Energy balance residual (EBR) correction

In its simplest form, the energy balance equation at the surface of earth is

5‑1

where the left-hand side term is the energy (per unit of time and area) available at each given time at the interface between the earth surface and the atmosphere, described as the net solar radiation (Rn, W m-2) reaching the surface minus the energy leaving the surface through the soil (G, W m-2). The right-hand side describes the energy leaving the surface towards the atmosphere, in terms of sensible (H, W m-2) and latent (LE, W m-2) heat flux. A more complete formulation may include terms (which are in general deemed minor) such as heat stored in the canopy and stands, energy used for photosynthesis, etc.

Non-closure of the surface energy balance is a frequently observed phenomenon of hydrometeorological field measurements, when using the eddy covariance method, which can be ascribed to an underestimation of the turbulent fluxes (Mauder et al., 2018). The adjustment methods available in Tovi seek to correct H and LE by imposing closure of the surface energy balance. The available four methods differ for two main aspects:

  1. The time scale over which the Energy Balance Closure (EBC) is forced, and
  2. The way residual energy is partitioned between half-hourly values of H and LE.

The method of Mauder et al., (2013) forces the EBC on a daily time scale and partitions residual energy in such a way as to preserve the Bowen Ratio (H/LE) before and after the correction on a 30-minute basis.

The method of Charuchittipan et al., (2014) forces the EBC on a 30-minute basis and ascribes a larger portion of the residual energy to H, and hence significantly increases the Bowen Ratio for most periods. The rationale is that the large-scale coherent structures not captured by the eddy covariance method are driven by buoyancy and hence entail primarily additional sensible heat fluxes.

De Roo et al. (2018) propose two methods. With both methods, informed by a Large Eddy Simulation (LES) study, the partition between H and LE varies with the measurement height and ascribes a larger portion of the energy to H with respect to LE, than the Bowen ratio would dictate, but not quite as much as the method by Charuchittipan et al. (2014). The two methods differ for the fact that one is purely derived from the LES results without any consideration of the observed non-closure, while the second is constrained by the non-closure evaluated on a daily basis.