Turbulence Estimates in Continental Stratocumulus Using ARM Cloud Radar Observations

Observations from the Atmospheric Radiation Measurement (ARM) millimeter cloud radar (MMCR) are used to examine the turbulence structure associated with 18 hours of stratocumulus observations over the Southern Great Plains site. The energy dissipation rate (EDR) calculated from spectrum width agrees well with that calculated from vertical velocity power spectra. The EDR of the unresolved small scale turbulence increases from cloud base to 0.8 of the cloud depth and then decreases to cloud top whereas the resolved turbulence intensity and the vertical integral length scale decrease with the height. In a normalized coordinate system, the averaged coherent structure of updrafts is characterized by low unresolved small scale EDR in the updraft core; larger EDR values surround the updraft core and its top and edges. In contrast to that in the updraft, the unresolved small scale EDR is large inside downdraft core. Compared to the updraft, the downdraft is more turbulent. For both updraft and downdraft, the maximum EDR occurs at 0.8 normalized height where the maximum reflectivity is observed. Day-night differences in the EDR structures in the cloud are also documented and related to surface buoyancy fluxes and radiative cooling at cloud top. This study illustrates the utility of using Doppler spectrum width from millimeter wavelength radars to provide high temporal resolution EDR in non-precipitating clouds.