Longwave radiative cooling is the primary forcing for sustaining stratocumulus clouds. In cloud models, this forcing is typically calculated under the Independent Pixel Approximation (IPA), which may not properly account for horizontal variability. We evaluate the effect of cloud inhomogeneity on instantaneous cooling rates by applying the Spherical Harmonics Discrete Ordinate Method by Evans (1998) to a non-precipitating marine stratocumulus cloud field produced by a Large Eddy Simulation model. The longwave spectrum between 4 and 100 microns is divided into eleven broad bands for which the radiative transfer (RT) is calculated with a correlated k-distribution. The RT model accounts for thermal emission, absorption, and scattering.

The three dimensional RT results in smoother fields of flux divergence compared to the IPA calculations. The difference between instantaneous 3D and IPA maximum cooling rates is shown to reach several degrees per hour locally. Averaged over the horizontal domain of 4 x 4 km, however, the difference is reduced to a fraction of a degree per hour, with weaker cooling in the 3D case. This change in cooling rate could affect cloud evolution, but the effect must be evaluated in a dynamical framework including feedbacks among radiation, dynamics, and cloud microphysics. The spatial distribution of the differential cooling rate is highly correlated with the cloud top geometry and, to a lesser degree, with the liquid water distribution inside the cloud. This correlation provides a possibility to parameterize the 3D radiation transfer effect of the cooling rates in terms of IPA calculations and cloud structure. The results so far are limited to the simulated case of a shallow marine stratocumulus, and more simulations will be made to generalize the conclusions.