Orographic Precipitation Processes in Midlatitudes and Tropics

当斜压降水系统经过山脉时，以及当深对流发生在地形改变的流动区域时，地形强烈影响降水。 本研究探讨这两种类型的地形降水的重点是：（1）物理机制，斜压风暴系统的广泛降水是增强的迎风坡上的中纬度山脉障碍;（2）影响的因素，在热带山脉附近的强降水对流系统的位置，强度和结构。对于中纬度斜压系统，首席研究员（PI）将测试来自中尺度阿尔卑斯山方案的假设（MAP，以阿尔卑斯山为基地）和通过观测验证试验改进微物理参数化（IMPROVE，总部位于俄勒冈州），迎风坡增强可能发生在：1）当接近中性或轻微静力不稳定的强低空气流在地形上容易上升，从而在地形的第一次急剧上升上形成单体时，（2）当弱的低空气流被一层剪切和翻转单元包围时。 在这两种情况下，细胞导致降水的地形较低的斜坡上的增强。PI将进一步检查MAP和IMPROVE数据，以更好地了解这些过程，并将调查范围扩大到包括来自北方加州水文气象试验台（HMT）的数据集，这将有助于确定MAP和IMPROVE II中确定的过程对美国西海岸风暴的更广泛适用性。对于热带对流系统，PI将继续探索喜马拉雅地区的对流，该地区具有高山，明确的上游水分来源和不同类型的地表特性的强大组合。在对已编制的卫星数据库和高分辨率模型输出的初步研究中，PI发现西北次大陆的极深对流是由来自高原的气流控制的，覆盖着低空潮湿气流。低层气流在干燥炎热的陆地表面上以长的收敛轨迹被山障的形状引导，对流在地形的第一个尖峰处释放，因此强降水对流系统发生在障的底部附近。当大型海上对流系统上岸的潮湿的河流三角洲的东北次大陆，它出现的层状组成部分，这些系统的地形上的迎风坡和前面的地形地形隆起增强。这项研究将扩大卫星数据库，以确定这些过程如何解释喜马拉雅山附近的气候降水模式，以及它们如何在日和季节上变化。在该项目的后半部分，PI可能会为安第斯山脉开发一个卫星数据库，以测试从喜马拉雅山脉得出的概念的普遍性。在这项研究的中纬度和热带阶段，PI与运行高分辨率预报模型的小组建立了合作关系。上述研究将使用这些模型的输出来测试和扩展从实地项目和卫星数据集获得的物理理解。PI还将评估模型在代表中纬度和热带地区地形降水过程方面的准确性。学术价值：这项研究建立在PI先前在降水云的动力学和物理学方面的专业知识基础上。它将有助于填补大气科学中关于地形对降水系统影响的知识空白。更广泛的影响：这项研究将有助于实现改善山区强降水和洪水预测的社会目标。

Orography strongly affects precipitation when baroclinic precipitation systems pass over mountain ranges and when deep convection occurs in regions of terrain modified flow. This research investigates both of these types of orographic precipitation by focusing on: (1) Physical mechanisms by which the widespread precipitation of a baroclinic storm system is enhanced on the windward slope of a midlatitude mountain barrier; (2) Factors affecting the location, intensity, and structure of heavily precipitating convective systems in the vicinity of tropical mountain ranges. For midlatitude baroclinic systems, the Principal Investigator (PI) will test hypotheses, derived from the Mesoscale Alpine Programme (MAP, based in the Alps) and Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE, based in Oregon), that windward slope enhancement can occur either: 1) when strong low level flow of near neutral or slight static instability rises easily over orography such that cells form over the first sharp rise of terrain, or 2) when weak low-level flow is bounded above by a layer of shear and overturning cells. In either case, the cellularity leads to enhancement of precipitation over the lower slope of the terrain. The PI will further examine the MAP and IMPROVE data to better understand these processes, and expand the investigation to include a dataset from the Hydrometeorological Testbed (HMT) in northern California, which will help determine the broader applicability of the processes identified in MAP and IMPROVE II to west coast U.S. storms in general. For tropical convective systems, the PI will continue to explore convection in the Himalayan region, which has a robust combination of high mountains, well-defined upstream moisture sources, and different types of land surface properties. In preliminary studies of a satellite database that have been compiled and high-resolution model output, the PI has found that extremely deep convection in the northwestern subcontinent is controlled by flow down from high plateaus overriding low-level moist flow. The low-level flow is channeled in long converging trajectories over dry, hot land surfaces by the shape of the mountain barrier, and convection is released at the first sharp peak of terrain, so that the heavily precipitating convective systems occur near the base of the barrier. When large maritime convective systems come ashore over the wet river delta of the northeastern subcontinent, it appears that the stratiform components of these systems are enhanced by orographic uplift on and ahead of the windward slopes of the terrain. The research will broaden the satellite database to determine how these processes account for the climatological precipitation pattern near the Himalayas and how they vary diurnally and seasonally. In the latter half of the project the PI may develop a satellite data base for the Andes to test the generality of concepts derived from the Himalayas. In both the midlatitude and tropical phases of this research the PI has established collaborations with groups running high-resolution forecast models. The above described research will use output from these models to test and extend the physical understanding gained from the field project and satellite datasets. The PI also will be evaluating the models' accuracy in representing the orographic precipitation processes in both midlatitudes and tropics. Intellectual merit: This research builds on the PI's previously established expertise in the dynamics and physics of precipitating clouds. It will help fill a knowledge gap in atmospheric sciences regarding orographic effects on precipitating systems. Broader impacts: This study will contribute to the societal goal of improving predictions of heavy precipitation and flooding in mountainous regions.