喜马拉雅山脉中段中小尺度地形对水汽输送和降水过程的影响研究

The Central Himalayas (CH) is a natural barrier of moisture transport from South Asia into the Tibet Plateau (TP). Because of its steep and complex topography, it's one of the difficult regions of observing and climate modeling. There are still significant biases in precipitation simulation of the WRF experiments, which are at a coarse spatial resolution and with a meso-micro-scale Orographic Form Drag schem. Aiming at the aforementioned issue, this project will focus on the following research contents: 1) Firstly, we will implement in-situ measurements of precipitation, traditional meteorological variables and ground-based GPS water vapor over the CH, and these data could form the data foundation of further model development; 2) Secondly, we will perform WRF modeling experiments at a high spatial resolution for describing meso-scale terrain and with sub-grid parameterization scheme for expressing micro-scale Orographic Form Drag, and then optimize meso- and micro-scale Orographic Form Drag scheme by using in-situ measurements, and finally minimize the biases of simulated wind, water vapor and precipitation; 3) Thirdly, we will analyze the impact of meso-micro-scale orography on the moisture transport and precipitation allocation under conditions of thermal force dominated summer period, dynamic force dominated winter period and extreme precipitations. This project can directly improve the ability of regional climate models to simulate precipitation in the complex mountains, enhance the understanding of water vapor transport processes in the complex mountain area, and finally contribute to the modeling of weather and water cycle processes in the Himalayan mountains.

喜马拉雅山脉中段是阻挡南亚水汽进入高原的天然屏障。该地区地形复杂，中小尺度地形作用不容忽视，是目前观测和模拟最困难的区域之一。即使在粗分辨率WRF模式中使用中小尺度湍流阻力参数化方案，模式仍然存在很大的降水模拟误差。针对该问题，本项目将开展以下研究内容：1）在喜马拉雅山脉中段开展沿亚东河谷的，从南坡到高原，降水断面和气象观测以及地基GPS水汽观测，作为模式发展基础；2）在WRF模式中，使用高分辨率(~km)直接表达中尺度地形，采用次网格参数化表达小尺度地形湍流阻力，并借助观测优化小尺度地形湍流阻力参数化方案，减小风速、水汽输送和降水的模拟误差；3）分析以热力为主的夏季、动力为主的冬季以及极端降水情况下中小尺度地形对喜马拉雅山区南北水汽输送和降水分配的影响。本项目将直接改善区域气候模式对复杂山区的降水模拟能力，加深对复杂山区水汽输送过程的理解，从而贡献于喜马拉雅山区天气和水循环过程研究。

无论是全球还是区域气候模拟对复杂多变区域降水模拟均存在系统性偏差，关键原因之一在于无法准确描述复杂地形对水汽输送过程的影响。针对该问题，本项目在公里级尺度上优化地形拖曳参数化方案，显著提高WRF模式在复杂山区的模拟精度；可将喜马拉雅中段降水湿偏差减小200%以上，也可将青藏高原降雨和风速平均偏差降低至0.7mm/dy和0.16m/s。同时，项目量化多尺度地形对水汽输送的降水效应，发现雅江大峡谷内的水汽输送主要作用域大峡谷内降水，但其上层水汽可作用于高原腹地降水。此外，项目发现喜马拉雅以南的季风水汽对高原北部降水的贡献比西风携带降水更大。项目的实施加深对复杂山区水汽输送过程的理解，同时进一步推动高分辨率气候模拟数据在山地降水研究中的应用。