The main driver of snowmelt in forests is the amount of solar and thermal radiation that reaches the snow surface. Forest structure creates "hot" and "cold" radiation spots very close together due to shading and forest heating.
Solar and thermal radiation is typically measured using individual radiation sensors, but placing these instruments in a single location in the forest means the radiation gradients between the "hot" and "cold" areas are not captured. Our solution to this was to develop moving radiometer platforms to directly measure radiation in as many locations as possible, and utilize thermal imaging to improve understanding of forest heating.
We developed a motorized cable car platform that can perform continuous measurements of solar and thermal radiation to the snow surface along linear transects in forests. A second platform, which consists of radiation sensors mounted on a handheld gimbal (a motorized camera stabilizer), allows us to capture spatial radiation patterns. With these two systems, we observe how radiation to the snow surface varies in both space and time. Linking these measurements to forest structure information such as hemispherical images taken along the same transects improves our understanding of how radiation is controlled by forest structure. The snow surface receives more thermal radiation at locations of denser forest. Solar radiation patterns depend on the exact position of the sun relative to the tree crowns at any given point in time and are therefore highly complex.
During springtime, the amount of solar radiation penetrating the forest and heating the trees increases as a consequence of the sun rising higher and higher. Thermal imagery has demonstrated that in gaps and sparse forests exposed to high amount of solar energy, tree trunks can be as much as 20°C higher than air temperature. However, in denser forests and shaded edges, tree trunks remain colder than air temperature due to a lack of solar heating.