Sensitivity of Convection Permitting Simulations to Lateral Boundary Conditions in Idealized Experiments

Limited‐area convection‐permitting climate models (CPMs) with horizontal grid‐spacing less than 4 km and not relying on deep convection parameterisations (CPs) are being used more and more frequently. CPMs represent small‐scale features such as deep convection more realistically than coarser regional climate models (RCMs) with deep CPs. Because of computational costs, CPMs tend to use smaller horizontal domains than RCMs. As all limited‐area models (LAMs), CPMs suffer issues with lateral boundary conditions (LBCs) and nesting. We investigated these issues using idealized Big‐Brother (BB) experiments with the LAM COSMO‐CLM. Grid‐spacing of the reference BB simulation was 2.4 km. Deep convection was triggered by idealized hills with driving data from simulations with different spatial resolutions, with/without deep CP, and with different nesting frequencies and LBC formulations. All our nested idealized 2.4‐km Little‐Brother (LB) experiments performed worse than a coarser CPM simulation (4.9 km) which used a four times larger computational domain and yet spent only half the computational cost. A boundary zone of >100 grid‐points of the LBs could not be interpreted meteorologically because of spin‐up of convection and boundary inconsistencies. Hosts with grid‐spacing in the so‐called gray zone of convection (ca. 4–20 km) were not advantageous to the LB performance. The LB's performance was insensitive to the applied LBC formulation and updating (if ≤ 3‐hourly). Therefore, our idealized experiments suggested to opt for a larger domain instead of a higher resolution even if coarser than usual (∼5 $\sim 5$ km) as a compromise between the harmful boundary problems, computational cost and improved representation of processes by CPMs.Plain Language Summary: Recently, very high resolution (grid‐spacing 100 grid points cannot be interpreted reliably. The boundary data should be given to the CPM every 3 hr or more often. Small‐domain CPM simulations all performed worse than a reference simulation on a larger domain with the same resolution or with an even two times lower resolution. We tested different resolutions of the driving data for the CPMs and found that driving data from a model in the “gray zone” of convection (about 4–20 km) is not advantageous for the CPM performance. We concluded that it often might be better to opt for a larger domain with an unusually coarse CPM resolution (ca. 5 km) than for a much smaller domain with grid‐spacing < 4 km.Key Points: The nesting challenge of convection‐permitting climate modeling (CPM) is investigated with idealized simulation experiments. Nesting the CPM into host simulations with grid‐spacing in the gray zone of convection is not better than into coarser simulations. Large boundary areas with poor simulation quality suggest using large domains even with grid‐spacing coarser than usually accepted for CPM.European Union's H2020 Research and Innovation Programme http://dx.doi.org/10.13039/100010661Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165