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Volume 11, issue 8 | Copyright
Geosci. Model Dev., 11, 3465-3479, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model evaluation paper 29 Aug 2018

Model evaluation paper | 29 Aug 2018

Closing the energy balance using a canopy heat capacity and storage concept – a physically based approach for the land component JSBACHv3.11

Marvin Heidkamp1,2, Andreas Chlond1, and Felix Ament1,3 Marvin Heidkamp et al.
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2International Max Planck Research School on Earth System Modeling, Hamburg, Germany
  • 3Meteorological Institute, CEN, University of Hamburg, Hamburg, Germany

Abstract. Land surface–atmosphere interaction is one of the most important characteristic for understanding the terrestrial climate system, as it determines the exchange fluxes of energy and water between the land and the overlying air mass.

In several current climate models, it is common practice to use an unphysical approach to close the surface energy balance within the uppermost soil layer with finite thickness and heat capacity. In this study, a different approach is investigated by means of a physically based estimation of the canopy heat storage (SkIn+).

Therefore, as a first step, results of an offline simulation of the land component JSBACH of the Max Planck Institute Earth system model (MPI-ESM) – constrained with atmospheric observations – are compared to energy fluxes and water fluxes derived from eddy covariance measurements observed at the CASES-99 field experiment in Kansas, where shallow vegetation prevails. This comparison of energy and evapotranspiration fluxes with observations at the site-level provides an assessment of the model's capacity to correctly reproduce the diurnal cycle. Following this, a global coupled land–atmosphere experiment is performed using an AMIP (Atmospheric Model Intercomparison Project) type simulation over 30 years to evaluate the regional impact of the SkIn+ scheme on a longer timescale, in particular, with respect to the effect of the canopy heat storage.

The results of the offline experiment show that SkIn+ leads to a warming during the day and to a cooling at night relative to the old reference scheme, thereby improving the performance in the representation of the modeled surface fluxes on diurnal timescales. In particular: nocturnal heat releases unrealistically destroying the stable boundary layer disappear and phase errors are removed. On the global scale, for regions with no or low vegetation and a pronounced diurnal cycle, the nocturnal cooling prevails due to the fact that stable conditions at night maintain the delayed response in temperature, whereas the daytime turbulent exchange amplifies it. For the tropics and boreal forests as well as high latitudes, the scheme tends to warm the system.

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Short summary
The core of every climate model is the solution of the surface energy balance. Numerical approaches are mandatory to calculate the land's response to solar input. However, different numerical approaches should not affect the physical results. Here we develop a physical approach that determines how the available energy is divided into radiative and heat fluxes. A key element of this scheme is the inclusion of different types of heat storages in the canopy layer.
The core of every climate model is the solution of the surface energy balance. Numerical...