Radiation in numerical weather prediction

Radiation is a fundamental process that drives atmospheric flows at all scales, and is key to both improving short-range surface temperature forecasts, and meeting ECMWF’s strategic aim of pushing the boundaries of predictability at the medium-range and longer timescales. This paper provides a detailed assessment of recent radiation developments at ECMWF and the priority areas where progress needs to be made in the coming years. A particular focus is placed on the new ‘ecRad’ radiation scheme that became operational in 2017 and will facilitate future developments. Five ‘Grand Challenges’ for radiation in NWP are then presented: the surface (particularly coastlines, forests and urban areas), clouds (particularly cloud structure, longwave scattering and 3D radiative effects), clear-sky absorption (particularly the impact of aerosols on monsoon systems, and the water vapour continuum), the middle atmosphere (particularly how reducing the large stratospheric temperature biases has the potential to improve predictive skill on monthly timescales) and efficiency (particularly optimizations in ecRad, and how future effort should reassess the balance between the spectral, temporal and spatial resolution of the radiation calculations).