Studies of Orographic Precipitation in Cool and Warm Climates

本研究探讨了在不同稳定性、风暴类型和地形条件下，具有代表性的各种风暴中地形增强降水的物理机制。该研究将侧重于了解促进地形降水增长和沉降的动力和微物理过程，从而为理解降水增长和沉降的时间尺度和参数化提供物理基础。本研究将利用来自中尺度高山计划（MAP）和通过观测验证实验（IMPROVE）改进的微物理参数化的数据，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。在MAP和IMPROVE的中纬度气旋体系中，首席研究员将对比风暴锋面阶段稳定到接近中性条件下的地形降水过程和风暴锋面后期中等不稳定条件下的地形降水过程。后者是美国西部山区重要的降水产生阶段，但以前在文献中被忽视。迄今为止，MAP和IMPROVE的研究只集中在这些风暴的锋面阶段。PI将扩展这些研究，包括额后对流阶段。MAP和IMPROVE显示了聚结和边缘对缩短降水增长和沉降的时间尺度的重要性，潮湿的几乎中性的无阻塞流通过有利于地形的快速上升来促进这些过程的重要性，以及嵌入的垂直运动细胞（由于浮力和/或切变）在增强微物理过程中的作用。在风暴的锋后阶段，将研究并结和边缘的关键生长机制，以及它们如何受到锋后对流垂直运动的影响。该研究将评估模型在风暴锋面和锋后阶段与MAP和IMPROVE收集的飞机和雷达数据相关的小尺度动力学和微物理过程的表现。地形降水机制的分析将扩展到两个重要的较暖地形。首先，首席研究员将研究热带气旋与山脉的平衡环流的相互作用。当温暖的有组织的气旋环流遇到地形障碍时，经过崎岖地形的热带气旋会产生地球上最具破坏性的洪水。作为该制度的代表，首席研究员将研究台风娜莉（2001）通过的模型模拟，这代表了台湾的破坏性气旋。模拟的风暴遇到了海面上突然升起的一个巨大的二维山脉。初步研究表明，当眼壁涡旋环流越过山障时，形成山波环流并阻断眼壁垂直运动。山波似乎暂时将强烈的上升运动和降水锁定在地形上。低层合并和高空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上。本研究将探讨模式结果中热带风暴/山波相互作用的假设，并进行台风从不同方向接近障壁的实验。要研究的第二个暖态将是一个非常不稳定的流与一个巨大的山障壁相互作用。这种机制在美国的大汤普森和布莱克山洪水以及2004年印度的洪水等主要洪水的产生中很有名。首席研究员将通过研究喜马拉雅山上游和上空的对流来研究非常温暖、高度不稳定的地形。这个地区是理想的，因为持续的潮湿气流产生了许多深刻而强烈的对流。热带降雨测量任务（TRMM）卫星数据将检查该地区对流的垂直结构，并将其与喜马拉雅地区的模式模拟进行比较，以确定低空合并和高空边缘是如何缩短这种高度对流环境中降水增长和沉降的时间尺度的。对TRMM数据的初步分析表明，边缘非常强大，以至于霰粒子在高达17公里的高度产生高反射率。拟议活动产生的更广泛影响：这项研究将有助于改善山区强降水和洪水预测的社会目标。

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.