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.
Closing the Energy Balance using a Canopy Heat Capacity – A
physically based Approach for the Land Component JSBACHv3.11
