Abstract. The Land Use and Climate Across Scales Flagship Pilot Study (LUCAS FPS) is a coordinated community effort to improve the integration of land use change (LUC) in regional climate models (RCMs) and to quantify the biogeophysical effects of LUC on local to regional climate in Europe. In the first phase of LUCAS, nine RCMs are used to explore the biogeophysical impacts of re-/afforestation over Europe: two idealized experiments representing respectively a non-forested and a maximally forested Europe are compared in order to quantify spatial and temporal variations in the regional climate sensitivity to forestation. We find some robust features in the simulated response to forestation. In particular, all models indicate a year-round decrease in surface albedo, which is most pronounced in winter and spring at high latitudes. This results in a winter warming effect, with values ranging from +0.2 to +1 K on average over Scandinavia depending on models. However, there are also a number of strongly diverging responses. For instance, there is no agreement on the sign of temperature changes in summer with some RCMs predicting a widespread cooling from forestation (well below −2 K in most regions), a widespread warming (around +2 K or above in most regions) or a mixed response. A large part of the inter-model spread is attributed to the representation of land processes. In particular, differences in the partitioning of sensible and latent heat are identified as a key source of uncertainty in summer. Atmospheric processes, such as changes in incoming radiation due to cloud cover feedbacks, also influence the simulated response in most seasons. In conclusion, the multi-model approach we use here has the potential to deliver more robust and reliable information to stakeholders involved in land use planning, as compared to results based on single models. However, given the contradictory responses identified, our results also show that there are still fundamental uncertainties that need to be tackled to better anticipate the possible intended or unintended consequences of LUC on regional climates.

Abstract. The Land Use and Climate Across Scales Flagship Pilot Study (LUCAS FPS) is a coordinated community effort to improve the integration of Land Use Change (LUC) in Regional Climate Models (RCMs) and to quantify the biogeophysical effects of LUC on local to regional climate in Europe. In the first phase of LUCAS, nine RCMs are used to explore the biogeophysical impacts of re-/afforestation over Europe. Namely, two idealized experiments representing respectively a non-forested and a maximally forested Europe are compared in order to quantify spatial and temporal variations in the regional climate sensitivity to forestation. We find some robust features in the simulated response to forestation. In particular, all models indicate a year-round decrease in surface albedo, which is most pronounced in winter and spring at high latitudes. This results in a winter warming effect, which is relatively robust across models. However, there are also a number of strongly diverging responses. For instance, there is no agreement on the sign of temperature changes in summer with some RCMs predicting a widespread cooling from forestation, a widespread warming, or a mixed response. A large part of the inter-model spread is attributed to the representation of land processes. In particular, differences in the partitioning of sensible and latent heat are identified as a key source of uncertainty. In contrast, for precipitation, the representation of atmospheric processes dictates more directly the simulated response. In conclusion, the multi-model approach we use here has the potential to deliver more robust and reliable information to stakeholders involved in land use planning, as compared to results based on single models. However, given the contradictory responses identified, our results also show that there are still fundamental uncertainties that need to be tackled to better anticipate the possible intended or unintended consequences of LUC on regional climates.

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.

Land use and land cover changes are important human forcings to the Earth's climate. This study examines the land-atmosphere coupling strength and the relationship between surface fluxes and clouds and precipitation for three land cover scenarios in the European summer. The WRF model was used to simulate one scenario with extreme afforestation, one with extreme deforestation, and one with realistic land cover for the time period between 1986 and 2015. The simulations were forced with ERA-Interim reanalysis data. The analysis followed a two-step approach. First, the convective triggering potential-low-level humidity index framework was applied to locate potential coupling hot spots, which were then analyzed with regard to their sensitivity toward land use and land cover changes. Second, actual feedbacks between evaporative fraction, cloud cover, and precipitation were analyzed statistically with focus on sign and location of the feedbacks. The results demonstrate that coupling hot spots, exhibiting predominantly positive feedbacks, were identified over parts of Eastern Europe and Scandinavia. In this strongly coupled region, afforestation and deforestation modified the atmospheric humidity and stability by changing the surface flux partitioning. Afforestation is associated with a net drying of the atmosphere due to a disproportionately strong increase in the sensible heat flux. In contrast, deforestation initiated a moistening of the atmosphere. The total precipitation changed only in limited areas significantly, which are mostly located in mountainous regions and the northeast of the domain. In summary, the results indicate a land surface influence on the atmospheric background conditions, and an impact on the potential strength of land surface-precipitation feedbacks.

Abstract. The quantification of land–atmosphere coupling strength is still challenging, particularly in the atmospheric segment of the local coupling process chain. This is in part caused by a lack of spatially comprehensive observations of atmospheric temperature and specific humidity which form the verification basis for the common process-based coupling metrics. In this study, we aim at investigating where uncertainty in the atmospheric temperature and moisture affects the land–atmosphere coupling strength over Europe, and how changes in the mean temperature and moisture, as well as their vertical gradients, influence the coupling. For this purpose, we implemented systematic a posteriori modifications to the temperature and moisture fields from a regional climate simulation to create a spread in the atmospheric conditions. Afterwards, the process-based coupling metric convective triggering potential – low-level humidity index framework was applied to each modification case. Comparing all modification cases to the unmodified control case revealed that a strong coupling hotspot region in northeastern Europe was insensitive to temperature and moisture changes, although the number of potential coupling days varied by up to 20 d per summer season. The predominance of positive feedbacks remained unchanged in the northern part of the hotspot, and none of the modifications changed the frequent inhibition of feedbacks due to dry conditions in the atmosphere over the Mediterranean and the Iberian Peninsula. However, in the southern hotspot region in the north of the Black Sea, the dominant coupling class frequently switched between wet soil advantage and transition zone. Thus, both the coupling strength and the predominant sign of feedbacks were sensitive to changes in temperature and moisture in this region. This implies not only uncertainty in the quantification of land–atmosphere coupling strength but also the potential that climate-change-induced temperature and moisture changes considerably impact the climate there, because they also change the predominant atmospheric response to land surface wetness.