Role of vegetation in representing land surface temperature in the CHTESSEL (CY45R1) and SURFEX-ISBA (v8.1) land surface models: a case study over Iberia
Authors: Nogueira Miguel , Albergel Clément , Boussetta Souhail , Johannsen Frederico , Trigo Isabel F. , Ermida Sofia L. , Martins João P. A. , Dutra Emanuel 
Published: September 2020

Abstract. Earth observations were used to evaluate the representation of
land surface temperature (LST) and vegetation coverage over Iberia in two
state-of-the-art land surface models (LSMs) – the European Centre for
Medium-Range Weather Forecasts (ECMWF) Carbon-Hydrology Tiled ECMWF Scheme
for Surface Exchanges over Land (CHTESSEL) and the Météo-France
Interaction between Soil Biosphere and Atmosphere model (ISBA) within the
SURface EXternalisée modeling platform (SURFEX-ISBA) for the 2004–2015
period. The results showed that the daily maximum LST simulated by CHTESSEL
over Iberia was affected by a large cold bias during summer months when
compared against the Satellite Application Facility on Land Surface Analysis
(LSA-SAF), reaching magnitudes larger than 10 ∘C over wide
portions of central and southwestern Iberia. This error was shown to be
tightly linked to a misrepresentation of the vegetation cover.  In contrast,
SURFEX simulations did not display such a cold bias. We show that this was
due to the better representation of vegetation cover in SURFEX, which uses
an updated land cover dataset (ECOCLIMAP-II) and an interactive vegetation
evolution, representing seasonality. The representation of vegetation over
Iberia in CHTESSEL was improved by combining information from the European
Space Agency Climate Change Initiative (ESA-CCI) land cover dataset with the
Copernicus Global Land Service (CGLS) leaf area index (LAI) and fraction of
vegetation coverage (FCOVER). The proposed improvement in vegetation also
included a clumping approach that introduces seasonality to the vegetation
cover. The results showed significant added value, removing the daily
maximum LST summer cold bias completely, without reducing the accuracy of
the simulated LST, regardless of season or time of the day. The striking
performance differences between SURFEX and CHTESSEL were fundamental to
guiding the developments in CHTESSEL highlighting the importance of using
different models. This work has important implications: first, it takes
advantage of LST, a key variable in surface–atmosphere energy and water
exchanges, which is closely related to satellite top-of-atmosphere
observations, to improve the model's representation of land surface
processes. Second, CHTESSEL is the land surface model employed by ECMWF in
the production of their weather forecasts and reanalysis; hence systematic
errors in land surface variables and fluxes are then propagated into those
products. Indeed, we showed that the summer daily maximum LST cold bias over
Iberia in CHTESSEL is present in the widely used ECMWF fifth-generation
reanalysis (ERA5). Finally, our results provided hints about the interaction
between vegetation land–atmosphere exchanges, highlighting the relevance of
the vegetation cover and respective seasonality in representing land surface
temperature in both CHTESSEL and SURFEX. As a whole, this work demonstrated
the added value of using multiple earth observation products for
constraining and improving weather and climate simulations.