Andrea Vajda

Significance of snowpack for root-zone water and temperature cycles in subarctic Lapland.

Abstract Snowmelt timing is a critical factor for tree growth in high latitudes, but threshold conditions with respect to soil moisture availability and soil temperature for the root-zone processes are not well known. We monitored snowpack thickness, air and soil temperature, and water content in the soil, sapwood, and roots of downy birch (Betula pubescens Roth.) in Finnish Lapland through 1999–2003. An extreme cold event in January 1999 (TAIR  =  −49°C) resulted in soil freezing (at 10-cm depth) down to T10  =  −26°C at a snow-free site, but beneath the 50-cm-thick snowpack the soil temperature was T10  =  −0.5°C. Snowmelt water was able to infiltrate partially frozen soil sequences, such that an increase in water content of the soil and birch roots occurred two to six weeks before soil temperatures rose notably above 0°C. The soil T10 reached +0°C a week after the disappearance of snow. The increase in water content of birch trunks was coincidental with the air temperature rises notably above 0°C. The systematic interseasonal pattern of water content in the birch root-trunk system, i.e. high peaks in late winter–early spring and fall, suggests sap flow in downy birch.

Severe weather affecting European transport systems: the identification, classification and frequencies of events

Severe weather can have serious repercussions in the transport sector as a whole by increasing the number of accidents, injuries and other damage, as well as leading to highly increased travel times. This study, a component of the EU FP7 Project EWENT, delineates a Europe-wide climatology of adverse and extreme weather events that can be expected to affect the transport network. We first define and classify the relevant severe weather events by investigating the effects of hazardous conditions on different transportation modes and the infrastructure. Consideration is given to individual phenomena such as snowfall, heavy precipitation, heat waves, cold spells, wind gusts; a combined phenomenon, the blizzard, is also considered. The frequency of severe weather events, together with the changes in their spatial extension and intensity, is analyzed based on the E-OBS dataset (1971–2000) and the ERA-Interim reanalysis dataset (1989–2010). Northern Europe and the Alpine region are the areas most impacted by winter extremes, such as snowfall, cold spells and winter storms, the frequency of heavy snowfall. The frequency of hot days is highest in Southern Europe. Severe winds and blizzards are the most common over the Atlantic and along its shores. Although heavy rainfall may affect the whole continent on an annual basis, extreme precipitation events are relative sparse, affecting particularly the Alps and the Atlantic coastline. A European regionalization covering similar impacts on the transport network is performed.