Optical Depth of Overcast Cloud across Canada: Estimates Based on Surface Pyranometer and Satellite Measurements

Abstract

Overcast cloud optical depths τ are inferred from hourly, broadband surface pyranometer measurements of global irradiance for 21 Canadian stations. A radiative transfer model that treats the atmosphere as plane-parallel and horizontally homogeneous is used so inferred τ are effective values that should resemble those used by GCM radiation routines. Results are presented mostly for June, July, and August (JJA), thus minimizing the impact of surface albedo errors that arise from unreported sea?ice and ground snow. Measurement periods for several sites exceed 20 yr. Frequency distributions of τ for JJA can be described well by gamma distributions with mean values τ that tend to be largest (30?35) for southern continental sites and smallest (20?25) for continental subarctic sites. Corresponding standard deviations are generally near ?1.4τ. Diurnal amplitudes of hourly mean τ (dayside) are negligible for most sites but exceed 5 for some. There is some evidence of weak, yet occasionally significant, increases in monthly mean τ from the late 1960s to early 1990s. Annual results for four coastal sites exhibit maximum monthly mean τ during autumn. The International Satellite Cloud Climatology Project (ISCCP)-CX optical depths τsat (means of ?10 km cloudy pixel values inside 1° ? 1° cells centered roughly on pyranometers) are compared with collocated surface-inferred values τsrf (means of two hourly values that flank ISCCP snapshots). Data for JJA of 1989 at a continental and a maritime site are considered. For the majority of cases, 1.25 < τsrf/τsat < 2.25 echoes an independent study. Many occurrences of τsat ? τsrf have ISCCP IR temperatures >273 K so cloud phase is not an issue. Moreover, for most of these cases, variances for the 10-km pixel optical depths suggest weak horizontal variability of cloud. A full explanation of this systematic discrepancy is beyond the scope of this study.