Proceedings of the 10th Conference on Mountain Meteorology 17-21 June 2002, Park City, Utah, pp. 360-363 American Meteorological Society 360 P3.14 DYNAMIC AIRFLOW CHANNELLING OVER THE SNAKE RIVER PLAIN, IDAHO M. Kossmann 1 , C. D. Whiteman 2 , and X. Bian 2 1 University of Canterbury, Department of Geography, Christchurch, New Zealand 2 Pacific Northwest National Laboratory, P.O. Box 999, Richland WA 99352, USA 1. INTRODUCTION Surface and upper air meteorological data gathered by the MesoWest cooperative networks (Horel et al. 2002) for the period 1 June 1997 - 31 May 2001 are used to study the effects of dynamic channelling of airflow over the Snake River Plain (SRP) in Idaho, USA. Previous wind studies over the SRP either focused only on the eastern SRP (e.g. Clawson et al. 1989) or dealt with thermally driven local winds (Stewart et al. 2002). The SRP is a curved valley with a broad, u-shaped cross section. Mountain ranges bounding the valley to the north and south reach up to about 3000 m and 2100 m MSL, respectively. The ranges to the south are broken by several gaps of substantial size, which may lead to disturbances in airflow channelling along the SRP by dynamically generated cross-valley winds. On average, the valley floor height decreases from about 1500 m in the east to about 900 m MSL in the west. However, local differences in the valley floor height on the order of several hundred meters are associated with smaller scale terrain irregularities within the valley, particularly in the eastern SRP. 2. AIRFLOW CHANNELLING IN VALLEYS Previous channelling studies showed that in long and broad valleys so-called pressure driven channelling (PDF) is the dominant airflow channelling process (Fiedler 1983, Wippermann 1984, Whiteman and Doran 1993). This means that the wind direction in the valley is determined by the along-valley component of the synoptic or regional scale horizontal pressure gradient, with the air in the valley flowing from high to low pressure (Figure 1). In contrast, a study published by Weber and Kaufmann (1998) suggests that in short and narrow valleys and over mountain saddles, the pure deflection of winds by the mountain ranges, i.e. so- called forced channelling (FC), determines the direction of airflow channelled along the valley’s axis (Figure 1). In medium-sized valleys both PDF and FC might be important (Smedman et al. 1997), with wind speeds expected to be high for synoptic wind directions for which PDF and FC act in the same along-valley direction and wind speeds being low for the synoptic wind directions for which the two channelling mechan- isms act in opposite directions (Figure 2). ____________________________________________ Corresponding author address: Meinolf Kossmann, University of Canterbury, Department of Geography, PB 4800, Christchurch, New Zealand. E-mail: meinolf. kossmann@geog.canterbury.ac.nz Figure 1. Pressure driven channelling (top) and forced channelling (bottom) in an east-west oriented valley in the Northern Hemisphere for ridge-level geostrophic wind directions from southwest, northwest, northeast, and southeast. Dashed lines indicate the valley sidewalls and solid lines are isobars with pi+1 > pi . VG is the geostrophic wind vector and V0 is the surface wind vector in the valley. With significant small scale terrain features existing in the long and broad SRP, the scale dependency of channelling mechanisms may result in complex interactions of channelled airflow, similar to Ekman’s (1998) findings for small scale corrugations on the valley floor of the long and broad Tennessee Valley. 3. DATA PROCESSING For our analyses, 700 hPa rawinsonde observations from Boise, located in the western SRP, are assumed to be representative of the geostrophic wind directions along the entire SRP at ridge height (Figures 3 and 4). Rawinsonde data are available twice daily at Boise at 00 and 12 UTC (i.e. 17 and 05 MST,