Vertical velocity variance and skewness in clear and cloud‐topped boundary layers as revealed by Doppler lidar

Abstract

AbstractIn this paper, observations by a ground‐based vertically pointing Doppler lidar and sonic anemometer are used to investigate the diurnal evolution of boundary‐layer turbulence in cloudless, cumulus and stratocumulus conditions. When turbulence is driven primarily by surface heating, such as in cloudless and cumulus‐topped boundary layers, both the vertical velocity variance and skewness follow similar profiles, on average, to previous observational studies of turbulence in convective conditions, with a peak skewness of around 0.8 in the upper third of the mixed layer. When the turbulence is driven primarily by cloud‐top radiative cooling, such as in the presence of nocturnal stratocumulus, it is found that the skewness is inverted in both sign and height: its minimum value of around −0.9 occurs in the lower third of the mixed layer. The profile of variance is consistent with a cloud‐top cooling rate of around 30 W m−2. This is also consistent with the evolution of the thermodynamic profile and the rate of growth of the mixed layer into the stable nocturnal boundary layer from above. In conditions where surface heating occurs simultaneously with cloud‐top cooling, the skewness is found to be useful for diagnosing the source of the turbulence, suggesting that long‐term Doppler lidar observations would be valuable for evaluating boundary‐layer parametrization schemes. Copyright © 2009 Royal Meteorological Society