Airborne Doppler radar observations of turbulence in marine stratocumulus

The 94 GHz Wyoming Cloud Radar was deployed on the NCAR C-130 aircraft during the Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS-II) experiment. DYCOMS-II was conducted off the coast of southern California during July 2001. Both downward- and backward-looking antennas were used to estimate the turbulence characteristics from the fine-structure of the Doppler velocities in clouds and drizzle-filled regions of the stratocumulus-topped boundary layer (STBL). Thus, during the most uniform drizzling case in DYCOMS, integral scales and turbulence dissipation were estimated throughout the STBL from a radar leg flown above it using, respectively, autocorrelation functions and structure functions, and taking into account the pulse volume averaging. The results were compared with the in situ measurements at four levels within the STBL. We find that the fluctuations of the vertical velocity of the air are underestimated by the Doppler measurements in the upper part of the cloud, likely due to a positive correlation between upward vertical air velocity and effective radar-weighted drizzle drop diameter. Obtaining dissipation from the radar measurements using the structure function remains questionable because we do not know the cross-spectrum between the fall velocity and the air vertical velocity. The method shows good agreement with the in situ measurements when the correlation between updrafts and drizzle is small. The vertical profiles of the integral scales agree well with the in situ observations and clearly show "squashed" turbulence near the surface and the cloud-top inversion.