The impact of European forests on cloud cover: an observation-based study

Abstract:

The impact of temperate forests on their environment is uncertain [1]. While forests generally have a lower albedo, the flux partitioning over forests and its relation to weather conditions is still poorly understood [2,3], complicating modeling attempts. Effects of land surface conditions on boundary-layer humidity and cloud formation can also be very non-linear [4]. Furthermore, the study of hydrological and climate impacts of temperate European forests is complicated because forests are fragmented and often can be found on hilly terrain, making it impossible to attribute differences in cloud cover directly to forest cover.

In this study, we analyse 10 years (2004-2013) of cloud cover data from the SEVIRI instrument aboard the Meteosat Second Generation satellite platform at a 15-minute temporal resolution [5]. We focus on two regions in France where large forests are found which satisfy the following criteria: a) absence of strong topography, and b) presence of sharp contrast between forest and non-forest regions. Cloud occurrence is expressed by the fraction of the daytime that clouds are detected within a ∼6 km MSG pixel. We find that in particular in summer and late summer, clouds are more likely to occur over forest than over the surrounding non-forest land (order of 20%). An opposite signal, but of weaker magnitude, is found in spring, when clouds are less likely to develop over forest. Difference in cloud occurrence is consistent with MODIS-derived differences in EVI, which reflects a more pronounced soil moisture reduction in the non-forested areas.

In addition to investigating seasonal and diurnal patterns, we also investigate the effects of windthrow on cloud occurrence. In 2009, storm Klaus caused extensive damage in southern France, resulting in a large-scale disturbance of the forest cover conditions. This disturbance lead to a significantly lower cloud cover over the forest region in the period after the storm in comparison to the period before the storm, suggesting that storm damage to forests can have unexpected long-term climate impacts through a reduced cloud cover.

References

[1] Bonan, Science 320 (2008)

[2] Teuling et al., Nature Geosci. 3 (2010)

[3] Van Heerwaarden & Teuling, Biogeosci., 11 (2014)

[4] Ek & Holtslag, J. Hydrometeorol., 5 (2004)

[5] Roebeling & van Meijgaard, J. Climate, 22 (2009)