第三极西风和季风主导流域源区降水呈现不同梯度特征

Contrasting precipitation gradient characteristics between westerlies and monsoon dominated upstream river basins in the Third Pole

利用位于第三极东南部受季风主导的长江、黄河、澜沧江、怒江上游和雅鲁藏布江流域,以及位于西部受西风主导的叶尔羌河、印度河、阿姆河和锡尔河上游流域源区256个气象站的降水数据,分析了各流域降水随海拔变化的梯度关系;基于ERA5数据,通过分析水汽含量、对流有效势能和抬升凝结高度与各流域内海拔的变化关系,探讨了不同气候系统主导的流域呈现不同梯度特征的原因;通过水文模型模拟径流反向验证降水梯度校正方法在推算高海拔山区降水时的可行性.结果表明:(1)位于季风区的长江上游、黄河上游、澜沧江、怒江和雅鲁藏布江流域降水随海拔增加而降低(17~128 mm/100 m),地形效应仅在小尺度呈现;西风主导的叶尔羌河、印度河、阿姆河和锡尔河上游流域降水随海拔增加而增加(5~64 mm/100 m),地形效应明显.(2)ERA5与气象站观测降水数据在不同流域源区表现出一致的降水梯度特征.季风区流域降水随海拔增加而减少,主要由水汽含量随海拔增加而减少所致,地形效应在局地尺度依然有所反映;西风区流域降水随海拔增加而增加,主要受抬升凝结高度降低和对流有效势能增加的影响.(3)陆面水文模型反向验证结果表明,在降水地形效应明显的流域,对低海拔站点降水进行地形校正是提高通过降水变率推算高海拔区域降水可靠性、提高水文模拟精度的一个有效途径.研究结果对第三极流域高海拔山区降水数据的地形校正有参考价值;对第二次青藏高原综合科学考察中降水观测的选点有指导意义.

Based on precipitation observations from 256 gauges in the westerly and monsoon dominated upstream river basins of the Third Pole(TP),this study determined the relationships between precipitation and elevation.The basins include the upper basins of the Yangtze,Yellow,Lancang,Nujiang,Yarlung Zangbo,Yarkant,Indus,Amu Darya,and Syr Darya.Using the ERA5 data,this work examined the possible reasons for the difference in the characteristics of precipitation gradient,i.e.analyzing the relationships between the total column water vapor(TCWV),convective available potential energy(CAPE),lifting condensation level(LCL)and elevation,respectively.The feasibility of orographic corrections of precipitation data or observation is validated with the improved VIC land surface hydrological model in two mountain basins in the TP.Mean annual precipitation from gauges generally show decreasing trends(17–128 mm/100 m)with increased elevation(2500–5500 m a.s.l.)in the monsoon dominated basins,i.e.upper Yangtze,Yellow,Lancang,Nujiang,and Yarlung Zangbo,while the orographic enhancements are observed at relatively smaller scales,such as,in the very source regions of the upper Lancang and Nujiang,and Rikaze sub-basin with areas of 11000-67740 km2.On the other hand,in the westerly dominated basins,mean annual precipitation tends to increase with elevation(5–64 mm/100 m)in the upper Yarkant,Indus,Amu Darya,and Syr Darya.The precipitation estimates from ERA5 show a good correspondence with the gauge data(R=0.6-0.9,P<0.05),and exhibit a general consistent precipitation gradient pattern with the gauge observations.The ERA5 variables of TCWV,CAPE,and LCL are useful to understand the factors for the spatial pattern of precipitation vertical gradients in the TP basins with different climate control.The TCWV,CAPE,and LCL represent the vertically integrated moisture,instability and condensation necessary for the generation and development of precipitation.The larger TCWV,higher CAPE and lower LCL enhance precipitation.The decrease of precipitation with elevation in monsoon basins is caused by the decrease of TCWV with elevation,while the increase of precipitation with elevation in westerly dominated basins is a result of increasing CAPE and decreasing LCL with elevation.Hydrological modeling results in the upper Yarkant basin and Rikaze basin indicate that the orographic correction of precipitation data significantly improves the model accuracy,reducing the biases to less than 5%relative to flow observations.This work demonstrates that precipitation correction through vertical gradients is an effective way to derive high mountainous precipitation estimates for hydrological modeling from lowland gauges in the TP,especially in the westerly dominated basins,and in monsoon basins at regional or local scales.Knowledge of the spatial and temporal characters and variations of precipitation over the TP is greatly incomplete,which largely hampers the understanding on climate variability and water availability projections in the TP.This study offers a useful reference to derive reliable mountain precipitation through orographic correction,and also provides a scientific basis to establish precipitation observation network in the Second Tibetan Plateau Scientific Expedition and Research.