Markus Niklaus

Two-dimensional adaptive mesh refinement simulations of colliding flows

Context. Colliding flows are a commonly used scenario for the formation of molecular clouds in numerical simulations. Turbulence is produced by cooling, because of the thermal instability of the warm neutral medium. Aims. We carried out a two-dimensional numerical study of colliding flows to test whether statistical properties inferred from adaptive mesh refinement (AMR) simulations are robust with respect to the applied refinement criteria. Methods. We compare probability density functions of various quantities, as well as the clump statistics and fractal dimension of the density fields in AMR simulations to a static-grid simulation. The static grid with 2048 2 cells matches the resolution of the most refined subgrids in the AMR simulations. Results. The density statistics are reproduced fairly well by AMR. Refinement criteria based on the cooling time or the turbulence intensity appear to be superior to the standard technique of refinement by overdensity. Nevertheless, substantial differences in the flow structure become apparent. Conclusions. In general, it is difficult to separate numerical effects from genuine physical processes in AMR simulations.

Land Degradation in South Africa - A Degradation Index Derived from 10 Years of net Primary Production Data

Dry regions such as arid southern Africa are strained by unfavourable climatic conditions. Intensive land use as rangeland and for livestock farming leads to additional encroachment of these ecosystems. The consequence of this long-time stress is degradation in terms of loss of the vegetative cover and productivity. Albeit these are known facts there is still a lack of objectiveness in the long term assessment of degradation on a larger scale. We present a method of applying remote sensing time-series in a vegetation model that helps to fill this gap. The approach is based on time-series of the vegetative productivity computed by our vegetation model BETHY/DLR (Biosphere Energy Transfer Hydrology Model). The used data included SPOT-VGT LAI (Leaf area index) and ECMWF meteorology time-series for the period of 1999–2010. The trend-analysis of model output and climatic input results in a new land degradation index (LDI) that distinguishes between climatic and human-induced reduction of vegetative productivity.

Numerical Models of Turbulence in Isothermal and Thermally Bistable Interstellar Gas

We present a novel subgrid scale model that is applicable to highly compressible turbulence. In large eddy simulation of forced supersonic turbulence, this subgrid scale model reduces the bottleneck effect in the turbulence energy spectra and it allows for a prediction of the local rate of turbulent energy dissipation as well as the turbulent pressure. The latter has an impact, for instance, on the gravitational support of turbulent gas and the efficiency of star formation in elaborate large-scale simulations of disk galaxies. We also report results from turbulence simulations with parameterized radiative cooling, in which the effects of thermal processes on the scale of star forming clouds was investigated.