In this first part of the study, cloud radiation fluxes in a tropical cloud cluster were retrieved using airborne weather radar data. Cloud particles distributions were parameterized from the intensity of radar reflectivity. Even though weather radar detects only precipitation, the apparent correlation between cloud optical depth and radar reflectivity enabled a useful retrieval of cloud optical depth, radiative fluxes, and radiative heating rates. The retrieved cloud radiation fluxes compared well with observations along an aircraft flight track, and the retrieval captured the observed horizontal gradients of solar fluxes through the cloud cluster.
Radiative effects of large, precipitation-sized particles were examined. It is shown that, when the background cloud optical depth was large (e.g., underneath deep clouds), the spatial inhomogeneity of precipitation-sized particles did not contribute much to the observed horizontal variations in downwelling solar flux. The contribution from precipitation-sized particles, however, became significant when the background cloud optical depth was small (e.g., underneath a shallow cloud or in the upper part of a deep cloud). To accurately estimate three dimensional distributions of radiative fluxes for cloud clusters with embedded deep convective and stratiform clouds, the radiative effects of precipitation-sized particles should not be ignored. This study demonstrates that retrieval of cloud radiation fluxes from airborne radar data can be a useful tool for studying cloud radiative effects