Diagnostic Studies of Heavy Convective Rainfall Events in Complex Terrain: A Contribution to the Mesoscale Alpine Program

暴雨和（山洪）洪水往往是由对流系统造成的，这些对流系统要么是准静止的，要么是缓慢移动的，单个单元在同一区域反复跟踪。平流的相互作用（即，细胞运动）和传播（即，新单体的形成）以产生缓慢的净风暴运动，以及由非常潮湿的风暴环境推动的高降水效率，是极端降雨事件的突出特征。 强降雨事件经常发生在复杂地形中，其中风暴动力学与经过地形特征的空气运动相互作用。 观测表明，这种事件往往伴随着强烈的边界层风，将湿空气引向地形障碍。 虽然有一些基本的了解，空气如何移动过去的地形特征和天气尺度的情况下，有利于严重的降水，很少有人知道的精确机制，导致触发，组织，位置，并持续了许多小时的这些类型的降水系统。 对10公里或更小尺度的机制缺乏了解，是因为缺乏这些尺度的详细观测。 从水文学的角度来看，在几分钟和几公里的尺度上评估降雨的时间和空间分布对于评估水文响应至关重要（例如，中尺度阿尔卑斯山方案是一项山区气象学研究倡议，将利用先进的观测技术和数值模式探索复杂地形对天气的影响。 拟议的研究需要对暴雨和山洪暴发事件进行诊断研究，其依据是：（a）在MAP特别观测期对阿尔卑斯山外的观测;（B）在美国落基山脉和阿巴拉契亚地区的Front Range进行的可比观测。将进行分析，以确定从天气尺度到风暴尺度的地形对流中暴雨的关键要素。 通过诊断研究要解决的问题是：（1）阿尔卑斯山南侧地形对流的准定常风暴运动的机制是什么？(2)这些机制与在美国落基山脉和阿巴拉契亚地区的Front Range中观察到的机制有何不同？(3)阿尔卑斯山南侧地形对流导致极端降雨率的微物理过程是什么？(4)这些微物理过程与美国落基山脉和阿巴拉契亚地区的前山脉有何不同？将通过对流开始、风暴平流和风暴传播之间的相互关系来研究山区准定常风暴运动。 山区地形的一个主要困难是，将风暴运动与转向层风和低空急流形态联系起来的标准程序不成立。 风暴运动研究的一个主要目标将是制定评估山区风暴传播的程序。 对于分析MAP SOP事件，多普勒车轮（DOW）可以提供关于风暴传播和低空风场之间关系的特别有用的信息。地形对流极端降雨率的微物理研究将集中在低回波质心（LEC）风暴，如1976年7月的大汤普森风暴和1997年7月的柯林斯堡风暴，以及1995年6月美国阿巴拉契亚地区的拉皮丹风暴。 将特别注意的假设，极端降雨率与积累区，其中一个耦合的暖雨过程和雾凇的结果，在有效的降水生产。 极化雷达观测的MAP SOP和美国暴雨的情况下，将发挥核心作用，分析极端降雨率。

Heavy rainfall and (flash) flooding often result from convective systems that are either quasi-stationary or slowly moving with individual cells repeatedly tracking over the same area. The interplay of advection (i.e., cell motion) and propagation (i.e., new cell formation) to produce a slow net storm motion, together with a high precipitation efficiency that is fueled by a very moist storm environment, are prominent features of extreme rainfall events. Heavy rainfall events frequently occur in complex terrain, where the storm dynamics interact with the air motion past terrain features. Observations indicate that such events are frequently accompanied by strong boundary-layer winds directing moist air towards a topographical barrier. Although there is some basic understanding of how air moves past terrain features and what synoptic-scale situations are favorable for severe precipitation, little is known about the precise mechanisms that lead to the triggering, organization, location, and persistence over many hours of these types of precipitation systems. This lack of understanding of mechanisms at scales of 10km or less has been attributed to a lack of detailed observations at those scales. From a hydrologic perspective, assessment of the temporal and spatial distribution of rainfall at scales of few minutes and kilometers is crucial to evaluate the hydrologic response (e.g., flooding) of precipitating cloud systems at the land surface.The Mesoscale Alpine Program (MAP) is a research initiative in mountain meteorology that will make use of advanced observing technology and numerical models to explore the effects of complex topography on weather. The proposed research entails diagnostic studies of heavy rainfall and flash flooding events based on: (a) Observations on the soutside of the Alps made during the Special Observing Period (S OP) of MAP and (b) comparable observations in the Front Range of the Rocky Mountains and Appalachian region of the United States (US). Analyses will be performed to characterize key elements of heavy rainfall from orographic convection at scales ranging from the synoptic to the storm scale. The questions to be addressed through diagnostic studies are: (1) What are the mechanisms responsible for quasi-stationary storm motion of orographic convection on the south side of the Alps? (2) How do these mechanisms differ from those observed in the Front Range of the Rocky Mountains and Appalachian region of the US? (3) What are the microphysical processes responsible for extreme rainfall rates from orographic convection on the south side of the Alps? (4) How do these microphysical processes differ from those in the Front Range of the Rocky Mountains and Appalachian region of the US? Quasi-stationary storm motion in mountainous regions will be examined through the interrelationships between convective initiation, storm advection and storm propagation. A major difficulty in mountainous terrain is that standard procedures for relating storm motion to steering-level wind and low-level jet configuration do not hold. A principal objective of storm motion studies will be to develop procedures for assessing storm propagation in mountainous terrain. For analyses of MAP SOP events, the Doppler-on-Wheels (DOW) may provide especially useful information on the relationship between storm propagation and the low-level wind field.Microphysical studies of extreme rainfall rates from orographic convection will focus on Low Echo Centroid (LEC) storms, like the Big Thompson (July 1976) and Fort Collins (July 1997) storms in the Front Range of the Rocky Mountains and the Rapidan storm (June 1995) in the Appalachian region of the US. Particular attention will be given to the hypothesis that extreme rainfall rates are linked to accumulation zones, in which a coupling of warm rain processes and riming results in efficient precipitation production. Polarimetric radar observations of MAP SOP and US heavy rain cases will play a central role in analyses of extreme rainfall rates.