Distribution of precipitation

In the mountains, snow is distributed very unevenly; enormous variations frequently occur over a small area. In order to assess both the availability of water resources for power generation and the likelihood of flood alerts, as well as to forecast the avalanche risk, it is crucial to understand this.

Although weather radar is a useful tool for measuring the amount of precipitation over large areas, studies have shown that the distribution of precipitation a few hundred metres above the ground is much more even than the distribution of snow deposits on the ground. Several possible causes are conceivable, including wind direction, weather situation, topographical features and slope angle. On a scale of around one hundred metres, statistical models are already capable of describing the variability of snow distribution with reasonable accuracy. Steepness, aspect, altitude zone and surface roughness are among the characteristics of the bare terrain that can serve as explanatory variables. These parameters can be derived from a digital terrain model. Once calibrated for a specific area, a statistical model can calculate large portions of the variability even without referencing the current snow distribution. Despite the research that has already been carried out, however, quantitative understanding of small-scale distribution remains in its infancy. In order to examine the small-scale snowfall patterns in Alpine terrain, in the Dischma experiment we trace the path of snow particles from their initial formation to the moment of contact with the ground. In our investigations we distinguish between processes that influence and determine the distribution of precipitation at three different elevations above the ground:

1.      The subject of interest at the highest elevation is the  formation of precipitation. What influence does the terrain exert on cloud microphysics and cloud dynamics? The rising of near-ground air masses upwind from a mountain ridge, for example, can give rise to condensation and local cloud formation. These constitute an additional source of moisture to promote the growth of snowflakes falling from higher layers of air. At the same time, however, consideration must also be given to the local wind field, which deposits the light snowflakes predominantly on the leeward side of the obstacle.

2.      Up to an elevation of 200 m above the ground, preferential depositing is the dominant process. Precipitation is deposited predominantly on the leeward side, typically after passing over a mountain ridge. We are endeavoring to establish the quantitative effect of this process on precipitation variability.

3.      On the ground we examine snow transport – looking at deposited snow grains that are dislodged from the snowpack and carried away by the wind.

Our aim is to understand and quantify these processes so well that they can be integrated in hydrological and meteorological models.


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