Added Value and regional effects in the multidecadal trends of a very high-resolution regional climate long-term model simulation at the coasts of Northern Germany (Dissertation)
AbstractWind speed is a very important atmospheric variable, because it may cause storm surges, damages and high economic losses. Therefore, particularly for coastal areas and preventive protection measures, it is essential to learn more about changes in storminess, wherefore a long, homogeneous meteorological time series is needed. A consistent, gridded data set was computed with a regional climate model with a very high convection-permitting resolution. This is one of the very first atmospheric model simulations with such high resolution covering several decades.
This thesis aims to extend the knowledge of the added value and regional effects in the multidecadal trends with the focus on wind speed in a very high-resolution hindcast. Firstly, the effect of spectral nudging on the simulation with a small model domain size of about 700 x 500 km was examined. Subsequently it was investigated if the high-resolution regional climate model adds value in terms of high wind speeds in comparison to the driving regional climate model data set with a coarser grid distance. Furthermore, the question is answered weather the variability and long-term changes of different variables have any crucial regional effects, which justify such expensive high-resolution and long-term simulations.
In the first part of the thesis, it is shown that spectral nudging has only little effect on the simulation when the model region is relatively small and that it is not necessary for regional climate model domain sizes of only several hundred kilometers in diameter and over flat and homogeneous terrain.
Concerning the added value of the high-resolution simulation, it was found that the convection-permitting resolution simulation has, in most cases, a better agreement with observational data than the coarse grid resolution simulation. The added value is more distinct for the synoptic comparisons than for the multiple storm studies analyzed with statistical measures like the Brier Skill Score. This is mainly a result of the convection, which was not parameterized in this simulation. The high-resolution model simulation is also able to capture small-scale features, which indicates an added value for atmospheric phenomena such as convective precipitation or post-frontal cloud cover. However, the added value strongly depends on region, time period and variable.
Findings related to the long term changes show the most clear and significant positive trends in temperature and wind speed. Spatial structures in the trends are weak. The strongest regional details were found in the precipitation and 10 m wind speed like the city effects with lower trends in the mean wind.