However, other quality measures like the root mean square error (

However, other quality measures like the root mean square error (RMSE) could deteriorate. Wind observations at coastal stations used for the development of the wind adjustment are described by Höglund et al. (2009) (see the previous section). In this study we focused on observations from Landsort for the period 1996–2008 after the recording switched from manual to automatic measurements (Figure 4). Sea ice observations are compiled from BASIS – a data bank for Baltic sea ice and sea surface temperatures (Udin et al. 1981). The digital data base was constructed by extracting information from Vorinostat ic50 reanalysed

ice and surface temperature maps from SMHI and the former Finnish Institute of Marine Research (today, the Finnish Meteorological Institute, FMI). Data are usually measured with a frequency of two maps per week during the ice season. The digital data were interpolated between measurements in order to obtain a daily time series for each year. When measurements were missing at the beginning (end) of the year, the first (last) available recording was used to fill in the dates for the daily time series. The data shown in

the present study are from the years 1980 to 2008. From the sea ice concentration data, the ice extent was calculated by summing all the grid areas with a sea ice concentration greater than 10%. At SMHI gridded SLP, 2 m air temperature, 2 m relative humidity and total cloud cover with a temporal resolution of three hours were compiled from observations since 1980 (e.g. Kauker & Meier 2003, Omstedt et al. 2005). In addition, 12 hourly accumulated precipitation fields are available at 06 and 18 UTC. Geostrophic wind speed was calculated and reduced to 10 m

wind speed by using a varying factor in the range between 0.5 and 0.6, depending on the distance to the coast (Bumke & Hasse 1989). Note that mean 10 m wind speeds calculated from geostrophic wind fields very likely overestimate mean observed 10 m wind speeds. Data from all available synoptic stations (about 700 to 800) covering the whole Baltic Sea drainage basin are interpolated on a 1° times 1° regular horizontal grid with respective latitude and longitude ranges of 50°N to 72°N and 8°E to 40°E. Thus, a two-dimensional univariate optimum interpolation scheme is utilized. Note that all stations are land-based: the data therefore suffer from a land-sea bias. For instance, air temperatures over the sea are expected to be slightly too high during summer and slightly too low during winter. However, the comparison between the ERA40 and the SMHI data bases suggests that the SMHI data also are of high quality over the sea (Omstedt et al. 2005). In the following we will refer to this gridded meteorological data set as the SMHI data.

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