Studies of Coastal Orographic Precipitation Processes

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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.