The turbulence structure in a continental stratocumulus cloud from millimeter-wavelength radar observations

The turbulent-scale vertical velocity structure in a continental stratucumu lus cloud is studied using a 3-mm wavelength Doppler radar operating in a v ertically pointing mode, The radar observations provided 30-m sampling in t he vertical with 2-s averages of 10 000 samples. Vertical velocity measurem ents 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 observationa l period, the cloud layer extended between 200 and 800 m. The vertical velo city variance profiles evolved systematically over the period from a well-d efined peal; in the upper part of the cloud layer of similar to 0.7 m(2) s( -2) to a peak in the lower part of the cloud Layer of 0.2 m(2) s(-2) as the layer became decoupled later in the observing period. The vertical velocit y 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 relative ly narrow penetrating updrafts at this level. The radar vertical velocities are used to compare the directly observed upd raft fractional coverage and mass flux with those obtained from the bulk st atistics, These comparisons are consistent with similar comparisons mode us ing a large eddy simulation model. The Fractional coverage and the mass flu x associated with coherent updraft structures are obtained fur a range of c riteria used to define the updrafts. A more detailed analysis of the vertic al velocities in the cloud confirms the existence of well-defined downdraft s extending through the entire cloud depth. There downdrafts art: estimated to have horizontal dimensions of about 200 m and appear to originate on th e 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 t he vertical structure of coherent eddies.