Evaluation of four global reanalysis products using in-situ observations in the Amundsen Sea Embayment, Antarctica

The glaciers within the Amundsen Sea Embayment (ASE), West Antarctica, are amongst the most rapidly retreating in Antarctica. Meteorological reanalysis products are widely used to help understand and simulate the processes causing this retreat. Here we provide an evaluation against observations of four of the latest global reanalysis products within the ASE region – the ECMWF Interim Re-analysis (ERA-I), Japanese 55-year Reanalysis (JRA-55), Climate Forecast System Reanalysis (CFSR) and Modern Era Retrospective-Analysis for Research and Applications (MERRA). The observations comprise data from four automatic weather stations (AWS), three research-vessel cruises and a new set of 38 radiosondes all within the period 2009-2014. All four reanalyses produce 2 m temperature fields that are colder than AWS observations, with the biases varying from approximately -1.8 °C (ERA-I) to -6.8 °C (MERRA). Over the Amundsen Sea, spatially averaged summertime biases are between -0.4 °C (JRA-55) and -2.1 °C (MERRA) with notably larger cold biases close to the continent (up to -6 °C) in all reanalyses. All four reanalyses underestimate near surface wind speed at high wind speeds (>15 m s-1) and exhibit dry biases and relatively large root-mean-square errors (RMSE) in specific humidity. A comparison to the radiosonde soundings shows that the cold, dry bias at the surface extends into the lower troposphere; here ERA-I and CFSR reanalyses provide the most accurate profiles. The reanalyses generally contain larger temperature and humidity biases, (and RMSE) when a temperature inversion is observed; and contain larger wind speed biases (~2 to 3 m s-1) when a low-level jet is observed