From the clouds to the ground - Effects of flow-precipitation interactions on snow distribution in complex alpine terrain

Knowledge about the spatial distribution of seasonal snow is essential e.g. to efficiently manage fresh water resources or for hydro-power companies. The large-scale gradient of snow accumulation over mountain ranges is mainly determined by lifting condensation and downstream drying. On the slope-scale, snow redistribution by wind and avalanches is the main source of variability. On a mountain-ridge to mountain-valley scale, small-scale orographic precipitation enhancement and preferential deposition interact and lead to asymmetric snow distribution across mountain ridges. However, their relative importance is barely known and the characteristics of preferential deposition are still under debate. Yet, especially in a changing climate, which may go along with modified dominant wind directions, it is important to understand precipitation processes shaping the snow cover. Therefore, we investigate terrain-flow-precipitation interactions and their effect on mountain-ridge to mountain-valley scale snow precipitation and deposition in complex alpine terrain. To this end, the Weather Research and Forecasting (WRF) model is set up to downscale Consortium for Small-Scale Modeling (COSMO) analysis to a horizontal resolution of 50 m using a nesting approach. At 450 m resolution these simulations fairly represent large-scale precipitation variability with respect to high-resolution operational weather radar precipitation estimates, capturing the effect of large-scale orographic enhancement. Although, the model misses substantial small-scale precipitation variability even at a 50 m resolution, we demonstrate that the lee-side flow field and mountain-ridge scale precipitation processes start to be represented at this resolution. Thus, a model resolution of at least 50 m is required to represent mountain-ridge to mountain-valley scale precipitation patterns, which is far higher than model resolutions conventionally used to simulate snow water resources in alpine regions. Small-scale orographic precipitation enhancement and mean advection are estimated to increase lee-side precipitation by up to 20%, while a conservative estimate of (near-surface) preferential deposition reveals lee-side snow deposition enhancement on the order of 10%. However, both processes strongly depend on atmospheric conditions such as atmospheric humidity or the strength of mean advection. The peculiarity of the lee-side flow field is of particular importance for the spatial distribution of snow accumulation, especially with regards to preferential deposition. This is further demonstrated by a very persistent eddy-like structure on the leeward side of the Sattelhorn ridge in the Dischma valley (Davos, CH), as reported based on Doppler wind lidar measurements and with corresponding flow field simulations at a resolution of 25 m by the Advanced Regional Prediction System (ARPS). Corresponding snow accumulation, assessed by the means of terrestrial laser scanning, confirms that snow distribution in very steep terrain is strongly influenced by post-depositional snow redistribution. Nevertheless, we can report a certain agreement of simulated pre-depositional precipitation patterns across mountain ridges with photogrammetrically determined snow distribution. Overall, we demonstrate the necessity and value of high-resolution snow precipitation measurements and simulations, and we contribute to the understanding of the small-scale variability of snow distribution in alpine terrain.

The effect of seasonal soil frost on the alpine groundwater recharge including climate change aspects

