Surface summertime radiative forcing by shallow cumuli at the ARM SGP

Description

Shallow cumuli are common over large areas of the globe, especially over the oceans. Their impact on the surface cloud radiative forcing (CRF), however, has not been carefully evaluated in the past. This study addresses this shortcoming by analyzing data collected during selected periods with single-layer shallow cumuli over eight summers (2000 through 2007) at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. During periods with shallow cumuli, the average shortwave and longwave CRF at the surface are 45.5 W m-2 (out of 612 W m-2 estimated for clear sky conditions) and +15.9 W m-2 (out of -105.2 W m-2 estimated for clear sky conditions), respectively. Instances of cloud-induced enhancement of the shortwave irradiance over that estimated for clear skies are observed to occur nearly 20% of the time and are caused by the spatial and temporal inhomogeneity of fields of cumuli. Such enhancement is responsible for occurrences of positive shortwave CRF with instantaneous values as large as +75 W m-2. The total amount of shortwave and longwave energy deposited at the surface is found to depend non-linearly on the fractional sky cover, and the largest values of the deposited energy occur for intermediate cloud amounts between 0.4 and 0.6.