Atmospheric reanalyses covering the European region are mainly available as part of relatively coarse global reanalyses. The aim of this article is to present the development and evaluation of a next generation regional reanalysis for the European CORDEX EUR-11 domain with a horizontal grid spacing of approximately 6 km. In this context, a reanalysis is understood to be an assimilation of heterogeneous observations with a physical model such as a numerical weather prediction (NWP) model. The reanalysis system presented here is based on the NWP model COSMO by the German Meteorological Service (Deutscher Wetterdienst) using a continuous nudging scheme. In order to assess the added value of data assimilation, a dynamical downscaling experiment has been conducted, i.e. an identical model set-up but without data assimilation. Both systems have been evaluated for a 1 year test period, employing standard measures such as analysis increments, biases, or log-odds ratios, as well as tests for distributional characteristics. An important aspect is the evaluation from different perspectives and with independent measurements such as satellite infrared brightness temperatures using forward operators, integrated water vapour from GPS stations, and ceilometer cloud cover. It can be shown that the reanalysis better resolves local extreme events; this is basically an effect of the higher spatio-temporal resolution, as known from dynamical downscaling approaches. However, an important criterion for regional reanalyses is the coherence with independent observations of high temporal and spatial resolution, resulting in significant improvement over dynamical downscaling. The system is intended to become operational within a year, continuously reprocessing and evaluating longer time periods. The reanalysis data are planned to become available to the research community within a year.
Atmospheric reanalysis data sets are a major tool to assess climate and its variability for past decades in a broad spectrum of applications. Nowadays, global reanalysis systems are available in third and fourth generation. They benefit from continuous improvements of numerical models and data assimilation schemes thus utilizing a growing number of observing systems. In the past decade, global reanalyses have been complemented by several regional reanalyses with enhanced horizontal resolution and appropriate model physics. Due to the considerable technical overhead associated with the comparison of reanalyses, it is difficult for users to identify an optimal data set for their needs. Therefore, this study strives to provide a comprehensive intercomparison of the representation of climate in various reanalyses. The study compares four global reanalyses and three regional reanalyses over Europe and North America with in-situ observations. The focus lies on the representation of near-surface temperature and precipitation expressed through various climate indices. The comparison shows that performances of the different reanalyses vary considerably depending on the parameter(s) of interest, the region and the type of aggregation. The main findings are that regional reanalyses provide added value especially with regard to precipitation. While ECMWF's newest reanalysis ERA5 is in better agreement with observations compared to the other global reanalyses for most climate indices over Europe, it is still outperformed by its predecessor ERA-Interim over North America. In order to support users in identifying an appropriate reanalysis for their needs, the study is complemented by comprehensive supplemental material.
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