Daniel Bayard

In alpine areas, the snow cover plays an important role as a water reservoir. Water is stored as snow over the winter and released in spring, recharging mountain aquifers through infiltration. These aquifers are essential, especially for supplying water for human activities during dry seasons. Numerous studies have shown that locally soil frost can drastically reduce the water infiltration. However, we know much less about the hydrological impact of soil frost at a larger scale, in particular with regard to groundwater recharge. This was our motivation for initiating an extensive field experiment in the southern Swiss Alps. Several methods at different spatial scales were adapted to explore (a) the local effect of a partially frozen ground on the snowmelt discharge in an alpine area, (b) the key processes influencing groundwater recharge during snowmelt periods, such as snow cover and soil frost evolution, snowmelt water runoff types, (c) the large-scale effect of soil frost on the aquifer recharge, and (d) winter situations that are critical with respect to flooding. In order to take into account spatial, altitudinal and climatic differences, the field study was run for two winter seasons at Gd St Bernard (2500 m), a site with very high winter precipitation and strong winds, and at Hannigalp above Grächen (2100 m), a ski resort with a rather dry winter climate. The different components of the soil water balance (lateral runoff, deep percolation, liquid soil water content) were measured on delimited plots of 5 m2. Additionally, a dye tracer experiment visualized the snowmelt infiltration patterns into the frozen, respectively non-frozen ground. At Gd St Bernard, investigations of the liquid water content and the soil temperature were additionally conducted at two secondary sites, differing in their orientation, so as to gain a better insight of the spatial soil frost expansion. Finally, an analysis of the water-table variation at the village of Grächen (some 500 m below Hannigalp) during the snowmelt was carried out for a 10-year time period, including our experimental seasons. We complemented our field investigations by using a numerical soil-snow-atmosphere transfer model that simulates the seasonal development of the soil frost and the snow cover, as well as lateral and vertical water flow in the top soil. This model was used as an upper boundary condition to a 2-D groundwater model, to calculate the outlet flow at the base of a simplified alpine aquifer. The two winters investigated had contrasting meteorological conditions, which resulted in intriguing differences and similarities with respect to the soil physical conditions: the first one having a thick snowpack and hardly any soil frost, and the second one with hardly any snow until January and a deep and persistent soil frost. In spite of a very thick snowpack, the water balance measurements from winter 2000/2001 showed for both sites zero to low surface and subsurface flow due to the fact that soil frost was local and the soil permeability high. During the next winter, approximately 25% of the total meltwater run off laterally. The soil infiltration capacity was mainly reduced by the presence of a basal ice sheet, caused by the freezing of meltwater after a mid-winter snowmelt event. The field data were used to calibrate and validate a physically-based one-dimensional model. To allow an accurate parameter setting for the water repellent soil of Hannigalp, laboratory infiltration experiments on soil columns were additionally carried out. Using the resultant information, the model predicted correctly most measured soil parameters, in particular good matching was observed between measured and simulated snowmelt discharge. At Gd St Bernard, the simulated results were less satisfactory, especially during the first winter, when large discrepancies were noted between measured and simulated lateral runoff. Apart from the shortage in the soil parameter characterization, the deviation was a result of the simplified description of the experimental field. The model was, for example, not able to simulate surface runoff generated by a steep slope under unfrozen conditions, as the soil surface is considered by the model to be flat. Nevertheless, results were accurate enough to reproduce the occurrences of snowmelt events and the accumulated values of snow-melt discharge pathways under frozen conditions. Long-term weather data were used to simulate the soil frost depth at both sites. The simulation results showed that the probability of the occurrence of frozen soils was lowest on slopes with south or north exposure. On southerly exposed plots, the heat stored in the soil during the summer inhibited the development of deep and persistent soil frost, while most pore ice thawed due to underneath heating. On north plots, the building of soil frost was rare, as early snowfall insulated the ground from the atmosphere. However, during snow-poor winters, the frost penetrated deep into the ground, and the low underneath heat flux at that location hardly affected the frost depth until snowmelt. The snow depth and average soil temperature were hence the two components affecting the frost depth extent. The altitude soil frost variation was related to the spatial extent of these two components. The higher the altitude, the higher the precipitation and the lower the mean air/surface temperature. Simulating the whole Grächen recharge area, seasonal soil frost was rare between 1800 m and 2000 m, as the snowpack was thick enough to insulate the ground, and the soil was warm enough to inhibit deep soil frost. In lower areas, the shallow snowpack enabled the soil to freeze during most winters, whereas at higher areas, deep and persistent soil frost was simulated due to the low underneath heating. A 10-year water-table depth record was used to examine possible large-scale effects of seasonal soil frost. We noted that the water-table rise at snowmelt was lowest at the end of winters with extensive soil frost. However, such winters were characterized by low precipitation, and reduced snow cover. In contrast, winters with little soil frost were mostly snow rich. Consequently, only a reduced number of winters were directly comparable. Considering these winters, the reduction in the water-table rise at snowmelt due to seasonal soil frost varied only between 10 and 30%, as, during snowmelt, most meltwater was able to re-infiltrate into the very permeable ground in lower unfrozen areas. We may assume that a soil with a smaller infiltration capacity can increase the influence of the soil frost on the aquifer recharge. Finally, we investigated the effect of a changing climate on the hydraulic and thermic regimes at different altitudes in the area of Grächen. We assume an increase in the air temperature of 2°C generates deeper soil frost on lower areas, as the resultant smaller snow cover does not insulate the soil from the atmosphere any more. However, it does reduce the soil frost on higher, snow rich areas, due to the warmer air temperature. As a whole, minor changes were simulated on the mean groundwater recharge, as more/less meltwater was able to infiltrate in higher/lower areas. By increasing additionally the precipitation by 15%, the soil frost diminished slightly over the whole catchment. By combining both climate change scenarios, we may expect an increase in extreme events, as, on the one hand, the rainfall intensity increases, and, on the other hand, rain on snow events over frozen soils occurs more often in lower areas. In practice, this work shows the importance of better investigating the hydrothermal behaviour of alpine regions. Such a knowledge is necessary, as more and more efforts are put into managing efficiently, durably and globally the available water resources. This work stresses also the necessity to consider seasonally soil frost in alpine risk management. As an example, flooding is mostly a consequence of strong rain precipitation at high altitudes combined with heavy snowmelt. Obviously, the risk of extreme runoff events increases during frozen winters, when the soil infiltration capacity is further reduced by the soil ice. Also, it has been shown that unstable slopes are mostly related to specific hydrogeological situations, like strong precipitation, which induces a sharp increase in the underneath water circulation. The melt water in spring is hence a potential activating factor, and a frozen soil may reduce the risk of sliding, as less water infiltrates into the ground.

