Jp2.13 a Re-evaluation of Upper Tropospheric Winds in Reanalyses near Svalbard

The two most widely used reanalysis data sets are the collaborative effort of the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) Reanalysis (hereafter, abbreviated to NNR) (Kalnay et al. 1996; Kistler et al. 2001), and the European Centre for Medium-Range Weather Forecasts (ECMWF) 15-year Reanalysis (ERA15) (Gibson et al. 1999). Both reanalyses have been evaluated extensively for a variety fields. The European Centre started a new reanalysis project (ERA-40) in 1999, and completed it in early 2003. The ERA-40 data set covers the period from September 1957 to August 2002, overlapping the earlier ERA-15 (Simmons and Gibson 2000). Many aspects of the reanalysis data are of high quality over regions with sufficiently abundant data. However, the accuracy of reanalyses is uncertain over areas with sparse data, particularly at high latitudes. Recently, Francis (2002) used rawinsonde data from two Arctic field programs, which were not assimilated into NNR and ERA-15, and compared them to reanalysis wind products for five layers between 1000 and 300 hPa. Both reanalyses exhibit large biases and are significantly too westerly and too northerly relative to these rawinsonde data. In addition, total wind speeds are too strong by 25 to 65 %. This study undertakes a more comprehensive evaluation of the accuracy of reanalysis atmospheric variables (wind components, as well as temperature, geopotential height and moisture fields) by comparing the two independent observed data sets of Francis (2002), the Coordinated Eastern Arctic Research Experiment (CEAREX) and the Lead Experiment (LeadEx), to the NNR and ERA-15 over the Arctic Ocean. The analysis is extended by considering the performance of ERA-40. Both average conditions and variability are considered.