Reference period

1. Objective

The wind atlas should represent the present-day wind conditions in Finland. Statistical analyses have shown that in the period from 1989-2007 there has not been significant trends in the wind climate over Finland, and this period was therefore chosen as the reference period. Due to limited computer power, it is, however, not possible to run a high-resolution mesoscale model for a large domain (covering the whole Finland and a surrounding ~300 km in all directions) for a 19 year period. Hence, a method is needed to represent the monthly wind climate over the 1989-2007 period by an optimal selection of months from various years

2. Method

The "average reference year" was decided to be produced from 48 selected months, representing in principle four years. The selection of months was done though analyses of the ERA-40 and ERA-Interim data base from the ECMWF.

The following calculations were made separately for each month at 9 stations/sites, which were located at Lapland (Käsivarsi and Sodankylä), coast of the Gulf of Bothnia (Hailuoto, Vaasa, and Rauma), coast of the Gulf of Finland (Hanko and Kotka), and inland Finland (Keski-Suomi and Kainuu).

First we search the minimum values for parameters RS and RD. The minimum values have to be selected among 194 = 130 321 possible combinations of years.


RS measures the representativeness with respect to the wind speed distribution: k is the shape parameter and A the scale parameter of the Weibull distribution, i refers to the parameter value in a combination of four years, and the overbar refers to the parameter value in the 19 year data set. The factor 6 is applied to make the deviations of A and k equally important for RS.


RD measures the representativeness with respect to the distribution of wind directions. D is the wind direction sector: D1 = 0-30°, D2 = 30-60°, and so on. V is the wind speed, and P is the percentage of cases in each wind direction sector.
The final parameter to be minimized is