Two hindcast simulations for 1961–2007 and four transient simulations for 1961–2100 of RCAO driven with either reanalysis data, ECHAM5 or HadCM3_ref with two different horizontal resolutions (25 or 50 km) were performed (Tables 1 and 2). In the scenario simulations the greenhouse gas emission scenario A1B
is assumed (Nakićenović et al. 2000). Unfortunately, the majority of the ensemble simulations described in section 2.1 were performed with RCA3 using Selleckchem CAL 101 a horizontal resolution of 50 km only. For the purpose of wind speed modelling this horizontal resolution is not sufficient because the orography and the spatial land-sea distribution are not properly resolved. The impact of the horizontal resolution on the mean wind speed (without modification) is shown in Figure 3. Mean wind speeds over the Baltic Sea simulated with 25 km resolution are up to 60% larger than those simulated with 50 km resolution. However, even with a horizontal
resolution of 25 km wind speed is still underestimated in RCA3 and in many other RCMs (Rockel & Woth 2007). This is true both for mean wind speed and even more so for high wind speed extremes. Most often these high wind speed extremes are associated with wind gusts. Therefore, many RCMs have been equipped with gustiness parameterizations to better represent wind extremes. In RCA3 gustiness is calculated following the wind gust estimate method by Brasseur (2001), assuming that wind gusts develop when air parcels higher up in the Everolimus solubility dmso boundary layer are deflected down to the surface by turbulent eddies (Nordström 2006). about According to Davis & Newstein
(1968) the measured mean wind is the maximum 10-minute mean wind over the last three hours, and the measured wind gust is the maximum two second mean wind over the last 10 minute period. Observations indicate that the relationship between peak gusts and mean wind speeds is linear, suggesting an approximately constant factor of 1.6 at 10 m height (Davis & Newstein 1968). This observed relation between gusts and mean wind speed makes it possible to use output from the gustiness parameterization to adjust the simulated wind speed extremes. Thus, we modified the simulated mean wind speed at 10 m height U10, utilizing simulated wind gusts Ugust, according to U10new=max(Ugust/1.6,U10). There is no adjustment for the wind direction. An example of the improvement is shown for the coastal station Landsort (Figure 4). Landsort is a well suited coastal station because for onshore winds (directions between 45 and 225°) the surrounding terrain causes relatively little disturbance. For further details of the method and results from other stations, the reader is referred to Höglund et al. (2009).