EPFL2003

Modelling Drifting Snow Sublimation and Investigating Small-Scale Snow Transport

Christine Dorothea Groot Zwaaftink

Drifting snow has a large influence on the mountain snow cover and the overlying atmospheric boundary layer. Consequently, drifting snow also affects avalanches, snow hydrology, vegetation and climate. Considerable effort is devoted to understanding the process in order to model the effects on the mountain snow cover or the overlying atmosphere. This thesis work aims to I) provide estimates of drifting snow sublimation within a small catchment in the Swiss Alps and II) test an alternative method to describe snow transport on short time and small spatial scales. Drifting snow sublimation moistens and cools the surrounding air and causes a loss of snow mass. Based on the feedback mechanisms integrating air temperature, humidity and snow concentration, it should be seen as a self-limiting process. Drifting snow sublimation (including these feedback mechanisms) has been implemented in Alpine3D, which is a high-resolution snow transport model for alpine terrain. This model has been applied to a small area in the Swiss Alps for a case study of approximately two days. Results show that, in general, the negative feedbacks of sublimation on the snow mass concentration, air temperature, and humidity are small. They are, however, relevant on the slope scale, as could be retrieved from analyzing the deposition on a leeward slope. Compared to a reference simulation without sublimation, drifting snow sublimation reduced the deposition by approximately 12% on this leeward slope when omitting all feedback mechanisms. In a simulation where all feedbacks were included, drifting snow sublimation reduced the snow deposition on this slope by 10%. Drifting snow sublimation thus appears to be a significant process for a leeward slope influenced by drifting snow. On the contrary, the spatially averaged reduction of deposition due to drifting snow sublimation in the complete modelled domain was 2.3%. During this case study, this value was comparable to surface sublimation. At the same field site a simulation of a complete season was performed and in this case, sublimation of drifting snow was accounted for only in the absence of concurrent precipitation. As for the case study, the slope affected the most by drifting snow sublimation was shown to be a leeward slope for SE wind. This could be explained by generally warmer and dryer conditions during periods with SE wind. In this particular slope, the snow amount was reduced by about 1.8% due to drifting snow sublimation. For the complete domain however, the model results indicate that drifting snow sublimation is much smaller than surface sublimation and negligible for the total water equivalent. Thus, at the studied field site drifting snow sublimation seems only significant locally or on short time scales. Limitations of drifting snow simulations based on mean wind fields are seen as the variability of the snow cover is underestimated on scales of few meters. Moreover, these models are also not able to represent snow transport on very short time scales. High-frequency observations of drifting snow and sand saltation have shown that turbulent structures have an influence on the snow or sand mass flux. Improvements in modelling drifting snow on such time scales can thus be reached by considering turbulence. Here, a combination of wind fields from large eddy simulations and a Lagrangian stochastic model was used to describe snow particle trajectories over flat terrain. This model reproduced a time series of the snow mass flux that qualitatively corresponds to observations from the Weissfluhjoch Versuchsfeld. Furthermore, results show that the use of the surface shear stress rather than the friction velocity, as in conventional models, resulted in a spatial variation of drifting snow despite homogeneous snow properties and the flatness of the considered terrain. The distribution of the surface shear stress in space and time was found reasonable yet narrow based on the ratio between the standard deviation and the mean shear stress, which was 0.17 compared to a value of 0.4 reported for measurements. Nonetheless, this variable surface shear stress played a key role in the modelled spatial variation of drifting snow. This also points at a relevant influence of the particle aerodynamic entrainment process for drifting snow. The results thus show that this model seems to overcome limitations of conventional three-dimensional snow transport models as it gives a more realistic description of drifting snow on short time scales and it may be used to gain further insight in drifting snow.

