Optimizing Large-eddy Simulations for Investigating the Energy-balance Closure Problem at Typical Eddy-covariance Measurement Heights
Abstract
Large-eddy simulations are well suited to investigate the energy-balance closure problem because they yield spatially distributed information and capture motions from the meter to the kilometer scale. Since improvements in computation allowed for large-eddy simulations with higher resolution that provide information closer to the surface, many studies found the energy balance gap to vanish near the surface. However, this contradicts eddy-covariance measurements typically showing daily energy balance ratios of approximately 0.8 even for measurement heights of a few meters. The Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors (CHEESEHEAD) project is ideally suited to compare numerous spatially distributed field measurements in a heterogeneous area to large-eddy simulations. Because most eddy-covariance towers at the CHEESEHEAD site are lower than 20 meters, these simulations need to be optimized for observations at low heights. Therefore, the aim of this preliminary study is to find a simulation setup that yields realistic virtual tower measurements at the height of 10 to 20 meters to investigate the energy balance closure problem. The study is carried out with PALM and is based on a set of idealized large-eddy simulations over a homogeneous surface to address two possible reasons for the underestimation of the energy balance gap: These are (1) the inability of prescribed surface fluxes to react to atmospheric motions which prevents self-reinforcement of mesoscale structures and (2) the use of Monin-Obukhov similarity theory even though grid points might be located within the roughness sublayer as a consequence of high resolution. Different combinations of prescribed surface fluxes, land surface model and plant canopy model, as well as an alternative to Monin-Obukhov similarity theory, will be used to simulate different vegetation types and atmospheric stabilities. To validate the simulations, we compare the resulting spatial and temporal spectra and cospectra to model spectra and investigate latent and sensible heat fluxes, as well as the resulting energy balance ratio. The most realistic set-up will be used for the investigation of the energy-balance closure of the CHEESEHEAD site.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B41K2497W
- Keywords:
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- 3315 Data assimilation;
- ATMOSPHERIC PROCESSES;
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES;
- 0430 Computational methods and data processing;
- BIOGEOSCIENCES