On the Observed Near Cancellation between Longwave and Shortwave Cloud Forcing in Tropical Regions

Observations based on Earth Radiation Budget Experiment (ERBE) satellite data indicate that there is a near cancellation between tropical longwave and shortwave cloud forcing in regions of deep convective activity. Cloud forcing depends on both cloud macrophysical properties (e.g., cloud amount, cloud height, etc.) and on microphysical properties (e.g., cloud particle size, particle shape, etc.). Hence, the near cancellation in the tropics could be due to either the macrophysical or the microphysical properties of these clouds, or a combination of these effects. By using satellite data from the ERBE and International Satellite Cloud Climatology Project (ISCCP) and recent in situ observations of tropical anvils, it is argued that the observed near cancellation in the tropics is mainly a result of the tropical tropopause height. This conclusion depends on two observational results from comparison of ERBE and ISCCP data: 1) Both the longwave and shortwave cloud forcing are predominately due to high cloud and 2) these clouds are optically thick in both the visible and infrared region.