Winds on the slope and in the valley cold pool

The initial formation of drainage flows and subsequent interaction with the cold pool are examined by contrasting winds measured with sonic anemometers at different locations within the cold pool and sidewall slopes. The interplay between the drainage flow, large-scale flow and submeso motions leads to frequent large shifts of wind direction that are poorly represented or completely omitted in dispersion models. The character of the wind-direction variability varies substantially between the stations. Nonstationary submeso motions in the cold pool and their intermittent generation of turbulent diffusion is also examined. The study is then extended to common marginal nights where the cold pool and slope winds only partially form or form for only part of the night. These conditions have not been studied previously and do not yield to simple description. Future modification of air pollution models is discussed.