Evaluation of the convection‐resolving climate modeling approach on continental scales

Abstract

Convection‐resolving models allow to explicitly resolve deep convection at horizontal grid spacings of O(1 km). On current supercomputers, refining the grid spacing to the kilometer scale is computationally still extremely demanding, and therefore, climate simulations at this resolution have so far largely been limited to subcontinental computational domains. However, new supercomputers that mix conventional multicore CPUs and accelerators possess properties beneficial for climate codes. Exploiting these capabilities allows expansion of the size of the computational domains to continental scales. Here we present such a convection‐resolving climate simulation, using a version of the COSMO model, capable of exploiting GPU accelerators. The simulation has a grid spacing of 2.2 km, 1536 × 1536 × 60 grid points, covers the period 1999–2008, and is driven by the ERA‐Interim reanalysis. An assessment of the 10‐year‐long simulation is conducted using a wide range of data sets, including several rain gauge networks, energy balance stations, and a remotely sensed lightning data set. Substantial improvements are found for the 2 km simulation in terms of the diurnal cycles of precipitation. This confirms results found in studies using smaller computational domains. However, the continental‐scale simulations also reveal deficiencies such as substantial performance differences between regions with and without strong orographic forcing. Analysis of the statistical distribution of updrafts and downdrafts shows an increase of the amplitude in seasons with convection and a pronounced asymmetry between updrafts and downdrafts. Furthermore, the analysis of lightning data shows that the convection‐resolving simulation is able to reproduce important features of the annual cycle of deep convection in Europe.

Projected changes in extreme precipitation over Scotland and Northern England using a high-resolution regional climate model

“…Both 1.5 and 12 km present-climate simulations have diurnal peaks earlier than observed, but the timing of the peak is better for the 1.5 km simulation. Similar early diurnal peaking of precipitation is found in ETHZürich CPM simulations over the Alps (Keller et al 2016;Leutwyler et al 2017). This early peaking bias is not found Table 1 Average "wet hour" ( ≥ 1mm/h ) intensity for radar observations, 1.5 and 12 km simulations.…”

“…A good diurnal cycle is indicative that convective processes are well captured, and hence gives us more confidence in the 1.5 km CPM projections for convective extremes. However, there is still a discrepancy in the timing of maximum and minimum daily rainfall, which is still found in newer CPM simulations (Keller et al 2016;Leutwyler et al 2017).…”

Projections of Future Precipitation Extremes Over Europe: A Multimodel Assessment of Climate Simulations

“…Hence, the present study merely focused on the daily time scale that is simulated in a skillful manner. Pioneering studies using high‐resolution (i.e., 1–3 km grid spacing) convection‐resolving simulations demonstrate that the representation of subdaily precipitation is significantly improved (Ban et al, , ; Kendon et al, ; Prein et al, ), thus guiding the way to the provision of hourly estimates in future modeling frameworks (Leutwyler et al, , ).…”

The devil in the detail of storms

“…Little has been done to assess potential changes in their frequency and intensity in the future. Convection-resolving, long-term climate change experiments as those by Leutwyler et al (2017) offer new possibilities to analyse storms in much more detail, without the uncertainty coming from any precursor diagnostics. However, such models still struggle to simulate the full lifecycle of low-pressure systems over a sufficiently long time-period.…”