Studies of Orographic Precipitation in Cool and Warm Climates

这项研究探讨了在不同的稳定度、风暴类型和地形条件下，典型风暴的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落下的动力和微物理过程，从而为理解和参数化降水增长和落下的时间尺度奠定物理基础。这项研究将利用中尺度阿尔卑斯方案(MAP)的数据和通过观测验证实验改进微物理参数化，通过充分利用这些数据集并将分析扩展到更温暖和更不稳定的地形降水制度，对中纬度气旋在山区移动的实地研究进行改进。在中纬度气旋MAP和ENVELINE中，首席调查员将把风暴锋面阶段稳定到近中性条件下的地形降水过程与风暴锋后阶段中等不稳定条件下的地形降水过程进行对比。后者是美国西部山区重要的降水产生阶段，但在以往的文献中一直被忽视。到目前为止，对MAP和改进的研究仅集中在这些风暴的锋面阶段。PI将把这些研究扩展到包括锋后对流阶段。MAP和ENVANCED显示了合并和翻转对于缩短降水增长和落下的时间尺度的重要性，潮湿的近中性畅通气流通过有利于地形上的快速上升而促进这些过程的重要性，以及嵌入的垂直运动单元(由于浮力和/或切变)在加强微物理过程中的作用。在风暴的锋后阶段，我们将研究与它们如何受锋后对流中的垂直运动影响有关的合并和翻边的关键增长机制。这项研究将评估模型与MAP中收集的飞机和雷达数据有关的小尺度动力和微物理过程的表现情况，并改进风暴的锋面阶段和锋面后阶段。对地形降水机制的分析将扩展到两个重要的变暖地形区域。首先，首席调查员将研究热带气旋的平衡环流与山脉的相互作用。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屏障时，会产生一些地球上最具破坏性的洪水。作为这一制度的代表，首席调查员将研究台风纳丽(2001)过境的模型模拟，这代表了台湾上空的毁灭性气旋。模拟的风暴遇到了一个突然从海面上冒出来的大2D山脉。初步研究表明，当眼壁涡旋环流穿过山体屏障时，形成山波环流，中断眼壁垂直运动。山浪似乎暂时将强烈的上升运动和降水锁定在地形上。低层汇合和高空碎屑形成似乎都缩短了降水增长的时间尺度，使降水集中在迎风坡上。拟议的研究将在模式结果中探索热带风暴/山波相互作用的假设，并在台风从不同方向接近屏障的情况下进行实验。要研究的第二个暖流将是一种非常不稳定的气流，它与一个巨大的山脉屏障相互作用。这种类型的制度以产生大洪水而闻名，例如美国的大汤普森和黑山洪水，以及最近2004年印度的洪水。首席调查员将通过研究喜马拉雅山脉上游和上方的对流来解决非常温暖、极不稳定的地形状况。这一区域是理想的，因为持续的潮湿气流会产生许多深而强烈的对流。将在热带降雨测量任务卫星数据中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相比较，以确定低海拔合并和高海拔边缘运动是如何在这一高度对流环境中缩短降水增长和落下的时间尺度的。对TRMM数据的初步分析表明，边缘是如此坚固，以至于海鸥粒子在高达17公里的高度产生高反射率。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.