Eddy-Covariance and the Surface Energy Imbalance: When Will the Imbalance Ever Go Away and Does Anyone Care?.

Eddy-covariance has been extensively used for decades, and is a fundamental technique used in global networks designed to measure trace gas exchange from surfaces, such as evapotranspiration, carbon dioxide fluxes, and other greenhouse gas emissions including those of methane and nitrous oxide.  Despite its ubiquitous usage,  eddy-covariance yields sensible heat and latent energy flux densities  that usually fail to balance the available energy, identified as net radiation and thermal storage (in the soil, biomass, and atmosphere).  The various explanations for this, ranging from inadequate estimates of (1) thermal storage calculations, (2) turbulent exchange because of fast-response sensor frequency and averaging limitations, (3) mean advective divergences and convergences, and (4) problems arising from footprints of turbulent processes and radiation measurements, are discussed.  The usage of the standard deviation of vertical velocity instead of the friction velocity for filtering data, in addition to the proper interpretation of mean advection processes are elaborated on.