ECRAD: A new radiation scheme for the IFS

A new radiation scheme for the IFS is described. It is flexible, allowing the spectral resolution, the description of cloud and aerosol optical properties, and the solver, to be changed independently of one another. In its default configuration, the new ‘McICA’ solver produces less noise in heating rates, which has been found to lead to a small but significant reduction in forecast temperature errors. Moreover, the new scheme is around two-thirds the computational cost of the old (with the same spectral resolution), so additional forecast skill is possible in model configurations that could only afford to call the radiation scheme only every 3 h by reducing this interval to 2 h. A correction to the way the longwave equations are solved has led to an improvement in stratospheric mean temperatures. The new scheme has been used to test the impact of various changes to the description of clouds, including improvement of the ice and liquid optical properties, representation of longwave scattering, use of cloud overlap assumptions more consistent with observations, changing the width and shape of the sub-grid cloud water distribution, and for the first time representing 3D radiative effects.