Kilometer-Scale Climate Modeling

Convection-resolving climate simulations for the greater Alpine region

In order to better understand the regional climate system and to improve regional climate change projections, a climate change simulation has been conducted using a convection-resolving model at horizontal resolution of 2.2 km across a greater Alpine domain, covering the region from Northern Italy to Northern Germany. To our knowledge, this is worldwide the first convection-resolving climate change simulation covering such a large domain (1100km x 1100km). Due to the computational cost, the simulations have been limited to 10-year long periods.

Validation against precipitation observations shows that the convection-resolving approach strongly improves the simulation of sub-daily precipitation in many aspects. This includes the diurnal cycle of mean and extreme summer precipitation, the diurnal cycle of wet-hour frequency, and the frequency-intensity distribution of hourly precipitation. This improved representation of short-term precipitation is a key for the assessment of flash flooding.

Validation Ban et al. 2014 — Validation of summer precipitation for reanalysis-driven simulations over 10-year long period. Diurnal cycle of mean precipitation (left), wet-hour frequency (middle) and heavy hourly (p99H, right) precipitation in observation (Obs, black line), convection-parametrizing model at horizontal resolution of 12km (CPM12, dashed blue), and convection-resolving model at horizontal resolution of 2km (CRM2, solid blue). Figure adapted from Ban et al., 2015

For the Alpine summer season, convection-resolving climate change simulations reveal that projected increases in both extreme daily and hourly summer precipitation follow theoretical expectations from the Clausius-Clapeyron relation (which expresses the increase of the water holding capacity of air with temperature), and amount to about 6-7% per degree warming. This result is of significant importance and the applicability of the 6-7%/K rate provides a surprisingly simple tool to climate change adaptation with respect to heavy precipitation events.

Scaling Relationship after Ban et al. 2015 — Scaling rate at high percentiles of precipitation, calculated as a relative change of precipitation normalized by the local temperature change. Scaling rate is compared between convection-parametrizing (CPM12, dashed blue) and convection-resolving (CRM2, solid blue) model for precipitation on daily (left) and hourly (right) time scales in summer. It shows that both, extreme daily and hourly precipitation intensifies with the Clausius-Clapeyron rate in convection-resolving model. Figure adapted from Ban et al., 2015.

Continental-scale convection-resolving climate simulations

Our group is leading a major modeling project entitled "Convection-Resolving Climate Modeling on Future Supercomputing Platforms (crCLIM)" which is funded by the Swiss National Science Foundation through the Sinergia program. The crCLIM project aims to develop a European-scale convection-resolving climate modeling capability at a horizontal resolution of about 2 km using the next generation of supercomputing platforms. Our group performs, validates and analyzes convection-resolving climate simulations over Europe. Currently a first 10-year long simulation is about to be completed.

Enlarged view: CrClim David Leutwyler — Animation of European-scale simulations for a day in July 2006, with a visualization of the cloud cover and precipitation rates (in mm/h). The panels show a 2-km simulation with parameterized convection (left panel) and a 2-km simulation with explicit convection (right panel). Note the more realistic representation of short-term precipitation and the organization of convection in the 2 km simulation (from PhD of David Leutwyler, ETH Zurich, project crCLIM).

Publications

Leutwyler, D., Fuhrer, O., Lapillonne, X., Lüthi, D., and Schär, C.: Towards European-scale convection-resolving climate simulations with GPUs: a study with COSMO 4.19, Geosci. Model Dev., 9, 3393-3412, external pagedoi:10.5194/gmd-9-3393-2016, 2016.

Ban, N., J. Schmidli, and C. Schär (2015), Heavy precipitation in a changing climate: Does short-term summer precipitation increase faster? Geophys. Res. Lett., external pagedoi:10.1002/2014GL062588

Ban, N., J. Schmidli, and C. Schär (2014), Evaluation of the convection-resolving regional climate modeling approach in decade-long simulations, J. Geophys. Res. Atmos., 119, 7889–7907, external pagedoi:10.1002/2014JD021478

Leutwyler, D., O. Fuhrer, X. Lapillonne, D. Lüthi, C. Schär (2015): Continental-Scale Climate Simulation at Kilometer-Resolution. ETH Zurich e-collection, DOI: external pagehttp://dx.doi.org/10.3929/ethz-a-010483656 (short description and animation), external pageonline video on Vimeo.