Studies of Orographic Precipitation in Cool and Warm Climates

本研究探讨在不同的稳定度、风暴类型和地形条件下，地形增强降水的物理机制。 这项研究将侧重于了解有助于地形降水增长和沉降的动力学和微物理过程，从而为了解降水增长和沉降的时间尺度并将其参数化奠定物理基础。 这项研究将利用中尺度高山方案和通过观测验证实验改进微物理参数化实地研究的数据，充分利用这些数据集，并将分析范围扩大到较温暖和较不稳定的地形降水情况。在MAP和IMPROVE的中纬度气旋状态中，首席研究员将对风暴锋相期间稳定至接近中性条件下的地形降水过程与风暴锋后阶段的中度不稳定条件进行对比。 后者是美国西部山区的一个重要降水阶段，但以前在文献中被忽视。 迄今为止，对MAP和IMPROVE的研究仅集中在这些风暴的锋面阶段。 PI将扩展这些研究，以包括锋后对流阶段。 MAP和IMPROVE表明了聚结和雾凇对缩短降水增长和沉降的时间尺度的重要性，潮湿的接近中性的畅通气流对促进这些过程的重要性，因为它有利于在地形上快速上升，以及嵌入的垂直运动单元（由于浮力和/或切变）在增强微物理过程中的作用。 在锋面后阶段的风暴的聚结和雾凇的关键增长机制将检查有关它们是如何受到影响的锋面后对流的垂直运动。 该研究将评估模型在多大程度上代表了与MAP和IMPROVE中收集的飞机和雷达数据有关的小尺度动力学和微物理过程，包括风暴的锋面和锋面后阶段。地形降水机制的分析将扩展到两个重要的温暖的地形制度。 首先，首席研究员将研究热带气旋的平衡环流与山脉的相互作用。 当热带气旋经过崎岖的地形时，当温暖的有组织的气旋环流遇到地形障碍时，就会产生地球上最具破坏性的洪水。 作为这一制度的代表，首席研究员将检查台风纳莉（2001年），这是一个破坏性的气旋在台湾通过的模型模拟。 模拟的风暴遇到了一个巨大的二维山脉突然从海上升起。 初步研究表明，当眼壁涡环流越过山障时，形成山波环流，中断眼壁垂直运动。 山波似乎暂时将强烈的上升运动和降水锁定在地形上。 低层的合并和高空的霰的形成都缩短了降水增长的时间尺度，使降水集中在迎风坡。 拟议的研究将探讨这一假设的热带风暴/山波相互作用的模式结果，并进行实验与台风接近障碍物从不同的方向。第二个温暖的制度将被审查是一个非常不稳定的流动与一个大的山脉障碍。 这种类型的制度是值得注意的生产重大洪水，如大汤普森和黑山洪水在美国和最近的2004年洪水在印度。 首席研究员将通过检查喜马拉雅山上游和上方的对流来解决非常温暖，高度不稳定的地形状况。 这个区域是理想的，因为持续的潮湿气流产生了许多深而强烈的对流。 将利用热带降雨测量使命卫星数据研究该区域对流的垂直结构，并将其与喜马拉雅地区的模型模拟进行比较，以确定低空聚合和高空成云是如何缩短这种强对流环境中降水增长和沉降的时间尺度的。 TRMM数据的初步分析表明，雾凇是如此强大，在高达17公里的高度产生高的反射率的粒子。拟议活动产生的更广泛影响：这项研究将有助于实现改善山区强降水和洪水预报的社会目标。

This study investigates the physical mechanisms of orographic enhancement of precipitation in a representative variety of storms, under different regimes of stability, type of storm, and topography. The study will focus on understanding the dynamical and microphysical processes contributing to growth and fallout of orographic precipitation, and hence to a physical basis for understanding and parameterizing the time scales of precipitation growth and fallout. This study will utilize data from the Mesoscale Alpine Programme (MAP) and Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) field studies of midlatitude cyclones moving over mountains by fully exploiting those datasets and by extending the analysis to warmer and more unstable orographic precipitation regimes. In the midlatitude cyclone regime of MAP and IMPROVE, the Principal Investigator will contrast the orographic precipitation processes in the stable to nearly neutral conditions during the frontal phase of the storm to the moderately unstable conditions in the postfrontal phase of the storm. The latter is an important precipitation producing stage over the western U.S. mountains but has been previously neglected in the literature. Studies of MAP and IMPROVE to date have focused only on the frontal phase of these storms. The PI will extend these studies to include the postfrontal convective stage. MAP and IMPROVE have shown the importance of coalescence and riming to shorten the time scales of growth and fallout of precipitation, the importance of moist nearly neutral unblocked flow in promoting these processes by favoring rapid ascent over the terrain, and the role embedded vertical-motion cells (owing to buoyancy and/or shear) in enhancing the microphysical processes. In the postfrontal phase of the storm the key growth mechanisms of coalescence and riming will be examined in relation to how they are affected by the vertical motions in postfrontal convection. The study will evaluate how well models represent the small-scale dynamical and microphysical processes in relation to aircraft and radar data collected in MAP and IMPROVE in both the frontal and post frontal phases of storms. The analysis of orographic precipitation mechanisms will be extended to two important warmer orographic regimes. First, the Principal Investigator will examine the interaction of the balanced circulation of a tropical cyclone with a mountain range. Tropical cyclones passing over rugged terrain produce some of the most devastating flooding on earth when the warm organized cyclonic circulation encounters a topographic barrier. As a representative of this regime, the Principal Investigator will examine a model simulation of the passage of Typhoon Nari (2001), which represents a devastating cyclone over Taiwan. The simulated storm encounters a large 2D mountain range rising abruptly out of the sea. Preliminary investigation suggests that when the eyewall vortex circulation extends across the mountain barrier, a mountain wave circulation forms and interrupts the eyewall vertical motion. The mountain wave appears to temporarily lock the strong upward motion and precipitation to the terrain. Both coalescence at low levels and graupel formation aloft appear to shorten the time scales of the precipitation growth and concentrate the rain on the windward slope. The proposed study will explore this hypothesis of tropical storm/mountain wave interaction in the model results and perform experiments with the typhoon approaching the barrier from different directions. The second warm regime to be examined will be a very highly unstable flow interacting with a large mountain barrier. This type of regime is notable for producing major floods such as the Big Thompson and Black Hills floods in the U.S. and the recent 2004 floods in India. The Principal Investigator will address the very warm, highly unstable orographic regime by examining convection upstream of and over the Himalayas. This region is ideal because a persistent moist flow produces many realizations of deep, intense convection. The vertical structure of the convection in this region will be examined in Tropical Rainfall Measuring Mission (TRMM) satellite data and compared to model simulations over the Himalayan region to determine how low-altitude coalescence and high-altitude riming, both of which shorten the time scales of the growth and fallout of precipitation in this highly convective environment. Preliminary analysis of the TRMM data suggests that the riming is so robust that graupel particles produce high reflectivity at altitudes up to 17 km. Broader impacts resulting from the proposed activity: This study will contribute to the societal goal of improving predictions of heavy precipitation and flooding over mountainous regions.