Diurnal to interannual variability in Cabauw simulated by the ECLand land surface model

Land surface plays a fundamental role in the earth system, mediating the water, energy and carbon fluxes between the land and the atmosphere. The land surface physical and biophysical processes act on time scales ranging from sub-daily to decades with relevant impacts from weather forecasts to climate change. However, there are very few available in-situ observations of land surface state and fluxes extending for several years to decades, limiting an integrated validation of the models on the different time scales. The long time series of Cabauw (Netherlands) observations provides a unique opportunity to evaluate land surface processes and their representation in land surface model at time scales ranging from sub-diurnal to interannual. In this study, we take advantage of the uniqueness of Cabauw observational record to investigate the performance of the ECMWF land surface model ECLand for the period 2001-2020 (20 years). Emphasis is given to the summer season and to evaporation and evaporative fraction. An idealized simulation without canopy resistance is performed along with other model configurations with changes to the constraints of canopy resistance (soil moisture availability and atmospheric humidity deficit) and the vertical discretization of the soil layers.

Observational uncertainties impact the surface energy budget closure. For example, the model shows a large overestimation of the ground heat flux diurnal cycle. However, part of this can be attributed to observational uncertainties associated with the sinking of the temperature sensors.  The default configuration of ECLand shows an underestimation of latent heat and evaporative fraction, which can be partially attributed to the model’s representation of canopy resistance. The increased vertical discretization of the soil layers has a neutral impact on the simulated turbulent fluxes, showing an improved representation of near-surface soil temperature. Our results show limitations in the representation of the summer interannual variability of the turbulent fluxes. These are associated with the representation of extreme events (droughts) and are not fully addressed in any of the model configurations tested. These results suggest that other processes relevant to the representation of evaporation in dryness stress conditions need to be further investigated.

This work was developed in the framework of the project NextGEMS funded through the European Union’s Horizon 2020 research and innovation program under the grant agreement number 101003470. Luis Frois was funded by the FCT Grant 2020.08478.BD. The authors also acknowledge the financial support of the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL.