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Clouds and Turbulence in the Arctic Autumnal Boundary Layers |
Naval Postgraduate School, Monterey, CA,
University Space Research Association, Huntsville, AL
We present a detailed analysis of the characteristics of boundary layer thermodynamics and turbulent mixing in both a clear and a cloudy boundary layer observed by a research aircraft, the NCAR C-130, during the Beaufort and Arctic Sea Experiment (BASE). The two regions were separated by less than 200 km and the measurements in the two regions were made during the same flight. Comparisons between the two regions reveal the roles of cloud in modifying the boundary layer and surface properties. The clear region was characterized by a deep layer of stable stratification over a shallow boundary layer of 40 to 200 m deep. This shallow boundary layer was found to be saturated and possibly to contain cloud condensate not detectable by the instruments onboard the aircraft. We also found strong horizontal variability of boundary layer thermodynamics in the cloudy region in response to varying cloud top height. Comparisons of the thermodynamic properties between the cloudy region and the clear region indicate modification of the stable stratification by cloud related processes, hence pointing to local formation of the cloud layers. The presence of cloud significantly modified the boundary layer thermodynamics and turbulence. Dominant wave motion is identified near the surface in the clear region, while turbulent mixing dominates the cloudy boundary layer. However, in case of multi-layered cloud structure, the presence of an upper cloud layer significantly reduced the radiative cooling as evidenced by the reduced turbulent kinetic energy in the lower layer. Scales of turbulent flux transport and scalar variance and correlation are also studied using spectral analysis.
