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Dataset - Assessing the Added Value of the Intermediate Complexity Atmospheric Research Model (ICAR) for Precipitation in Complex Topography

Horak, Johannes; Hofer, Marlis; Maussion, Fabien; Gutmann, Ethan; Gohm, Alexander; Rotach, Mathias

The coarse grid spacing of global circulation models necessitates the application of climate downscaling to investigate the local impact of a changing global climate. Difficulties arise for data sparse regions in complex topography which 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 model (ICAR) 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 multi-year precipitation time series generated with ICAR on a 4x4km² grid for the South Island of New Zealand for the eleven-year period from 2007 to 2017. It focuses on complex topography and evaluates ICAR at 16 weather stations, eleven of which are situated in the Southern Alps between 700 m MSL and 2150 m MSL. ICAR is diagnosed 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 4 km. Furthermore, ICAR is found to provide added value over its ERA-Interim reanalysis forcing data set for alpine weather stations, improving mean squared errors (MSE) up to 53%. It performs similar during all seasons with an MSE minimum during winter, while flow of higher linearity and atmospheric stability were 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. These results show the skill of ICAR in a real-world application, and may be improved upon by further observational tuning. Since ICAR is a computationally efficient and physics-based model, it has potential to generate downscaled fields for long term impact studies in data sparse regions with complex topography.

The research presented has been funded by the Austrian Science Fund (FWF) grant 28006-N32. The computational results have been achieved with the high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation and with the HPC infrastructure LEO of the University of Innsbruck. The National Center for Atmospheric Research is sponsored by the US National Science Foundation. Furthermore the authors thank the National Institute of Water and Atmospheric Research, New Zealand, and in particular Christian Zammit, for providing support, the weather pattern classifications, the VCS gridded rainfall data set and the data from the weather stations as specified in Tab. 1. Research on Brewster Glacier is supported by the Department of Geography, University of Otago, New Zealand; the National Institute of Water and Atmospheric Research (Climate Present and Past CLC01202); and the Department of Conservation under concession OT-32299-OTH.
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