EGU2020-21573
https://doi.org/10.5194/egusphere-egu2020-21573
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Effect of Forests on the Amount of Incoming Precipitation over Europe

Ronny Meier, Edouard Davin, Jonas Schwaab, and Sonia Seneviratne
Ronny Meier et al.
  • ETH Zürich, IAC, D - USYS, Zürich, Switzerland (ronny.meier@env.ethz.ch)

Numerous studies have demonstrated that forests considerably alter temperatures at the land surface. These alterations vary in space and time due to a complex interplay of several modified energy fluxes at the land surface. The effect of forests on the amount and pattern of precipitation has gained less attention, despite its high socio-economic relevance. Previous work has demonstrated that the high precipitation amounts in tropical rain forests are self-sustained by the abundance of those forests itself. Yet, the impact of forests on precipitation in extra-tropical regions has gained only little attention. This study attempts to identify a relationship between the amount of precipitation and the abundance of forests over the European continent. Such a relationship can originate from two kinds of interactions: (1) The amount of precipitation can drive the abundance of forests, as water is a crucial resource for forests ecosystems. (2) The energy and water redistribution at the land surface associated with forests can alter processes in the atmospheric air column, which in term could affect the amount of precipitation at the location of the forest. Here, we aim to isolate the second kind of effects, as those are more relevant for human decision making.

Establishing a causal relationship between the abundance of forests and the amount of precipitation is complex due this two-way interaction. Hence, three different data sources are employed to advance our understanding of how forests influence precipitation patterns. Firstly, a geographically weighted regression is applied to the spatially-continuous, observation-based precipitation data set MSWEP2.2 (Beck et al., 2017). Besides the forest fraction, a number of topographical variables are considered as predictor variables to account for potential confounding factors (i.e, to assure that interactions of the first kind are not misinterpreted as interactions of the second kind). Secondly, closely-located, paired sites that resemble in topography, but differ in forest fraction are identified in the GHCN (Menne et al., 2012) and the GSDR (Lewis et al., 2019) rain gauge data sets. This allows to evaluate the results based on MSWEP2.2. Thirdly, the same geographically weighted regression is applied to convection-resolving regional climate simulations. By artificially defining the forest fraction distribution in model simulations, interactions of the first kind can be disabled, further fostering the understanding about the causality on the relations identified using the observations. Further sensitivity experiments could be conducted, to improve the process understanding on interactions of the second kind. Overall, our results indicate, that the abundance of a forest increases the amount of precipitation in the order of 100 mm/yr in many locations of Europe. This increased amount of precipitation is more pronounced during the winter months, while the summer signal is more close to zero.

How to cite: Meier, R., Davin, E., Schwaab, J., and Seneviratne, S.: The Effect of Forests on the Amount of Incoming Precipitation over Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21573, https://doi.org/10.5194/egusphere-egu2020-21573, 2020

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  • CC1: Comment on EGU2020-21573, Timothy Hewson, 06 May 2020

    Hi - sorry I missed the chat. This sounds like very interesting work but I was surprised that you found a much greater influence in winter. Is this not counter-intuitive given the relatively dormant nature of evapotranspiration in winter-time? Do you have a physical explanation? Thanks!

    • AC1: Reply to CC1, Ronny Meier, 07 May 2020

      Hi,

      Thank you! It was also not what we expected. Our hypothesis is that the local signal is mainly caused by surface roughness. As forests and shrubs are typically rougher than agricultural land they might hold back precipitating air masses slightly longer. Such an effect could be enhanced during winter, when more of the precipitation is coming from large-scale features.

      Please note that the non-local effect behaves more like what one would expect (Fig. 5). The precipitation increase in summer is probably caused by higher ET over natural vegetation being exported to other regions. For the negative signal in winter, we again think this could be related to the rougher natural vegetation hindering the propagation of precipitating air masses further downwind.

      A very interesting modeling study on the effect of large-scale afforestation in Europe on the cyclone frequency: Belusic, D., Fuentes-Franco, R., Strandberg, G. & Jukimenko, A. Afforestation reduces cyclone intensity and precipitation extremes over europe. Environ. Res. Lett. 14, 074009, DOI: 10.1088/1748-9326/ab23b2 (2019).