Entrapment: An Important Mechanism for Shortwave 3D Cloud Radiative Effects and its Inclusion in the SPARTACUS Radiative Transfer Model

Several mechanisms have been proposed to explain differences between the albedo of realistic cloud scenes computed using the 1D Independent Column Approximation (ICA) and 3D solutions of the radiative transfer equation. For example, when the Sun is low in the sky, interception of sunlight by cloud sides tends to increase albedo relative to ICA estimates which neglect this effect. On the other hand, at high Sun 3D radiative transfer has been found to make clouds less reflective and it is this latter effect that appears to dominate in the global/annual average (e.g., Barker et al. 2015). We argue that this behavior is explained by the mechanism of "entrapment". Entrapment need not involve lateral transfer of radiation through cloud sides, but involves horizontal diffusion of radiation beneath a cloud layer that ups the chances, relative to the ICA, of light being absorbed either by cloud or the surface. The Speedy Algorithm for Radiative Transfer through Cloud Sides (SPARTACUS) is a fast shortwave and longwave radiation solver available for use in the radiation scheme of the ECMWF forecast model. SPARTACUS explicitly computes side irradiation effects and has recently been modified to represent the three entrapment scenarios depicted in the figure. Its representation of "realistic entrapment", which involves explicit calculation of the mean horizontal distance travelled by reflected light, has been verified against 3D Monte Carlo calculations for 59 contrasting scenes simulated by two cloud-resolving models. Global model calculations using SPARTACUS suggest that without the entrapment mechanism ("minimum entrapment" in the figure), domain-average surface solar irradiances are very nearly equal for 1D and 3D transfer. When entrapment is included, however, it increases by ~1 W m-2 as a global average, but with much larger differences on a regional scale. Coupled to estimates of longwave 3D effects, which are the same sign and about the same magnitude as in the shortwave, 3D effects exert a significant warming effect on the climate system.