Extremes temperatures and enthalpy in Finland and Sweden in a changing climate
Ari Venäläinen, Seppo Saku, Kirsti Jylhä
Though risks caused by harsh weather conditions are taken into account in the planning of nuclear power plants, some exceptional weather events or combination of different events may prevent normal power operation and simultaneously endanger safe shutdown of the plant. Extreme weather events could influence, for example, the external power grid connection, emergency diesel generators (blockage of air intakes), ventilation and cooling of electric and electronics equipment rooms and the seawater intake. Due to the influence of an intensified greenhouse effect the climate is changing rapidly during the coming decades and this change is expected to have an influence also on the occurrence of extreme weather events. In this report we have examined extreme temperatures. Enthalpy is a parameter that combines air temperature and air humidity and it is used in the design of air conditioning systems. Therefore, we have included also return levels of enthalpy in our analysis. The influence of climate change on extreme temperatures is analysed based on regional climate model simulations.
The reoccurrence times of high temperatures combined with high air humidity was analysed based on measurements made at five Finnish and three Swedish meteorological stations. Based on the observational records we find the 10 year return level of daily maximum temperature to be around 32°C and the 100 year return level around 35°C. If we look the return levels of warm and humid conditions then for example in Helsinki the 10 year return level of one week mean temperature in case mean air humidity is above 80% is 20.1°C. The 10 year return level of daily maximum enthalpy is around 60 kJ/kg and the 100 year return level almost 70 kJ/kg.
According to the climate model simulations the largest increase of 50-year return level of daily maximum temperature is found in southern Sweden and south-western Finland. By the end of this century the increase can be 3-5 °C. The largest change in the return levels of daily minimum temperature can be found in north-eastern Finland at the end of this century. This change can be even more than 10 degrees.