The turbulent-scale vertical velocity structure in a continental
stratocumulus cloud is studied using a 3-mm wavelength Doppler radar
operating in a vertically pointing mode. The radar observations provided 30 m resolution in the vertical with 2-second averages of 10,000 samples. Radar observations were made continuously for an 8-hour period and were further supported by measurements from a laser ceilometer and a microwave radiometer. Vertical-velocity perturbations in the cloud were obtained as the difference between individual 2-second values and a 60-minute averages extending over the entire observational period. During the beginning of the
observational period the cloud layer extended between 200 to 800 m. The vertical velocity variance profile at this time had a well-defined peak in the upper-part of the cloud layer of ~ 0.7 (m/s)2.
The variance profiles for each successive 1-hour period show an orderly evolution as the boundary layer becomes decoupled later in the day. The magnitude of the peak near cloud top decreases and a second peak in the variance is observed in the lower part of the cloud layer. Maximum variance at this time is reduced to 0.2 (m/s)2. The vertical velocity skewness during the well-coupled conditions is negative-- consistent with presence of relatively narrow downdrafts. A positive skewness in the top 100 m of the cloud is consistent with relatively narrow penetrating updrafts at this level. A more detailed
analysis of the vertical velocity perturbations confirms the existence of well-defined downdrafts extending through the entire cloud depth.
These downdrafts are estimated to have horizontal dimensions of about 200 m and appear to originate on the downshear side of updrafts. The reduction of radar reflectivity in the downdrafts is consistent with the entrainment of drier air at cloud top. This study further illustrates the utility of mm-wavelength radars for studying turbulence in boundary layer clouds and particularly in defining the vertical structure of coherent eddies