The Surface Cloud Grid VAP uses as input the 15-minute output from the Shortwave Flux Analysis VAP (Long 2001, Long and Ackerman 2000, Long et al. 1999) from the Southern Great Plains (SGP) Central and Extended Facilities. It applies a multi-pass weighted sum analytic approximation technique (Caracena 1987), which uses Gaussian weighting and an imposed scale length, to interpolate to a 0.25 degree by 0.25 degree lat/long grid over the SGP domain. The output, like the input, includes only solar elevation angles of 10 degrees or greater.
The Surface Cloud Grid VAP produces gridded output for cloud fraction, the ratio of measured over clear-sky short wave total (both unshaded pyranometer and sum of direct + diffuse) and direct irradiance, the estimated clear-sky fit total shortwave irradiance, and the estimated clear-sky fit direct shortwave irradiance. The ratio measured over clear-sky direct shortwave, in conjunction with the total shortwave ratio and clear-sky total and direct irradiance, allows for component evaluation of the irradiance and cloud effects. These output variables were chosen because the intent is to use the Surface Cloud Grid VAP output for model comparisons, as well as climatological and statistical research.
For more details, see Surface Cloud Grid (SfcCldGrid) Value-Added Product: Algorithm Operational Details and Explanations technical report.