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Models of heat radiation in a forest
As the days grow longer and warmer in spring, the snow begins to melt in the mountains. Given that forests cover around 30% of Switzerland's surface area, a large proportion of the annual melt water originates from these forested regions. Location, rate and manner of snow melt in the forests are among the factors that determine whether flooding will occur in spring.
A key driver of snow melt in the forest is the amount of longwave radiation that reaches the snow surface – in other words, the amount of heat emitted from the sky or the surface of the trees. Existing models simulate the longwave thermal radiation beneath the forest canopy on the basis of air temperature.
The forest as a cold sink
Clare Webster, a PhD candidate belonging to the snow hydrology research group, recently set herself the goal of examining how accurately such models calculate longwave radiation. She was aided by the availability of a large dataset - for approximately ten years the snow hydrology team have collected records of longwave and shortwave radiation reaching the snow surface in three different coniferous forest zones in the Swiss Alps. The researchers also measured air temperature both above the canopy (at a height of 35 m) and within the forest (at heights of 10 m and 2 m).
She discovered that certain deficiencies in the models were attributable to disparities between the air temperature above the canopy and that measured below – in other words, that the forest was behaving like a sink for cold air. Existing models were found to be unreliable particularly in springtime, when the amount of shortwave solar radiation penetrating the forest and heating the tree trunks increases as a consequence of the sun rising to higher elevations in the sky. In particular, the models were least reliable during sunny days when solar heating was at its highest.
Tree trunks far warmer than the surrounding air
As illustrated by thermal imaging data, trunk surface temperature can be as much as 30°C higher than the ambient air temperature, which demonstrates that trees emit far more longwave radiation than previously assumed. This effect has been factored into a new model that incorporates an additional parameter, the trunk-view fraction and its measured temperature.
Furthermore, a simple method of forecasting trunk temperature has been developed based on measured air temperature and shortwave radiation. This allows the use of the extended model even in the absence of thermal data of trunk temperature.
Links with other projects
In another project, SLF researchers have used remote sensing data to produce a detailed description of the forest structure. They have thus developed a new forest-snow interception model that is 30% more accurate than previous descriptions.
Thanks to the findings of these two projects, more reliable assessments can be made of the quantities of water that are stored in the snow in forested regions. This information is applied directly in the monitoring work performed by the operational snow-hydrological service at the SLF.