A Validation Study of Cloud Scene Simulation Model Temporal Performance
AIR FORCE INST OF TECH WRIGHT-PATTERSONAFB OH SCHOOL OF ENGINEERING
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Cloud Scene Simulation Model CSSM temporal performance was validated by comparing the cloud forcing signatures on observed radiometric time series with those derived from CSSM output for initial conditions similar to that for the observed data. Observed radiometric data was collected by a normal incidence pyraheliometer sensitive to wavelengths in the range .3mm to 3mm. Simulation radiometric time series data was derived by applying the following process to each case study. CSSM cloud liquid water content CLWC grids were converted to grids of slant path optical depth values by the Fast Map post processor to the CSSM. A ray tracing routine then integrated the slant path optical depth values along a path from the position of the sun through each cloud volume to a point at its base. The position of the sun was fixed by ephemeris calculations for the time and location of the case study. The integrated optical depth values were then used with a modified form of Beers Law to derive radiometric time series values. Subjective and statistical comparison of the observed and corresponding simulation time series data were then made. The statistical comparison used correlograms, variograms and power law analyses to determine whether CSSM cloud field evolution with time proceeded in a realistic manner. Results indicate that CSSM cloud fields do not evolve in a realistic manner. Radiometric traces derived from simulation data lack small scale variations present in the observed data, while large scale variations in the simulation data are exaggerated.