An investigation of the accuracy and long term trends of ERA-40 over the Baltic Sea

Abstract: Meteorological measurements from the research station at Östergarnsholm in the Baltic Sea between 1995 and 2001 are used to evaluate the ERA-40 data base over the Baltic Sea. This includes comparisons of mean parameters and turbulent fluxes. The statistics concern all the data available when the measuring site was exposed to open sea conditions, and includes about 1500 hours of data. Seasonal data is compared as well. Regarding all the mean parameters compared, ERA-40 is partly in line with the results from the measurements, with various scatter. Temperature is too warm, wind speed is lower as well as relative humidity and sea surface temperature is systematically overestimated. The heat fluxes are generally overestimated by ERA-40 when compared to heat fluxes calculated by the bulk formulae, sensible heat flux with a mean value of 5W/m2 and latent heat flux with 9W/m2. The overestimation is larger for high fluxes and during high wind speeds. Friction velocity is too large in ERA-40. There are indications that the ERA-40 data over the Baltic Sea is somewhat influenced by land. A second objective for this study is to detect possible trends occurring at Östergarnsholm over a long term perspective with the aid of the complete 44 year long data set. An increasing trend in temperature can be observed, which is in agreement with other long term temperature series in the region.