SURFACE MEASUREMENTS OF SOLAR IRRADIANCE - A STUDY OF THE SPATIAL CORRELATION BETWEEN SIMULTANEOUS MEASUREMENTS AT SEPARATED SITES

Pyranometers have been used for many years to measure broadband surfac e incoming solar irradiance, data that is necessary for surface energy budget, cloud forcing, and satellite validation research. Because suc h measurements are made at a specific location, it is unclear how repr esentative they may be of a larger area. This study attempts to determ ine a reasonable spacing between measurement sites for such research b y computing the correlation, and standard deviation from perfect corre lation, between simultaneous measurements of incoming solar irradiance for a network of surface measurement sites covering a 75 km X 75 km a rea. Using 1-min data collected from this network of 11 sites during t he NASA First ISSCP Radiation Experiment/Surface Radiation Budget Proj ect temporal averages were calculated. The correlation between any two of these sites was determined by comparing simultaneous measurement a verages for the 55 possible combinations of site pairs, along with the distances between them. In an attempt to remove the effect of the diu rnal cycle, thus leaving clouds as the primary influence on correlatio n of the radiation field, model results for a clear day were used to n ormalize measured irradiances and correlations were again calculated. For individual days, the correlation between sites varied widely, depe nding primarily on the type of cloud cover the region experienced that day. Removal of the diurnal cycle, as expected, significantly decreas ed these correlation values. Comparisons using the continuous experime nt records from 13 October through 2 November 1986, however, show that a relatively high degree of correlation existed with or without the d iurnal cycle removed. Plotting these correlation coefficients versus t he distance between sites, the expected trend for a decrease in correl ation with increasing distance is observed. Results also confirm that, whether using the complete record for the duration of the experiment or by individual day, the correlation between site station pairs incre ases with increasing averaging times. Finally, the standard deviation from perfect correlation suggests a predictive relationship within abo ut 6% of clear-sky irradiance for daily averages at a distance of 75 k m. Thus, a spacing of 150 km between measurement sites seems reasonabl e for studies of midlatitude frontal weather regimes using daily avera ges over periods of weeks or more.