Studies of Coastal Orographic Precipitation Processes

地形对降水的强度和空间分布有着深远的影响。一般来说，地形降水的发展有两个关键因素：天气状况或季节，它可以决定稳定降水与对流降水的相对重要性；屏障的宽度，它可以决定地形降水是落在屏障顶部还是落在其迎风或背风斜坡上。当然，这些相对简单的概念并不能阐明地形降水过程中隐含的许多复杂性，例如气流阻塞和云微物理。这项研究通过关注沿海地形降水过程来解决其中的一些复杂性。本研究的具体目标是：1。考察沿海地形如何使撞击的低空气流向上偏转，迫使地形降水发展；研究海岸地形馈电云的结构及其随上坡风速的变化。这些目标将通过使用机载多普勒雷达和现场微物理、地面极化多普勒雷达、剖面雷达、气球探测、地面气象和降水数据来实现，这些数据来自加利福尼亚降落喷气机（CALJET）和太平洋降落喷气机（PACJET）实验，这些实验在20世纪90年代末和21世纪初沿着加利福尼亚北部海岸进行。在过去的四十年里进行了几次野外观测实验，以帮助澄清地形降水过程中尚未解决的细节。这些努力大多集中在相对较大的障碍上，半宽超过50公里，高度超过上游平坦地形2公里。半宽和高度分别小于50公里和2公里的较小屏障也可显著影响降水的发展，特别是那些位于海岸线或海岸线附近、海洋湿气来源不受上游地形阻碍的屏障。事实上，这些地形地区的降水可能导致严重的洪水，造成数亿美元的财产损失。对CALJET和PACJET数据集的分析提供了一个机会来推进对沿海地形降水过程的基本理解。更具体地说，这项研究的重要意义在于，它将更好地阐明影响沿海地形沿线和近海降水分布和强度的强迫因素，并将为沿海地形馈电云中活跃的微物理过程的范围以及它们如何受到强迫因素的影响提供约束。更广泛的影响本研究项目具有各种更广泛的影响。更一般地说，由于增加了对沿海地形降水过程的认识，这些结果将具有社会效益。更好地了解导致海岸地形极端降雨的过程可以帮助改进这类事件的水文预报。这一领域的进展也具有经济和环境意义，因为水量对水管理、农业、水上娱乐、水质和保护濒危物种栖息地（如鲑鱼渔业）有重大影响。与此项目相关的更具体、更广泛的影响将是学生参与研究，并将研究结果应用于课堂教学和非正式科学教育展览的开发。

Orography has a profound influence on the intensity and spatial distribution of precipitation. Broadly speaking, there are two key factors in the development of orographic precipitation: the weather regime or season, which can determine the relative importance of stable versus convective precipitation and the width of the barrier, which can determine whether orographic precipitation falls on the barrier crest or its windward or lee slopes. Of course, these relatively simple concepts do not articulate many of the complexities implicit in orographic precipitation processes, such as airflow blocking and cloud microphysics. This research effort addresses some of these complexities by focusing on coastal orographic precipitation processes. Specific objectives of this research are to:1. Examine how the coastal terrain deflects impinging low-level airflow upward to force the development of orographic precipitation;2. Investigate the structure of coastal orographic feeder clouds and how it varies as a function of upslope wind speed.These objectives will be pursued by testing specific hypotheses using airborne Doppler radar and in-situ microphysics, ground-based polarimetric Doppler radar, profiling radar, balloon sounding and surface meteorology and precipitation data from the California Land-falling Jets (CALJET) and Pacific Landfalling Jets (PACJET) experiments operated along the northern California coast during the late 1990's and early 2000's.Intellectual MeritSeveral observational field experiments have been conducted over the last forty years to help clarify unresolved details about orographic precipitation processes. Most of these efforts have been focused on relatively large barriers, with half-widths exceeding 50 km and heights more than 2 km above upstream flat terrain. Smaller barriers, with half-widths and heights less than 50 km and 2 km, respectively, can also significantly influence the development of precipitation, particularly those on or near a coastline where sources of maritime moisture are unimpeded by upstream topography. In fact, precipitation in these orographic locales can lead to severe flooding that incurs hundreds of millions of dollars in property damage. Analysis of the datasets from CALJET and PACJET provides an opportunity to advance fundamental understanding of coastal orographic precipitation processes. More specifically, this research is important because it will better clarify the forcing factors that influence the distribution and intensity of precipitation along and offshore of coastal orography and it will provide constraints on the range of microphysical processes active in coastal orographic feeder clouds and how they are influenced by forcing factors.Broader ImpactsThis research project has a variety of broader impacts. More generally, the results will have societal benefit owing to the increased knowledge of coastal orographic precipitation processes. A better understanding of the processes that lead to extreme rainfall in coastal orography could help improve hydrologic forecasts of such events. Advancements in this area also have economic and environmental relevance as water quantity has a large impact on water management, agriculture, water-based recreation, water quality and protection of endangered species habitats such as salmon fisheries. More specific broader impacts associated with this project will be student participation in the research and application of the results to classroom instruction and development of exhibits for informal science education.