The Turbulence Structure in a Continental Stratocumulus Cloud from Millimeter-Wavelength Radar Observations

Abstract

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 sampling in the vertical with 2-s averages of 10?000 samples. Vertical velocity measurements were made continuously for an 8-h period and were further supported by measurements of cloud-base height from a laser ceilometer and liquid water path from a microwave radiometer. During the beginning of the observational period, the cloud layer extended between 200 and 800 m. The vertical velocity variance profiles evolved systematically over the period from a well-defined peak in the upper part of the cloud layer of ?0.7 m2 s?2 to a peak in the lower part of the cloud layer of 0.2 m2 s?2 as the layer became decoupled later in the observing period. The vertical velocity skewness during the well-coupled conditions was negative through most of the cloud, consistent with the 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. The radar vertical velocities are used to compare the directly observed updraft fractional coverage and mass flux with those obtained from the bulk statistics. These comparisons are consistent with similar comparisons made using a large eddy simulation model. The fractional coverage and the mass flux associated with coherent updraft structures are obtained for a range of criteria used to define the updrafts. A more detailed analysis of the vertical velocities in the cloud 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 at cloud top in the downdrafts is consistent with the entrainment of drier air. This study further illustrates the utility of millimeter-wavelength radars for studying turbulence in boundary layer clouds and particularly in defining the vertical structure of coherent eddies.