New insights in the capability of climate models to simulate the impact of LUC based on temperature decomposition of paired site observations

Abstract

In this study, we present a new methodology for evaluating the biogeophysical impact of land use change (LUC) in regional climate models. For this, we use observational data from paired eddy covariance flux towers in Europe, representing a LUC from forest to open land (deforestation). Two model simulations with the regional climate model COSMO-CLM 2 (The Consortium for Small-Scale Modelling model in climate mode COSMO-CLM coupled to the Community Land Model CLM) are performed which differ only in prescribed land use for site pair locations. The model is evaluated by comparing the observed and simulated difference in surface temperature (T s ) between open land and forests. Next, we identify the biogeophysical mechanisms responsible for T s differences by applying a decomposition method to both observations and model simulations. This allows us to determine which LUC-related mechanisms were well represented in COSMO-CLM 2 , and which were not. Results from observations show that deforestation leads to a significant cooling at night, which is severely underestimated by COSMO-CLM 2 . It appears that the model is missing one crucial impact of deforestation on the nighttime surface energy budget: a reduction in downwelling longwave radiation. Results are better for daytime, as the model is able to simulate the increase in albedo and associated surface cooling following deforestation reasonably well. Also well simulated, albeit underestimated slightly, is the decrease in sensible heat flux caused by reduced surface roughness. Overall, these results stress the importance of differentiating between daytime and nighttime climate when discussing the effect of LUC on climate. Finally, we believe that they provide new insights supporting a wider application of the methodology (to other regional climate models).

Pairing FLUXNET sites to validate model representations of land-use/land-cover change

“…Our results suggest that the CLM represents the observed changes in R net and G well, but there remain issues in simulating the energy partitioning between LE and H , which also further confirms the large uncertainties in simulated ET responses to LULCC revealed in several recent studies (e.g., Pitman et al, 2009;Boisier et al, 2012Boisier et al, , 2014de NobletDucoudré et al, 2012;Vanden Broucke et al, 2015). Based on the observations, deforestation generally leads to a decrease in summer daytime R net , accompanied by decreased LE and H .…”

“…In other words, the observed differences between sites can only be partially attributed to LULCC because their environmental conditions may also be different. As most of the current studies used paired sites to represent LULCC, we have assumed that the paired sites share similar background atmospheric conditions, and any observed differences in surface climate conditions can be attributed to LULCC (e.g., Lejeune et al, 2017;Luyssaert et al, 2014;Teuling et al, 2010;Vanden Broucke et al, 2015). Meanwhile, model simulations with the different forcings can effectively examine the effects of the local environment of individual sites, because their footprints can also be taken by the meteorological measurements.…”

“…Meanwhile, the development of FLUXNET (Baldocchi et al, 2001) enables the study of land surface responses to different land-cover types based on paired field observations from neighboring flux towers over forest and open land (Juang et al, 2007;Lee et al, 2011;Luyssaert et al, 2014;Teuling et al, 2010;Williams et al, 2012). In terms of LSM evaluation, the paired site observations have been mainly used to simulated impacts of LULCC on land surface temperature (Chen and Dirmeyer 2016;Lejeune et al, 2016;Vanden Broucke et al, 2015). However, a more fundamental question, "whether a model can represent the observed LULCCinduced changes in surface energy fluxes well", has not been thoroughly investigated, even though we know that the turbulent fluxes are tightly associated with both energy and water exchange between the land surface and atmosphere.…”

Carbon‐equivalent metrics for albedo changes in land management contexts: relevance of the time dimension

“…, Vanden Broucke et al. ). How the surface temperature responds, or the climate sensitivity, depends on the type of land perturbation, since this dictates the magnitude of any internal feedback arising from changes in vegetation structure and physiology (Davin et al.…”

“…Locally, the net effect of an albedo change forcing, often measured in terms of the response by surface temperature, is complicated by concomitant changes to non-radiative biogeophysical mechanisms at the surface influencing sensible heat dissipation, like changes in aerodynamic roughness or evapotranspiration (Boisier et al 2012, Vanden Broucke et al 2015. How the surface temperature responds, or the climate sensitivity, depends on the type of land perturbation, since this dictates the magnitude of any internal feedback arising from changes in vegetation structure and physiology (Davin et al 2007, Lee et al 2011.…”