Using wind fields from a high resolution atmospheric model for simulating snow dynamics in mountainous terrain

It is widely known that the snow cover has a major influence on the hydrology of Alpine watersheds. Snow acts as temporal storage for precipitation during the winter season. The stored water is later released as snowmelt and represents an important component of water supply for the downstream population of large mountain-foreland river systems worldwide. Modelling the amount and position of the snow water stored in the headwater catchments helps to quantify the available water resources and to estimate the timing of their release. The presented work investigates wind induced snow transport processes which are considered to be crucial for the snow distribution in Alpine catchments. In contradiction to the importance that is attributed to this process, there are only a few studies available which have quantified the transport intensities on the catchment scale. This can be attributed to the fact that the even today not much is known about the spatial characteristics of wind fields which are the driving force for snow transport processes. The presented thesis tries to overcome this lack of information by using physically based wind fields predicted by an atmospheric model (PSU_NCAR MM5 model) for the modelling of the snow cover (simulated by SnowModel). All of the used models are described in great detail in the literature, validated in many different regions, and can be seen as applicable with regard to the goal of this work. As snow transport processes are particularly important on a comparatively small scale a numerical inclusion of the responsible processes into regional models is inadequate. Hence, while this study itself mainly uses smaller scale physically based models, a parameterisation scheme is presented at the end of this thesis that is able to incorporate its main findings into larger scale models. All of the presented work was carried out at the Berchtesgaden National Park. The site is highly appropriate because of the extremely rough terrain and the good accessibility. Furthermore, the instrumentation of the area is comparatively good and the data sources (GIS, field campaign data) are excellent. The thesis deals with the winter seasons (August July) 2003/2004 and 2004/2005. For this period, data of 5 meteorological stations, 1 field campaign and two Landsat ETM+ images were available. As mentioned before, physically based wind fields were used as input for the snow transport modelling. An operational coupling between atmospheric model and snow transport model was not pursued because of the high computational costs of the atmospheric model. Thus, a library of representative wind fields was produced in advance and linked to the snow transport model via operational German weather service Lokalmodell results. This becomes possible because of the comparability of a MM5 model layer with one of the Lokalmodell model layers. To link the wind field library to the snow model all of the predicted MM5 wind fields were characterised by information available from the Lokalmodell. This enable an easy detection of the MM5 wind field which is closest to the real climatic wind conditions at any Lokalmodell time step (1 hour). The produced MM5 wind fields have a spatial resolution of 200 meters. As an initial check if the snow cover simulation of SnowModel in association with the wind field library delivers adequate results with respect to the snow distribution, model runs were first carried out at the 200m scale. An analysis of the results showed that the coupled routine delivers acceptable results. It could be seen that with the use of the MM5 wind fields, the snow cover becomes more anisotropic and that transport processes over crests as well as sublimation processes are predicted to become more intensive. Nevertheless, a higher resolution was needed to quatify the effects and to validate the results. In a subsequent step the MM5 wind fields were downscaled to a 30m resolution. The downscaling procedure lead to a better agreement between modelled and measured wind speeds. The resulting 30m wind fields were used for high resolution model runs which were validated on the basis of the field campaign and remotely sensed data. A comparison with model runs using wind fields interpolated from station data showed that the runs performed with the MM5 wind fields deliver more consistent and comprehensible results. Subsequently, the validity of the model is discussed on the basis of selected results. High resolution model results indicated that snow transport processes are effective at high elevations but virtually negligible for regions below of 1800m a.s.l.. Furthermore, it could be seen that the correct estimation of snow transport from the surrounding areas to glaciers becomes possible by using the MM5 wind fields. Very high modelled sublimation rates at the mountains crests are discusses with respect to their importance on the water balance. Furthermore, the influence of preferential snow deposition and snow slides which were not numerically predicted in this work were discusses. Additionally, the applicability of atmospheric model results as input for land-surface models could be confirmed. In a final step a model scheme is presented that would make the generated information available for regional scale models. This model parameterization scheme which is based on the modelled 30m snow water equivalent distribution within the test area was used for this area. The scheme allows for a quick and simple description of the subscale snow heterogeneity in regional scale models. This can lead to considerable model improvements with respect to the description of the energy and moisture fluxes to and from the surface. An accurate description of these fluxes is essential for an accurate simulation of the melt period and, therefore, for an acceptable calculation of the runoff generation in larger scale models.