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Qiang Fu
,
K. N. Liou
,
M. C. Cribb
,
T. P. Charlock
, and
A. Grossman

Abstract

A systematic formulation of various radiative transfer parameterizations is presented, including the absorption approximation (AA), δ-two-stream approximation (D2S), δ-four-stream approximation (D4S), and δ-two- and four-stream combination approximation (D2/4S), in a consistent manner for thermal infrared flux calculations. The D2/4S scheme uses a source function from the δ-two-stream approximation and evaluates intensities in the four-stream directions. A wide range of accuracy checks for monochromatic emissivity of a homogeneous layer and broadband heating rates and fluxes in nonhomogeneous atmospheres is performed with respect to the “exact” results computed from the δ-128-stream scheme for radiative transfer. The computer time required for the calculations using different radiative transfer parameterizations is compared. The results pertaining to the accuracy and efficiency of various radiative transfer approximations can be utilized to decide which approximate method is most appropriate for a particular application. In view of its overall high accuracy and computational economy, it is recommended that the D2/4S scheme is well suited for GCM and climate modeling applications.

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Qiang Fu
,
M. C. Cribb
,
H. W. Barker
,
S. K. Krueger
, and
A. Grossman

Abstract

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 subtropical marine boundary layer clouds that were generated by a 2D cloud-resolving model. The effects of cloud geometry on the radiative energy budget are examined by comparing the full-resolution Monte Carlo results with those from the independent column approximation (ICA) that applies the plane-parallel radiation model to each column.

For the tropical convective cloud system, it is found that cloud geometry effects always enhance atmospheric solar absorption regardless of solar zenith angle. In a large horizontal domain (512 km), differences in domain-averaged atmospheric absorption between the Monte Carlo and the ICA are less than 4 W m−2 in the daytime. However, for a smaller domain (e.g., 75 km) containing a cluster of deep convective towers, domain-averaged absorption can be enhanced by more than 20 W m−2. For a subtropical marine boundary layer cloud system during the stratus-to-cumulus transition, calculations show that the ICA works very well for 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 systems including black carbon aerosol and a water vapor continuum. It is found that cloud geometry produces no discernible effects on the absorption enhancement 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 not explain the excess absorption suggested by recent studies.

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