Airflow and Precipitation Fields within Deep Alpine Valleys Observed by Airborne Doppler Radar

Abstract

Although airborne Doppler radar is increasingly relied upon to provide detailed descriptions of mesoscale precipitation systems in remote and complex meteorological settings, the utility of these observations has often been limited by the considerable difficulty in their manual processing to remove ground clutter and other sources of contamination, which is a prerequisite to synthesis of reliable airflow and reflectivity fields. This difficulty is further magnified over mountainous terrain, where these sources of contamination take on increased spatial extent and geometric complexity. Removal of such contamination has traditionally required tedious and time-consuming manual editing. As such, routine retrieval of near-surface airflow and precipitation characteristics over steep orography and within hydrologically critical zones, such as deep valleys cutting through mountainous regions (along which population and transportation corridors are frequently concentrated), has been impractical. A new approach is described that largely automates this data-editing procedure for airborne radar platforms, achieving reliable elimination of corrupted data with minimal loss of meteorological signal. Subjective decisions are minimized through a judicious combination of data renavigation, pattern recognition, and reliance upon high-resolution digital terrain information. This technique is applied to data obtained over the Alps by the NCAR Electra and NOAA P-3 aircraft during the recent Mesoscale Alpine Programme field campaign. Three-dimensional airflow and reflectivity fields are shown to illustrate the power and fidelity of this new approach by capitalizing on data collected near, and even beneath, the aircraft track to provide a unique and highly illuminating description of airflow deep within Alpine river valleys and their tributaries during two contrasting orographic precipitation events. The validity of these results is explored through quantitative comparison of this output with independent kinematic measures obtained from ground-based Doppler radar. The utility of airborne radar to provide comprehensive and near-simultaneous views reaching into multiple valleys hidden from the view of ground-based radars is highlighted for a notable case of ?down valley? flow, more comprehensively illustrating the nature and extent of low-level upstream blocking during a widespread orographic precipitation event.