UHF Radar Detection and Numerical Simulation of an Episode of Foehn and Lee Waves over the Northern Coast of Iberia

Abstract

This work shows an episode of foehn- and lee-wave activity over the sea-facing slopes of the Cantabrian Mountain Range, on the northern coast of Iberia, as detected by a wind profiler radar (WPR?LAP 3000 at 1290 MHz) at Punta Galea-Bilbao. Surface meteorological data from the regional network operated by the Basque Meteorological Service and the National Oceanic and Atmospheric Administration (NOAA) satellite images were also used to document the atmospheric dynamics. The period analyzed?17?18 January 1997?is included within the 1997 European Cooperation in the Field of Scientific and Technical Research (COST) Wind Initiative for a Network Demonstration in Europe (CWINDE-97) project, during which two well-defined severe wind episodes were registered over the Bilbao area as a result of consecutive frontal passages over the northern coast of Iberia. The results of a mesoscale model, running with nested grids down to a resolution of 3 km ? 3 km, were used to understand the output of the profiler. Vertical winds at the WPR site showed a simultaneous pattern of upward and downward motion in the lower and upper layers, respectively, which was in accordance with the output of the model. The lee waves simulated by the high-resolution grid over the WPR site have been shown to be responsible for this pattern of vertical wind velocities. The vertical spatial distribution of dry and moist air, suggested by the signal-to-noise ratio of the radar, showed a ?reversed? pattern of relative humidity distribution in the lower troposphere (higher values at upper levels) coincident with the foehn episode. The lee waves and the dry- and moist-air vertical distribution, which are associated with the strong southwesterly winds preceding the frontal systems over the coastal area, have been found to be responsible for range aliasing in the WPR lower-pulse mode.