Importance of three-dimensional scene construction for computation of the shortwave radiation budget

Active sensors such as Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat provide detailed cloud vertical profile, but the information is limited to a narrow satellite track. As a consequence, once CALIPSO and CloudSat derived cloud profiles are collocated with another instrument's observations with a larger field-of-view (FOV), they cover only a part of the other sensor's (FOV). Clouds and Earth's Radiant Energy System (CERES) instruments on Aqua have a FOV of about 20 km. When modeled radiances with CALIPSO and CloudSat cloud profiles are evaluated against CERES observations, it is expected that the effect of the FOV difference on evaluating the instantaneous irradiance will not be negligible even though it has a negligible impact on evaluating temporally and spatially averaged modeled irradiances. Using a scene construction algorithm developed by Barker et al. (2011), using NASA's A-train observations, the importance of three-dimensional SCA on instantaneous shortwave radiation computations are examined. The SCA expands cloud profiles obtained from the active sensors in the cross-track direction based on the narrow-band radiances observed by Moderate Resolution Imaging Spectroradiometer (MODIS) imager. 3D radiative transfer computations with and without SCA are compared to investigate how SCA improves modeling capability of shortwave radiation. To assess modeling accuracy, modeled top-of-atmosphere (TOA) radiances are compared with radiances observed by the CERES instrument. It is shown that SCA plays an important role in the instantaneous TOA radiance estimation by reducing the root-mean-square (RMS) error; mean bias error is essentially unaffected. Moreover, it is shown that the RMS error for horizontally inhomogeneous cloud types, such as stratocumulus (Sc), Cumulus (Cu) are reduced more by the SCA, compared to values for horizontally homogeneous cloud types such as Nimbostratus (Ns) and deep convective clouds (Dc). The importance of the SCA on estimating instantaneous surface irradiance and atmospheric absorption is also examined. Similar to TOA radiances, it is shown that the influence of the SCA on instantaneous surface irradiance becomes more important for horizontally inhomogeneous clouds such as Sc and Cu than for horizontally homogeneous clouds such as Ns and Dc. However, the SCA improves atmospheric absorption and heating rate profiles significantly for horizontally homogeneous clouds such as Ns and Dc because the absolute amount of absorption by these clouds is large.

Barker, H. W., M. P. Jerg, T. Wehr, S. Kato, D. P. Donovan, and R. J. Hogan, 2011: A 3D cloud-construction algorithm for the EarthCARE mission. Q. J. R. Meteorol. Soc., 137, 1042–1058.