Observations from a 94-GHz radar are used to define the vertical structure
of marine fair-weather cumuli. Doppler spectra obtained from the radar prov
ide mean vertical velocities as well as detailed spectral shapes that can b
e used to infer small-scale vertical velocity shear, illuminate cloud micro
physical processes, and provide estimates of turbulence dissipation rates.
These new observations facilitate the analysis and understanding of in-clou
d circulations and the physical processes involved, since the cloud boundar
ies and dimensions are mapped along with the internal structure of the clou
ds. Coincident observations from a 915-MHz radar (wind profiler) were used
to further define the turbulence structure in and around the clouds. The ob
servations document the detailed vertical and horizontal dimensions of updr
aft and downdraft circulations in the clouds observed. The two cumuli studi
ed in detail have similar circulation patterns-an updraft core surrounded b
y 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 consist
ent with adiabatic ascent, and penetrate about 150 m into the capping inver
sion. No penetrating downdrafts are observed within the updraft cores. The
downdrafts that flank the updraft on the downwind side of the cloud are rel
atively narrow (less than 100 m) and extend from cloud top to cloud base. T
he downdraft on the upwind side of the cloud is about 150 m across and pene
trates 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 wi
th entrainment mixing at cloud top penetrating through detraining, dynamica
lly inactive parts of the cloud matter. Analysis of the Doppler spectrum at
the updraft-downdraft interfaces indicates large Doppler spectral widths d
ue to turbulence and sharp shear zones in the radar sampling volume. Large
Doppler spectral widths in the detraining upwind part of the cloud are cons
istent 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 cum
ulus clouds are composed of successive bubbles that emerge from the subclou
d layer. Thus these small cumuli should be considered as convective complex
es rather than simple growing elements that later decay into passive clouds
. This study illustrates the potential of millimeter-wavelength radars for
studying small cumuli.