EPFL2013

Stochastic modeling of snow transport and hydrologic response in alpine terrain

Francesco Comola

The scientific community has developed a keen interest in the processes driving the hydrologic cycle in alpine regions. The concern mainly stems from the vulnerability of snow-covered environments to the warming temperatures, such that entire ecological and social systems are at stake. Snow and ice storages in alpine regions are, in fact, fundamental water resources for large and dry lowland areas of western Americas, central Asia, northern India, and southern Europe. Snowmelt is also the principal control on the hydrologic and thermal regimes of alpine streams, which act as ecological corridors for a wide range of aquatic species. Despite the growing body of literature on the subject, the dynamics of some relevant processes are still unclear. Here, we aim at providing a deeper insight into the preferential deposition of snowfall, the wind-driven erosion and redistribution of snow, the fragmentation of drifting snow crystals, the stream temperature dynamics, and the effects of the solar radiation pattern on the hydrologic response of alpine catchments. We address our scientific questions with theoretical and numerical models that cast the complex transport phenomena in stochastic frameworks. The first part of this thesis focuses on snow transport. We combine a Lagrangian-stochastic model of particle dynamics, large-eddy simulations, and an immersed boundary method to investigate the processes driving the heterogeneous snow distribution in alpine terrain. Our results suggest that near-surface flow-particle interactions reduce snowfall deposition on the wind- and leeward slopes of the mountains, while a larger amount of snow accumulates on the hilltop and the surrounding flat terrains. Moreover, drifting and blowing snow can significantly change the snow depth distribution by eroding the hilltop and replenishing the leeward side of the ridge. We then propose a snow crystal fragmentation theory, whose assumptions are tested with discrete element simulations, to understand the transition from the size distribution of snowflakes to that of blowing snow particles. Our findings suggest that a range of scale-invariance in the blowing-snow size distribution emerges from the fractal geometry of snow crystals. Moreover, we show that the fundamental laws of energy and momentum conservation allow us to predict the number of particles ejected upon collision of drifting grains with a snow surface, characterized by arbitrary particle size distribution and cohesion. The second part of the thesis focuses on hydrological processes in alpine catchments. We first derive a theoretical framework, based on a residence time distribution approach that describes the coupled water and energy transport processes at hillslope scale. We then account for the proposed theory in a spatially explicit hydrological model to simulate stream flow and stream temperature dynamics in alpine catchments having arbitrary degrees of geomorphological complexity. Our model results highlight that highly heterogeneous advective and non-advective energy fluxes in the stream network yield water temperatures of remarkable spatial and temporal variability. Finally, we show that the effects of different solar radiation patterns on the snow-dominated hydrologic response are scale-dependent, i.e., significant at small scales where slopes present a predominant orientation, and almost negligible for catchment sizes larger than the aspect correlation scale.

EPFL2017