduced low-level clouds during summer, coupled with intensified evapotranspiration, may drive seasonal drought prevalence. Low-level clouds slightly dominated from November to March. (2) Differences in macroscopic cloud properties between the two stations were primarily modulated by water vapor transport regimes and topographic forcing. Both sites predominantly featured single-layer clouds. However, with abundant water vapor and orographic uplift, the Xinyuan station exhibited a higher propensity for generating double-and multi-layer clouds than that of the Yining station. CBHOF exhibited significant seasonal variation: minimal changes occurred during spring and autumn, summer was dominated by mid-level clouds, and winter was characterized by concentrated low clouds with significant peak height differences between the two stations owing to topographic and moisture transport effects. (3) The eastern Ili River Valley received more precipitation than the west, with stratiform precipitation greatly exceeding convective precipitation. Convective clouds at the Xinyuan station exhibited stronger vertical growth and intensity than those at the Yining station, attributed to localized orographic lifting. In contrast, stratiform clouds exhibited similar characteristics at both stations, indicating that topography exerted negligible differential influence on their formation.

Key words: laser ceilometer, daily variation, cloud base height, topography, convective cloud, Ili River Valley