Sensitivity of Convection Permitting Simulations to Lateral Boundary Conditions in Idealised Experiments
• Bodo Ahrens,
• Nora Leps
Bodo Ahrens
IAU, Goethe University Frankfurt am Main

Corresponding Author:bodo.ahrens@iau.uni-frankfurt.de

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Nora Leps
Deutscher Wetterdienst
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## Abstract

Limited-area convection-permitting climate models (CPMs) with horizontal grid-spacing less than $4$\,km are being used more and more frequently. CPMs represent small-scale features such as deep convection more realistically than coarser regional climate models (RCMs), and thus do not apply deep convection parameterisations (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 idealised so-called Big-Brother (BB) experiments with the LAM COSMO-CLM ($\approx$ $2.4$\,km). Deep convection was triggered by idealised hills with driving data from simulations with different spatial resolutions, with/without a deep CP, and with different nesting frequencies and LBC formulations. All our nested idealised $2.4$\,km Little-Brother (LB) experiments performed worse than a coarser CPM simulation ($4.9$\,km) using a four times larger computational domain, but with only 50\% computational cost. A boundary zone of $>100$ grid-points of the LB could not be interpreted meteorologically because of spin-up of convection and boundary inconsistencies. A host with grid-spacing in the so-called grey zone of convection (ca. $4$ - $20$\,km) was not advantageous to the LB performance compared to an even coarser host. The LB performance was insensitive to the applied LBC formulation and updating (3-hourly or better). Therefore, our CPM experiments suggested opting for a larger domain instead of a higher resolution even if coarser than usual (i.e., $> 4$\,km). Better preconditioning the convectivity at the CPM inflow boundaries might decrease the spin-up zone’s depth.