D9.3: Report on surface radiation balance in SR-ESMs compared against observational products and coarse resolution simulation

About this document The report summarizes the evaluation of the representation of the components of the surface radiation balance from the SR-ESMs. Work package in charge WP9 Executive summary With the emergence of storm-resolving global climate models, approaching horizontal grid spacings of only a few kilometers, we expect a better representation of the surface irradiance climatology via three possible pathways i) synoptic variability may be better represented, ii) convective systems may be better resolved, and iii) landscape heterogeneity is represented in more detail. This report presents the results of work package 9.3, where we evaluate the performance of the nextGEMS simulations in produce accurate mean downwelling surface irradiances and investigate which pathways dominate the impact of horizontal resolution on surface irradiances. For the evaluation, we use all 5-year cycle 3 (IFS: 4.4 km, 9 km, and 28 km; ICON: 5 km) and 30-year cycle 4 (IFS: 9 km; ICON: 10 km) simulations. Most simulations have global annual mean biases within only 4.5 W m^−2 and 7.6 W m^−2 for solar and thermal irradiance, respectively, although local irradiance biases with respect to station measurements can be up to an order of magnitude higher. Furthermore, we find no clear and consistent improvement of mean surface irradiances as we move towards higher resolutions. We then focus on the cycle 3 IFS simulations, ranging from 4.4 km to 28 km horizontal resolution, to study in detail how differences in horizontal resolution affect the surface irradiance climatology. We find that differences in mean surface irradiance are most pronounced in areas with high orographic variability, suggesting that the better resolved orography is the main pathway of resolution-driven changes in the surface irradiance climatology. The representation of the cloud-driven local irradiance variability on hourly to daily time scales also improves with horizontal resolution, although these improvement do not carry over to larger spatiotemporal scales. Our findings suggest that for an accurate representation of the surface irradiance climatology, the added value of moving towards higher horizontal resolution among the nextGEMS simulations is still modest and may not yet outweigh the additional computational costs.