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

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

Gerber, Franziska  
2018

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.

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Type
doctoral thesis
DOI
10.5075/epfl-thesis-8636
Author(s)
Gerber, Franziska  
Advisors
Lehning, Michael  
•
Mott-Grünewald, Rebecca Maria  
Jury

Prof. Dimitrios Lignos (président) ; Prof. Michael Lehning, Dr Rebecca Maria Mott-Grünewald (directeurs) ; Prof. Satoshi Takahama, Prof. Heini Wernli, Dr Vincent Vionnet (rapporteurs)

Date Issued

2018

Publisher

EPFL

Publisher place

Lausanne

Public defense year

2018-06-29

Thesis number

8636

Total of pages

160

Subjects

Terrain-flow-precipitation interactions

•

Snow precipitation and accumulation distribution

•

Lee-side flow field

•

Preferential deposition

•

Cloud dynamics

•

Spatial variability

•

Remote sensing

•

High-resolution numerical simulations

EPFL units
CRYOS  
Faculty
ENAC  
School
IIE  
Doctoral School
EDCE  
Available on Infoscience
June 14, 2018
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/146858
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