Master's thesis defense by Andreas Pedersen

Title: Case Studies of Climate Change Impact on Heavy Precipitation

During the last one and a half century a rise in the temperature has been seen. This is mainly due to a rise in the concentration of greenhouse gasses. Various studies from the IPCC report show that the temperature rises with between 0.5 and 6 K, depending on the concentration of greenhouse gasses. When the temperature changes, so does the atmosphere’s ability to hold water vapour. This change is about 7% per degree Kelvin due to the Clausius-Clapeyron equation.

Other studies like [Berg, et al. 2013] and [Nikolina Ban, et al. 2015] have shown that the consequence of this change most likely is a doubling of the precipitation compared to the change in the amount of water in the atmosphere, according to the Clausius-Clapeyron equation. This will apply to both weather situations varying from a few hours to a full day. At the same time, it is seen that the convective precipitation responds more heavily to the change in temperature than the stratiform precipitation, which responds less active.

In this thesis it has been examined how changes in temperature at respectively 1.5, 3 and 6 K will affect the heavy precipitation and cloudbursts in Denmark. This examination has been done by making and analysing seven different weather situations. Five is based in the summer and two in the winter. The summer situations take place in the years 2010 to 2016. During these years there has been cloudbursts in Denmark. Simulations made in WRF are the basis of this study. The data is processed over 24 hours with the accumulated precipitation of both 30 minutes, 3 hours and 24 hours.

By generating the best straight line based on the precipitation, it is possible to examine the change in precipitation. A significant change in the amount of precipitation is shown in the results. This is for both the accumulated precipitation and all the simulations. Almost every simulation including the winter simulations showed an increase for the 30 minutes accumulated precipitation. This increase is 1.5-2 times bigger than the increase of water vapours as seen in the Clausius-Clapeyron equation. For one 1.5 K, one 3 K and some of the 6 K simulations the precipitation were three to five times higher than the change in the specific humidity. The winter simulations followed the best straight line, while a different pattern was seen in the summer simulations. Here almost every one of them showed an increase in heavy precipitation of more than 15 mm. This increase was above the best straight line.

In the summer simulations there were no significant increase of the slope of the best straight line at 3 hours accumulated precipitation. Compared to the 30 minutes accumulation, the slope is however still beneath the increase in heavy precipitation. At 3 and 24 hours accumulated precipitation, it is seen that it - for the winter simulations - match the Clausius-Clapeyron equation. However, for some of them a decrease in heavy precipitation is seen compared to the equation.

For the 24 hours accumulated precipitation in the summer simulations, the slope is steeper than the Clausius-Clapeyron ratio. However, a decrease is seen when compared to the 3 hours accumulated precipitation, but still it is about twice the theoretical values. When a Page iii time accumulation is made, the amount approaches the slope and has no deviations. These simulations have the same results as previous studies; that the precipitation during a meteorological day might have an increase twice that of the Clausius-Clapeyron equation. This thesis shows that a rise in temperature will intensify the frequency of cloudbursts and heavy precipitation.