Small-Scale Variability in the Nocturnal Boundary Layer

Nocturnal variations of temperature and wind are examined at three contrasting sites. After the early evening period of rapid cooling, the magnitude of the variations of temperature on a time scale of 10 min to an hour often become larger than the corresponding temperature change due to the nocturnal trend. These shorter-term temperature variations are forced by wave-like motions and more complex modes. Observations from a network of stations across a shallow valley at one of the sites are analyzed in more detail. Typically, decreasing wind speed corresponds to less mixing and lower temperature at the surface followed by increasing wind speed, increased mixing, and higher temperatures. The flow may continue to switch back and forth between these two states for much of the night. These non-stationary motions interact with motions induced by the gentle local topography, leading to intermittent local drainage flows, transient cold pools, and both propagating and semi-stationary microfronts.