Nonstationary drainage flows and the valley cold pool

The initial formation of drainage flows and subsequent interaction with the cold pool are examined by contrasting winds measured with 2D sonic anemometers at three stations along a gentle slope for a 45-day observational period in late summer and early fall. On clear nights with weak winds, the station at the bottom of the slope experiences downslope flow only at the beginning of the evening, which quickly yields to light winds of variable direction after formation of a cold pool on the valley floor. A second station, several hundred metres up the slope, experiences drainage flow in the first part of the evening, which diminishes and yields to light and variable winds in the middle of the night as the influence of the cold pool deepens and engulfs the station. Drainage flow continues throughout the night at the third station located still farther up the slope. The interplay between the drainage flow, large-scale flow and submeso motions leads to frequent large shifts of wind direction. The character of the wind-direction variability varies substantially between the three stations. Large remaining uncertainties and needed additional observations are noted. Concepts for more chaotic terrain are discussed.