Turbulence in the nocturnal boundary layer with light and variable winds

Turbulence quantities are analyzed from nine months of data collected on the floor of a valley where nocturnal cold pools are frequent. Often the speed of the vector-averaged wind over the night is less than 0.5 m s ^-1 (near-calm winds) with no preference for wind direction. The quantity of such near-calm data is sufficiently large to allow new types of analyses as well as the opportunity to examine statistics of the previously ignored near-neutral subclass of near-calm winds. For near-calm weak winds and strong stratification, the turbulence can be difficult to isolate from wave-like motions and more complex small-scale structures. The smallest scale perturbations on time-scales less than 5 s behave most like turbulence. Larger-scale perturbations are characterized by very weak vertical velocity fluctuations and large temperature fluctuations, but lead to systematic downward heat flux after extensive averaging. For near-calm nocturnal conditions, significant turbulence is mainly generated by short-term (minutes) accelerations of unknown origin. The turbulence between such infrequent mixing events is extremely weak, but not zero. While the turbulence in the events approximately follows similarity theory, the extremely weak turbulence scales neither with stratification nor bulk shear, and its inferred vertical length-scale is small compared to the distance from the ground, even at the 2 m level.