Vertical Retrieval of Solar and Infrared Irradiances in the Stratiform Regions of EMEX Cloud Clusters

Abstract

A technique is described for retrieving upward and downward solar and infrared irradiances in time and height within the stratiform precipitation region of two tropical cloud clusters. A broadband, multiple-scattering, radiative transfer program was used to retrieve irradiances from the ocean surface to 20 km, with horizontal resolution of 1 km. The radiative transfer scheme accounts for scattering by air molecules, nonprecipitating cloud ice and water, and precipitation-sized water and ice. Solar absorption by water vapor, ozone, cloud, and precipitation, and infrared absorption and emission by water vapor, cloud, and precipitation are included. Vertically scanning airborne weather radar provided measurements of radar reflectivity from which optical depths due to precipitation were determined, and cloud microphysical measurements were used to estimate nonprecipitating cloud optical depths. The retrieved values were compared against flight-level measurements of solar and infrared irradiances obtained between 1 and 7 km above sea level. Net infrared and solar irradiances were retrieved with mean biases up to 12 W m?2, while mean biases in the upward and downward components of solar and infrared irradiances were 10?33 W m?2. Retrieved net infrared fluxes at the ocean surface were horizontally uniform in the precipitation at ? 10 to ? 15 (i.e., upward) W m?2. Net solar fluxes into the ocean ranged from 25 to about 100 W m?2 over 60 km in horizontal distance. The horizontal variations in observed and retrieved solar irradiance were closely associated with variations in radar-detected precipitation structure. Solar and infrared heating and cooling rates near cloud top, determined for the two cloud clusters, were consistent with heating rates predicted by modeling studies. Infrared cooling within 1 km of cloud top produced net cooling in the presence of sunlight, while solar heating of the cloud extended several kilometers into the cloud. Thus, this retrieval technique may be a viable method for studying the radiative component of diabatic heating in tropical cloud clusters, and documenting the radiative fluxes into the ocean on fine spatial and temporal scales.