# Assessing the added value of the Intermediate Complexity Atmospheric Research (ICAR) model for precipitation in complex topography

The coarse grid spacing of global circulation models necessitates the application of downscaling techniques to investigate the local impact of a changing global climate. Difficulties arise for data-sparse regions in complex topography, as they are computationally demanding for dynamic downscaling and often not suitable for statistical downscaling due to the lack of high-quality observational data. The Intermediate Complexity Atmospheric Research (ICAR) model is a physics-based model that can be applied without relying on measurements for training and is computationally more efficient than dynamic downscaling models. This study presents the first in-depth evaluation of multiyear precipitation time series generated with ICAR on a inline-formula4×4 km2 grid for the South Island of New Zealand for an 11-year period, ranging from inline-formula2007 to inline-formula2017. It focuses on complex topography and evaluates ICAR at inline-formula16 weather stations, 11 of which are situated in the Southern Alps between inline-formula700 and inline-formula $M6inlinescrollmathmlnormal 2150\phantom{\rule{0ex}{0ex}}unit\mathrm{normal m}\phantom{\rule{0ex}{0ex}}\mathrm{normal m}.\mathrm{normal s}.\mathrm{normal l}$ 61pt10ptsvg-formulamathimgbccda4f3ebd8779f3fffbd3c5ab68286 hess-23-2715-2019-ie00001.svg61pt10pthess-23-2715-2019-ie00001.png (m m.s.l refers to meters above mean sea level). ICAR is assessed with standard skill scores, and the effect of model top elevation, topography, season, atmospheric background state and synoptic weather patterns on these scores are investigated. The results show a strong dependence of ICAR skill on the choice of the model top elevation, with the highest scores obtained for inline-formula4 km above topography. Furthermore, ICAR is found to provide added value over its ERA-Interim reanalysis forcing data set for alpine weather stations, improving the median of mean squared errors (MSEs) by inline-formula30 % and up to inline-formula53 %. It performs similarly during all seasons with a MSE minimum during winter, while flow linearity and atmospheric stability are found to increase skill scores. ICAR scores are highest during weather patterns associated with flow perpendicular to the Southern Alps and lowest for flow parallel to the alpine range. While measured precipitation is underestimated by ICAR, these results show the skill of ICAR in a real-world application, and may be improved upon by further observational calibration or bias correction techniques. Based on these findings ICAR shows the potential to generate downscaled fields for long-term impact studies in data-sparse regions with complex topography.

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Horak, Johannes / Hofer, Marlis / Maussion, Fabien / et al: Assessing the added value of the Intermediate Complexity Atmospheric Research (ICAR) model for precipitation in complex topography. 2019. Copernicus Publications.

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