LAUSR

A set of surface radiation measurements across the lower 48 United States (SURFRAD, Augustine et al 2000) has allowed a closer inspection of weather model representations of downward shortwave radiation in the last several years. In this study, it is found that downward shortwave radiation (SW↓) is excessive for the NOAA 3-km HRRR model at each of the 14 SURFRAD stations distributed across the lower United States when averaged over 2-month periods. Possible causes for this station-consistent SW↓ bias error were hypothesized. Three were eliminated by this study and two were then evaluated in this study. We found that this error was not from clear-sky errors but from insufficient attenuation by clouds. It was also found that this cloud deficiency was partly caused by a dry bias in atmospheric water vapor initial conditions. New experiments using the hourly cycled HRRR model-assimilation system were designed and carried out for three seasons with modified data assimilation addressing the dry bias problem and reduction of effective radius for cloud water droplets for both explicit and subgrid-scale clouds. The assimilation and cloud optical parameter changes contributed similarly toward a combined reduced SW↓ radiation bias by 80% in fall season and 84% in winter season but by only 35% in summer season. Even with the improved data assimilation, a dry bias contributing to deficient clouds continues which is a topic to be explored in a following study.