Orographic Effects on Precipitating Cloud Systems

对地形降水过程的广泛了解对暴雨、洪水和恶劣天气的预报至关重要。地球上的大部分降水与锋面系统、对流风暴或热带气旋有关。当风暴发生在山脉附近或上方时，风暴的降水被增强、重新分配和集中。这项研究试图增进对所有三种类型风暴的这些影响的理解。研究工作将基于来自几个实地活动的现有但尚未充分利用的数据集。用几个最新的冰相微物理方案进行的高分辨率模型模拟将补充现场观测研究。这项研究利用合作来促进对不同山脉(阿尔卑斯山、喀斯喀特山脉、加拿大海岸山脉、内华达山脉、安第斯山脉、喜马拉雅山脉和台湾中央山脉)附近的一系列风暴类型的检查。对于经过山脉的锋面系统，这项研究将检验地图建议的降水增强假说，并改进II现场计划，并确定这些想法是否适用于不同山脉附近的风暴。特别是，来自Snow-V10的数据将被用来确定加拿大西海岸山区中解耦流动、伴随切变和下谷流动的存在和特征。比较阿尔卑斯山、海岸山脉和内华达山脉的数据将被用来确定这些层中的湍流是否具有特定形式的开尔文-亥姆霍兹波。通过对接近安第斯山脉的锋面的模型模拟，将探索障碍射流形成的解耦层的发展。在对流系统方面，我们将研究低层气流冲击山区和中尺度对流系统增强通过大山所导致的强对流系统的触发和下游发展。将通过模型模拟和安第斯地区的数据分析来研究触发机制。模拟将检验这样一种假设，即地形下沉通过覆盖潮湿的低空湿急流，直到急流通过撞击山脚而打破盖子，从而阻止了深层对流。该模式将考察造山运动对流向中尺度系统的下游演化。通过模拟2010年喜马拉雅地区的Leh洪水个例，将检验穿越陡峭地形的中尺度对流系统的增强。至于热带气旋，则会分析台风莫拉克(Morakot)(2009年)的雷达资料。莫拉克台风在地形上增强时，会造成毁灭性的洪灾。我们将探索暖雨增长对增强至关重要的假设，模式模拟将检验结果。智力优势：这项研究的结果将增进对山脉在影响全球所有主要降水系统中所起作用的基本理解。这项研究将使用现有的但尚未开发的野外活动数据集和最新版本的高分辨率数值预报模式来检查所有主要风暴类型(锋面、对流风暴和热带气旋)在靠近山脉或越过山脉时发生的降水过程。将所有主要风暴类型包括在内，并对地理位置不同的山脉进行检查，将实现一般性。更广泛的影响：改进对山脉如何影响降水的观察和理解，将有助于改进对严重洪水、冰雹、龙卷风和大风的预测。本次研究中调查的风暴已造成数百人伤亡和巨大的财产损失，研究将纳入对其人类影响的调查。

A broad understanding of orographic precipitation processes is vital to the forecasting of heavy rain, floods, and severe weather. Most precipitation on earth occurs in connection with frontal systems, convective storms, or tropical cyclones. The precipitation of these storms is enhanced, redistributed and concentrated when the storms occur near or over mountains. This study tries to advance understanding of these effects, for all three types of storms. The research effort will be based on existing but not fully exploited datasets from several field campaigns. High-resolution model simulations with several of the latest ice-phase microphysical schemes will supplement the field observation studies. The study leverages collaborations to facilitate examination of a range of storm types near different mountain ranges (Alps, Cascades, Canadian Coast Mts., Sierra Nevada, Andes, Himalayas, and Taiwan Central Mts.).For frontal systems passing over mountain ranges, this research will test hypotheses of precipitation enhancement suggested by the MAP and IMPROVE II field programs and determine if these ideas have applicability to storms in the vicinities of diverse mountain ranges. In particular, data from SNOW-V10 will be used to determine the existence and characteristics of decoupled flows, accompanying shear, and down-valley flows in the mountain setting of the Canadian western coast. Comparison of data from Alps, Coast Mountains, and Sierra Nevada range will be used to determine whether the turbulence in these layers has the specific form of Kelvin-Helmholtz waves. The development of decoupled layers by barrier jet formation will be explored by model simulation of fronts approaching the Andes. For convective systems, we will examine the triggering and downstream development of intense convective systems as a result of the impingement of low-level flow on mountains and the enhancement of mesoscale convective systems passing over large mountains. The triggering mechanism will be examined with model simulations and data analysis in the Andean region. The modeling will test the hypothesis that orographic subsidence holds back the deep convection by capping of the moist low-level moist jet until the jet breaks the cap by striking a foothill. Downstream evolution of the orogenic convection into a mesoscale system will be examined with the model. The enhancement of a mesoscale convective system passing over steep terrain will be examined by simulating the 2010 Leh flood case over the Himalayas. The precipitation mechanisms in the orographically triggered and modified convective systems will be examined using radar data from operational and research radars in Taiwan.For tropical cyclones, Taiwan radar data will be analyzed for Typhoon Morakot (2009), which caused devastating flooding when its rain was orographically enhanced. We will explore the hypotheses that warm-rain growth was crucial to the enhancement, and model simulations will test the results.Intellectual merit: The results of this study will improve basic understanding of the role of mountains in affecting all major precipitation systems across the globe. The research will use existing but unexploited field campaign datasets and the latest versions of high-resolution numerical forecast models to examine the precipitation processes occurring in all major storm types (fronts, convective storms, and tropical cyclones) when they occur near or over mountains. Inclusion of all major storm types and examination over geographically separated mountain ranges will achieve generality.Broader impacts: Improved observations and understanding of how mountains affect precipitation will lead to improved prediction of severe flooding, hail, tornadoes, and winds. The storms investigated in this study have caused hundreds of casualties and great property damage, and the research will incorporate investigation of their human impacts.