Alpine Air Mass Climatology and its Relation to the changing Snow Line

A changing snow line - seasonal and annual - exhibits a strong societal and economical impact to the Alpine region. The annual hydrological cycle is being changed, with consequences e. g. to water supply and hydroelectric power generation. The danger of flooding is increased considerably when the snow line is increasing, as more runoff is generated even with an unchanged precipitation amount. Winter sports are heavily affected by a decreasing snow pack. Traffic is highly sensitive to snowy road conditions. There is hardly an economic sector in the Alpine region, which is not affected by (changing) snow conditions.

So far, scientific investigations have concentrated their focus mostly on the relation (air-)temperature versus snow cover (duration). This relation, however, is highly complex. The build up of a snow cover needs a single precipitation event with a sufficient low wet bulb temperature. The melting of a snow pack is dependent on a positive energy balance. Besides the radiation budget again the wet bulb temperature plays the determining role for melting.

From the VERACLIM data set, a high resolution reanalysis of the Alpine Atmosphere for the last 25 years, the wet bulb temperature with a 3 hourly resolution has been used to investigate the level of snow during precipitation and the melting conditions. The high temporal resolution is necessary as most precipitation events are related to fronts, with a significant short term air mass change. Some trends have been found frightingly large, up to +400 Meters during the last quarter century. Interestingly enough, the weaker increase has been found in winter as compared to the summer.

The presentation will give an overview of the trends at one Alpine grid point and an outlook to a further spatial evaluation of the data set.