Sensitivity of convective precipitation to model grid spacing and land-surface resolution in ICON

Abstract

The impact of model grid spacing and land-surface resolution (LSR) on convective precipitation are investigated for areas with different orographic complexities. ICOsahedral Nonhydrostatic (ICON) model simulations were performed for six days having weak large-scale forcing using six model grid spacings (in metres): Numerical Weather Prediction (NWP) ((Formula presented.), (Formula presented.)) and Large-Eddy Simulation (LES) physics simulations ((Formula presented.), (Formula presented.), (Formula presented.), and (Formula presented.)) in a nested set-up. Concerning LSR, we focused on simulations with LSRs of 1,250 and 5,000 m, keeping the model grid spacing at 156 m. The onset of precipitation in (Formula presented.) is earlier by 0.5–2 hr, while LSR modifications show a similar onset compared with (Formula presented.). The relative percentage difference (RPD) of areal mean daily precipitation across LES physics simulations decreases consistently with model grid spacing for most of the cases. The RPD of precipitation in (Formula presented.) is considerably higher (75th percentile: ≈155%) than that of the LSR runs at resolutions of both 1,250 and 5,000 m, with 75th percentiles of ≈7% and ≈22%, respectively. To investigate the processes causing the differences in precipitation characteristics, like onset time and amount, the heat and moisture budgets of (Formula presented.) and (Formula presented.) were compared. The results show that, at the initial stage of cloud formation, a higher number of smaller clouds are formed in (Formula presented.) compared with (Formula presented.). The small clouds in (Formula presented.) are subject to considerable evaporative cooling at their edges and shell regions, due to entrainment processes. As a result, these clouds often dissolve before they can grow deep. Later on, cloud aggregation in (Formula presented.) also enables precipitation. The delayed onset of precipitation and reduced areas of aggregated clouds having low precipitation rates are the main reasons for less precipitation in (Formula presented.) than in (Formula presented.).