PNNL

Abstract

Although shallow cumuli are common over large areas of the globe, their impact on the surface radiative forcing has not been carefully evaluated. This study addresses this shortcoming by analyzing data from days with shallow cumuli collected over eight summers (2000-2007) at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility (collectively ACRF) Southern Great Plains site. During periods with clouds, the average shortwave and longwave radiative forcings are 45.5 W m-2 and +11.6 W m-2, respectively. The forcing has been defined so that a negative (positive) forcing indicates a surface cooling (warming). On average, the shortwave forcing is negative, however, instances with positive shortwave forcing are observed approximately 20% of the time. These positive values of shortwave forcing are associated with three-dimensional radiative effects of the clouds. The three-dimensional effects are shown to be largest for intermediate cloud amounts. The magnitude of the three-dimensional effects decreased with averaging time, but it is not negligibly small even for large averaging times as long as four hours.