Abstract/Summary

Clouds have a decisive impact on the Earth’s radiation budget and on the temperature of the atmosphere and surface. In global weather and climate models, however, cloud-radiation interaction is treated in an approximate way that contributes to the large uncertainty due to clouds in climate predictions. One of the simplifications is to only consider radiative transfer in one vertical dimension and neglect horizontal radiative transfer. This thesis provides the first systematic estimates of the global impact of 3D cloud-radiation interactions in the shortwave and longwave. We show that 3D cloud effects consist of both horizontal transfer through cloud sides and horizontal transfer within regions. We develop the longwave part of the SPARTACUS radiation scheme that incorporates treatment of these 3D effects in a one-dimensional radiation calculation at a numerical cost suitable for a global weather and climate model, and validate the scheme. SPARTACUS includes the effects of cloud internal inhomogeneity, of horizontal in-region transport and of the spatial distribution of in-cloud radiative fluxes. Algorithm evaluation is facilitated by an exact theoretical benchmark: for idealised optically thick cubic clouds, we can reason analytically that neglecting longwave 3D cloud side effects leads to an underestimation of cloud radiative effect (CRE) by exactly a factor of three. We introduce a new measure of the cloud geometry information relevant to radiation in the ”effective cloud scale” CS, which only depends on cloud type. Analysis of the effective cloud scale of various cloud types demonstrates that CS = 1.0 ± 0.4 km is a good estimate for the cloud scale of boundary-layer clouds, irrespective of their cloud type and of data source. More variety of cloud types at middle and high levels leads to a greater uncertainty range of CS = 5 to 20 km for clouds above the boundary layer, with a best estimate of CS = 10 km. We conduct offline radiation calculations on atmospheric states from a year of ERAInterim re-analysis. We estimate that overall 3D cloud effects warm the Earth by about 4 W m−2, with warming effects in both the shortwave and the longwave, of 3 W m−2 and 1 W m−2 respectively at top-of-atmosphere and both about 2 W m−2 at the surface. Longwave heating and cooling in vertical layers is increased by up to 0.2 K d−1 and −0.3 K d−1 respectively. In the shortwave, we have separated two different 3D effects. We find that the effect of transport through cloud sides has a cooling effect of around −1 W m−2. This cooling is dominated by the previously rarely investigated effect of in-region horizontal transfer that significantly decreases cloud reflectance and warms the Earth’s system by 5 W m−2. These 3D effects are neglected by current models, but are noticeably stronger than the effect of anthropogenic greenhouse gases and therefore definitely worth considering in climate simulations. We have shown for the first time how this can be achieved in a computationally affordable way.