A Midlatitude Cirrus Cloud Climatology from the Facility for Atmospheric Remote Sensing. Part V: Cloud Structural Properties


In this fifth of a series of papers describing the extended-time high cloud observation program from the University of Utah Facility for Atmospheric Remote Sensing, the structural properties of cirrus clouds over Salt Lake City, Utah, are examined. Wavelet analysis is applied as a function of cloud height to a 10-y record of ruby (0.694 μm) lidar backscattering data collected from visually-identified cirrus clouds to study the presence of periodic cloud structures, such as Kelvin-Helmholtz waves, cirrus mammata, and uncinus cells (all with wavelengths of ~1-10 km), as well as longer mesoscale cloud organizations. Approximately 8.4% (18.8%, 30.8%) of the data display such periodic structures after passing a 95% (75%, 50%) confidence level test. This may signify that most cloud organizations are quasi-periodic in nature. The amount of lidar cloud data showing periodic structures does not change considerably with length scale between 0.2 to 200 km, although a preference for ~20-km mesoscale cloud structures is indicated. Using time series of vertically-integrated lidar backscattering profiles, we find a steady decrease in autocorrelation coefficients starting at a few kilometers as the length (or model grid) scale increases. Examining the variability of cirrus cloud optical depth τ from an earlier LIRAD (combined lidar and infrared radiometer) analysis reveals that the standard deviation σ of τ is related by σ= 0.45 τ. Reflecting the fundamental dynamical processes involved in midlatitude cirrus cloud generation, few if any cirrus layers in our sample can be considered horizontally homogeneous over our typical 3-h lidar data collection period. This reality must be accounted for in modeling the radiative transfer through simulated cirrus clouds through subgrid-scale parameterizations.
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