Energy Budget Closure, Are We There Yet? after so Many Years, It Still Depends

The surface energy budget (Rn = H + LE + G), conceived of decades ago in early meteorological and climatological studies, has been used as one tool for assessing energy budget flux measurement quality. Rn is net radiation, H is sensible heat, LE is latent energy, and G is the soil/biomass storage, expressed as energy flux densities. The surface energy budget is also a fundamental assumption in a multitude of energy budget flux estimation models, especially those used for ET (evapotranspiration) estimates with remotely sensed data. Unfortunately, it is well known that eddy-covariance sensors for velocity and scalars frequently result in lack of energy budget closure, usually with the sum of (H + LE) measured as less than (Rn – G), especially at ½ hourly time scales. A variety of postulated reasons have been offered, including footprint and fetch mismatches, inadequate spectral correction and related averaging periods (both for high frequency and low frequency challenges), advective convergence/divergence, sonic anemometer inaccuracies, density and related thermodynamic corrections being improperly addressed or ignored, infrared gas analyzer shortcomings, poor accounting for and measurement of storage, and radiometer inaccuracies, to name a few. I emphasize a version of the late Dr. Ray Leuning’s famous quote, “Know thy site,” and postulate that energy budget closure can be approximately achieved by appropriate correction and accommodations that are site specific. This means a single set of solutions for energy budget closure is unlikely as it is not true that “one size fits all”; rather a careful assessment of each site’s ecophysiology, surface characteristics, and regional scale biometeorology is critical to approaching energy budget closure. It may not be possible in a number of cases to approach energy budget closure because of the inability to sufficiently characterize some critical processes. Some examples of challenges and success are given.