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

Princeton University, Princeton NJ

Investigators

Abstract

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.