Radar Observations of Updrafts, Downdrafts, and Turbulence in Fair-Weather Cumuli

Abstract

Observations from a 94-GHz radar are used to define the vertical structure of marine fair-weather cumuli. Doppler spectra obtained from the radar provide mean vertical velocities as well as detailed spectral shapes that can be used to infer small-scale vertical velocity shear, illuminate cloud microphysical processes, and provide estimates of turbulence dissipation rates. These new observations facilitate the analysis and understanding of in-cloud circulations and the physical processes involved, since the cloud boundaries and dimensions are mapped along with the internal structure of the clouds. Coincident observations from a 915-MHz radar (wind profiler) were used to further define the turbulence structure in and around the clouds. The observations document the detailed vertical and horizontal dimensions of updraft and downdraft circulations in the clouds observed. The two cumuli studied in detail have similar circulation patterns?an updraft core surrounded by downdrafts. Although the clouds have a horizontal depth of only about 700 m, updraft velocities of about 5.5 m s?1 were observed. These updrafts, which are only about 400 m across, exhibit characteristics that are consistent with adiabatic ascent, and penetrate about 150 m into the capping inversion. No penetrating downdrafts are observed within the updraft cores. The downdrafts that flank the updraft on the downwind side of the cloud are relatively narrow (less than 100 m) and extend from cloud top to cloud base. The downdraft on the upwind side of the cloud is about 150 m across and penetrates about 200 m into the detraining cloud mass observed in this part of the cloud. This downdraft appears to be driven by the cooling associated with entrainment mixing at cloud top penetrating through detraining, dynamically inactive parts of the cloud matter. Analysis of the Doppler spectrum at the updraft?downdraft interfaces indicates large Doppler spectral widths due to turbulence and sharp shear zones in the radar sampling volume. Large Doppler spectral widths in the detraining upwind part of the cloud are consistent with the presence of larger droplets. The updraft core structure in one of the clouds has a structure that is consistent with the idea that cumulus clouds are composed of successive bubbles that emerge from the subcloud layer. Thus these small cumuli should be considered as convective complexes rather than simple growing elements that later decay into passive clouds. This study illustrates the potential of millimeter-wavelength radars for studying small cumuli.