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\begin{document}

\title{Implementation of orographic-drag anisotropy in all flow directions in
the Earth System Model CAS-ESM 2.0}



\author[1]{Jinbo Xie}%
\author[2]{Minghua Zhang}%
\author[1]{Qingcun Zeng}%
\author[1]{Zhenghui Xie}%
\author[1]{Hailong Liu}%
\author[1]{Zhaoyang Chai}%
\author[1]{Juanxiong He}%
\author[1]{He Zhang}%
\affil[1]{Institute of Atmospheric Physics, Chinese Academy of Sciences}%
\affil[2]{Stony Brook University}%


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  \date{May 27, 2024}


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\begin{abstract}
A reasonable representation of orographic anisotropy in earth system
models is vital for improving weather and climate modeling. In this
study, we implemented the orographic drag scheme, including 3-D
orographic anisotropy (3D-AFD), into the Chinese Academy of Sciences
Earth System Model version 2 (CAS-ESM 2.0). Three groups of simulations
named sensitivity run, medium-range forecast, and seasonal forecast
respectively were conducted using the updated CAS-ESM model together
with the original 2-D isotropic scheme (2-D) and the 3-D orographic
anisotropy for the eight-direction scheme (3D-8x) to validate its
performance. Sensitivity runs indicated that the simulated drag using
the original 2-D scheme did not change with the wind directions, while
the simulated drag using the updated 3D-AFD showed a smoother transition
than that using 3D-8x. The 3D-AFD and 3D-8x had also about 80\% larger
drag and smaller wind speed of 1m/s than that of the 2-D scheme.
Enhanced drag in the medium range and seasonal forecast using the
updated CAS-ESM both alleviated the bias of the overestimated wind speed
and the cold bias over mountain regions in the 2-D scheme. This was more
apparent in winter (0.4-0.5 m/s and \textasciitilde{}1K) than that in
summer (0.1 m/s and \textasciitilde{}0.1K) for the northern hemisphere
region, such as the Tibetan Plateau. The vertical wind profile was also
improved in the seasonal forecast. The results suggested that a
reasonable representation of the orographic anisotropy was important in
climate modeling, and the updated model of CAS-ESM with 3D-AFD
alleviated the bias of the mountain wind.%
\end{abstract}%



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