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

Abstract

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.

How Well Can Land-Surface Models Represent the Diurnal Cycle of Turbulent Heat Fluxes?

“…Land surface model parameterizations must resolve these interactions through the (i) surface energy balance solution (Viterbo and Beljaars 1995), and the subgrid-scale parameterizations of the (ii) surface or canopy conductance (Monteith 1965;Medlyn et al 2011), (iii) the turbulent exchange in the surface layer (Monin and Obukhov 1954;Businger et al 1971;Maronga and Reuder 2017), and (iv) the planetary boundary layer (Baklanov et al 2011). These subgrid-scale parameterizations are critical for accurate simulation of the surface climate (Baklanov et al 2011;Heidkamp et al 2018), including heat and mass exchange with atmosphere. Therefore, an evaluation of the diurnal phase lag of simulated turbulent heat fluxes can identify important model deficiencies and point toward improvements.…”

“…Given the lack of further data to calculate these heat storage components, we cannot further assess this issue within the scope of this paper. However, we strongly recommend a detailed measurement of subsurface and canopy heat storage components, which are important for the diurnal heat redistribution (Moderow et al 2009;Gao et al 2017;Heidkamp et al 2018;Meier et al 2019;Eshonkulov et al 2019). Furthermore, alternative measurement technologies should be used in parallel to assess the validity of the diurnal cycle of eddy covariance turbulence measurements.…”

“…This layer has a large heat capacity and dampens the temporal variation of the surface temperature depending on the depth of the layer. The second approach is to use a skin layer that has no heat capacity but requires iteration to get a stable solution (Viterbo and Beljaars 1995;Best et al 2005;Heidkamp et al 2018).…”

“…Next we illustrate the impact of surface energy balance schemes on the phase lags of the heat fluxes using output of two different versions of the JSBACH model, which is the LSM scheme of the ECHAM climate model. Heidkamp et al (2018) used the JSBACH model in an offline mode and simulated the diurnal cycle with the reference version of the JSBACH model (i.e., with upper soil layer to solve the surface energy balance) and the new version with a skin layer (JSBACH-Skin). For illustration Heidkamp et al (2018) used observational data from the Diurnal Land/Atmosphere Coupling Experiment (DICE, http://appconv.metoffice.com/dice/dice.html) project.…”

“…Heidkamp et al (2018) used the JSBACH model in an offline mode and simulated the diurnal cycle with the reference version of the JSBACH model (i.e., with upper soil layer to solve the surface energy balance) and the new version with a skin layer (JSBACH-Skin). For illustration Heidkamp et al (2018) used observational data from the Diurnal Land/Atmosphere Coupling Experiment (DICE, http://appconv.metoffice.com/dice/dice.html) project. This is based on the well-known CASES99 campaign at the Southern Great Plains sites in the United States over crop land.…”