Evaluation of Mass Flux Representations of Vertical Velocity Fluctuations in Continental Stratus Clouds Using a mm-Wavelength Doppler Radar

A cloud mass flux representation of the vertical turbulent fluxes provides a physical framework for understanding the effects of shallow convection in maintaining the vertical structure of the boundary layer. This approach is based on the assumption that coherent updrafts and downdraft structures are responsible for most of the turbulent transport. Previous evaluations of mass flux parameterizations have been based on aircraft observations (e.g., Penc and Albrecht 1987) and results from Large Eddy Simulations (LESs) (e.g., Siebesma and Cuijpers 1995). In this study, mm-wavelength radar observations of vertical velocities in a continental stratus cloud are used to evaluate assumptions in the mass flux representations of the vertical-velocity variance. As part of this evaluation, direct sampling and conditional sampling techniques are used to define updraft and downdraft structures.