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

暴雨和(暴发)洪水通常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，个别单体在同一地区反复跟踪。平流(即单体运动)和传播(即新单体的形成)的相互作用产生了缓慢的净风暴运动，以及由非常潮湿的风暴环境提供的高降水效率，这是极端降雨事件的显著特征。暴雨过程经常发生在复杂的地形中，风暴动力与经过地形地物的空气运动相互作用。观测表明，这种现象经常伴随着强烈的边界层风，将潮湿的空气引向地形屏障。虽然对空气如何通过地形特征以及哪些天气尺度的情况有利于强降水有一些基本的了解，但对导致这些类型降水系统的触发、组织、位置和持续数小时的确切机制知之甚少。缺乏对10公里或更小尺度的机制的了解，是因为在这些尺度上缺乏详细的观测。从水文学的角度来看，评估降雨在几分钟和几公里尺度上的时空分布对于评估地表降水云系的水文响应(例如洪水)至关重要。中尺度阿尔卑斯计划(MAP)是山地气象学中的一个研究项目，它将利用先进的观测技术和数值模式来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的特别观测期(S OP)期间对阿尔卑斯山以外的地区进行的观测；以及(B)在美国落基山脉和阿巴拉契亚地区(美国)前沿山脉的可比观测。将进行分析，以确定从天气尺度到风暴尺度的地形对流造成的强降雨的主要因素。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风暴准静止运动的机制是什么？(2)这些机制与美国落基山脉和阿巴拉契亚山脉前缘观察到的机制有什么不同？(3)阿尔卑斯山南侧地形对流引起的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前缘区域的微物理过程有什么不同？通过对流产生、风暴平流和风暴传播之间的相互关系来考察山区的准静止风暴运动。山区的一个主要困难是，将风暴运动与转向层风速和低空急流配置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开发评估风暴在山区传播的程序。对于MAP SOP事件的分析，车载多普勒(DOW)可以提供风暴传播和低层风场之间关系的特别有用的信息。地形对流引起的极端降雨率的微观物理研究将集中在低回声质心(LEC)风暴，如落基山脉前缘的大汤普森风暴(1976年7月)和柯林斯堡风暴(1997年7月)，以及美国阿巴拉契亚地区的拉皮丹风暴(1995年6月)。将特别注意极端降雨率与积累区有关的假设，在该假设中，暖雨过程和结冰的耦合导致有效的降水产生。对MAP、SOP和美国暴雨个例的偏振雷达观测将在极端降雨率分析中发挥核心作用。

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.