Diurnal Variations of Precipitation over the Steep Slopes of the Himalayas Observed by TRMM PR and VIRS

Abstract

This study investigates diurnal variations of precipitation during May–August, 1998–2012, over the steep slopes of the Himalayas and adjacent regions (flat Gangetic Plains–FGP, foothills of the Himalayas–FHH, the steep slope of the southern Himalayas–SSSH, and the Himalayas-Tibetan Plateau tableland–HTPT). Diurnal variations are analyzed at the pixel level utilizing collocated TRMM precipitation radar and visible infrared data. The results indicate that rain parameters (including rain frequency, rain rate, and storm top altitude) are predominantly characterized by afternoon maxima and morning minima at HTPT and FGP, whereas, maximum rain parameters at FHH typically occur in the early morning. Rain parameters at SSSH are characterized by double peaks; one in the afternoon and one at midnight. Over HTPT and FGP, convective activity is strongest in the afternoon with the thickest crystallization layer. Over FHH, the vertical structure of precipitation develops most vigorously in the early morning when the most intense collision and growth of precipitation particles occurs. Over SSSH, moist convection is stronger in the afternoon and at midnight with strong mixing of ice and water particles. The results of harmonic analysis show that rain bands move southward from lower elevation of SSSH to FHH with apparent southward propagation of the harmonic phase from midnight to early morning. Moreover, the strongest diurnal harmonic is located at HTPT, having a diurnal harmonic percentage variance of up to 90%. Large-scale atmospheric circulation patterns exhibit obvious diurnal variability and correspond well to the distribution of precipitation.

摘 要

本文利用 1998–2012 年热带测雨卫星搭载的测雨雷达, 可见光、 红外扫描仪和地形数据的融合资料, 研究了雨季 (5–8 月) 喜马拉雅山南侧陡峭地形及其周边区域 (flat Gangetic Plains–FGP, the foothills of Himalayas–FHH, the steep slope of south Himalayas–SSSH, the Himalayas-Tibetan Plateau tableland, HTPT) 的降水日变化特征. 研究结果表明, 在 FGP 和 HTPT, 各降水参量 (降水频次、 降水强度和雨顶高度) 的日峰值主要集中在午后, 在早晨最小, 而 FHH 各降水参量日峰值主要出现在凌晨. SSSH 各降水参量午后和午夜峰值并存. 在 HTPT 和 FGP, 对流活动均在午后发展最强, 冰晶层厚度最大. 在 FHH, 降水垂直结构在午夜至凌晨发展最旺盛, 冰水粒子碰并增长过程最为激烈. 在 SSSH, 对流活动在午后和午夜垂直运动发展较强, 伴随着冰水粒子的强烈混合. 谐波分析结果表明, 降水位相在午夜至凌晨时段从 SSSH 向南移动至 FHH, 有显著的南移特征. 此外, 降水日变化特征在 HTPT 最为显著, 降水日变化解释方差可达90%. 大尺度大气环流具有明显的日变化特征, 与降水的分布有很好的对应关系.