A study on the processes that cause extreme precipitation on the west coast of Norway

Abstract

The purpose of this thesis was to get a deeper understanding of extreme daily precipitation events on the west coast of Norway. To examine the extreme precipitation on the west coast of Norway, daily precipitation events are investigated by using observations from 114 stations in the time period 1961-2009. The main focus is given to three of the 50 most extreme daily precipitation events; one winter, one spring and one fall event. By using the Weather Research Forecast model and by changing the sea surface temperature, changes in pattern and intensity for the three events are investigated. Parameters such as daily precipitation, specific humidity, relative humidity, vertical velocity and static stability are used to validate the findings. A general shift in the distribution of the daily precipitation is seen for both the winter and fall events, where the combination of vertical velocity and relative humidity seems to be the largest contributors. The change in intensity of daily precipitation differs greatly between the events, with over 20\% increase for the spring event and under 5\% for the fall event. Static stability combined with relative humidity can explain the different responses in the intensity for the three events. Two simple methods for calculating theoretical precipitation are introduced, but the change in both intensity and distribution makes it difficult for the two methods to produce good predictions of individual extreme daily precipitation events. By introducing thresholds for parameters such as relative humidity, time delay for hydrometeor production and fall speed of the precipitation in the simple methods, more accurate presentation of the daily precipitation may be produced.