Cloud geometry effects on atmospheric solar absorption

A 3D broadband solar radiative transfer scheme is formulated by integrating a Monte Carlo photon transport algorithm with the Fu-Liou radiation model, it is applied to fields of tropical mesoscale convective clouds and subtro pical marine boundary layer clouds that were generated by a 2D cloud-resolv ing model. The effects of cloud geometry on the radiative energy budget are examined by comparing the full-resolution Monte Carlo results with those f rom the independent column approximation (ICA) that applies the plane-paral lel radiation model to each column. For the tropical convective cloud system, it is found that cloud geometry e ffects always enhance atmospheric solar absorption regardless of solar zeni th angle. In a large horizontal domain (512 km). differences in domain-aver aged atmospheric absorption between the Monte Carlo and the ICA are less th an 4 W m(2) in the daytime. However. for a smaller domain (e.g., 75 km) con taining a cluster of deep convective towers, domain-averaged absorption can be enhanced by more than 20 W m(-2). For a subtropical marine boundary lay er cloud system during the status-to-cumulus transition, calculations show that the ICA webs very well fur domain-averaged fluxes of the stratocumulus cloud fields even for a very small domain (4.8 km). For the trade cumulus cloud field, the effects of cloud sides and horizontal transport of photons become more significant. Calculations have also been made for both cloud s ystems including black carbon aerosol and a water vapor continuum. It is fo und that cloud geometry products no discernible effects on the absorption e nhancement due to the black carbon aerosol and water vapor continuum. The current study indicates that the atmospheric absorption enhancement due to cloud-related 3D photon transport is small. This enhancement could nor explain the excess absorption suggested by recent studies.