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\begin{document}

\title{Sensitivity of Convection Permitting Simulations to Lateral Boundary
Conditions in Idealised Experiments}



\author[1]{Bodo Ahrens}%
\author[2]{Nora Leps}%
\affil[1]{IAU, Goethe University Frankfurt am Main}%
\affil[2]{Deutscher Wetterdienst}%


\vspace{-1em}



  
  \date{May 17, 2024}


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\begin{abstract}
Limited-area convection-permitting climate models (CPMs) with horizontal
grid-spacing less than \$4\$\textbackslash{},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 (\$\textbackslash{}approx\$ \$
2.4\$\textbackslash{},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\$\textbackslash{},km Little-Brother (LB) experiments performed
worse than a coarser CPM simulation (\$4.9\$\textbackslash{},km) using a
four times larger computational domain, but with only
50\textbackslash{}\% computational cost. A boundary zone of
\$\textgreater{}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\$\textbackslash{},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., \$\textgreater{} 4\$\textbackslash{},km). Better
preconditioning the convectivity at the CPM inflow boundaries might
decrease the spin-up zone's depth.%
\end{abstract}%



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