The Opposing Effects of Reforestation and Afforestation on the Diurnal Temperature Cycle at the Surface and in the Lowest Atmospheric Model Level in the European Summer

Abstract

The biophysical effects of re/af-forestation on the diurnal temperature cycle in European summer are investigated by analyzing a Regional Climate Model (RCM) ensemble, established within the Land Use and Climate Across Scales Flagship Pilot Study (LUCAS FPS). With this RCM ensemble, two idealized experiments are performed for Europe, one with a continent with maximized forest cover, and one where all forests are turned into grassland. First, an in-depth analysis of one ensemble member (CCLM-VEG3D) is carried out, to reveal the complex process chain caused by such land use changes (LUCs). Based on these findings, the whole ensemble is analyzed and principle biophysical effects of re/af-forestation are derived. Results show that the diurnal temperature range is reduced at the surface (top of the vegetation) with re/af-forestation. Most RCMs simulate colder surface temperatures (Tsurf) during the day and warmer Tsurf during the night. Thus, for the first time, the principle temperature interrelations found in observation-based studies in the mid-latitudes could be reproduced within a model intercomparison study. On the contrary, the diurnal temperature range in the lowest atmospheric model level (Tair) is increased with re/af-forestation. This opposing temperature response is mainly caused by the higher surface roughness of forest, enhancing the turbulent heat exchange. Furthermore, these opposing temperature responses demonstrate that the use of the diagnostic 2 m temperature (weighted interpolation between Tsurf and Tair) has a limited potential to assess the effects of re/af-forestation. Thus, studies about the biophysical impacts of LUCs should investigate the whole near-surface temperature profile.

What determines the sign of the evapotranspiration response to afforestation in the European summer?

“…Bonan, 2008), since at least in Southern and Central Europe evapotranspiration is reduced in FOREST (Figure 2a). As stated by Breil et al, (2020), the lower surface temperatures in FOREST are mainly caused by increased sensible heat fluxes all over Europe (Figure 5c), which transform and transport the increased energy input from the net short-wave radiation into the atmosphere, without increasing the surface temperature.…”

“…CC BY 4.0 License. Due to a higher surface roughness, the daily maximum surface temperatures (top of the vegetation) of a forest are lower than 275 of grassland (Breil et al, 2020). The saturation deficit between the vegetation and the atmosphere (1), which depends on these surface temperatures (Eq.…”

Impact of Quasi‐Idealized Future Land Cover Scenarios at High Latitudes in Complex Terrain

“…(2020) and Breil et al. (2020). This is important for snow related variables (e.g., snow depth, cover/extent and snow water equivalent) that are critical for representing accumulation and ablation processes at the surface.…”

“…The comparison is further complicated by the use of 2 m air temperature in studies by Davin et al (2020). Recent studies such as the multimodel study of Breil et al (2020) have demonstrated the fallibility of using 2 m air temperature, a diagnostic variable, to assess LULCC. In particular, they show that different models can provide conflicting results in summer when using 2 m air temperature but that differences can be reconciled when surface temperature is used instead.…”

“…Our work extends on previous studies by contributing new information on the localized effects of LULCC on this and other variables, indices and extremes that can only be obtained via very high-resolution modeling. Benefits to such high-resolution models include their ability to separate snow and rain based on microphysical processes instead of the temperature based threshold that is applied in most land surface models at nonconvection permitting simulations such as those in Davin et al (2020) and Breil et al (2020). This is important for snow related variables (e.g., snow depth, cover/extent and snow water equivalent) that are critical for representing accumulation and ablation processes at the surface.…”