Capturing All Relevant Scales of Biosphere-Atmosphere Exchange – the Enigmatic Energy Balance Closure Problem

Quantitative knowledge about biosphere-atmosphere exchanges is essential to predict the evolution of the planet's ecosystems, weather and climate. The accuracy of all turbulent flow models depends on the quality of their closure parameterisations, and thus the underlying turbulence measurements. Measurements from all over the world show that the sum of turbulent energy fluxes between the biosphere and the atmosphere generally underestimate the non-convective terms by 10%-30%. This surface energy balance closure problem is recognized as one of the most important enigmas of micrometeorology, considerably limiting progress in atmospheric and climate sciences. A major reason for this underestimation lies in the fact that some scales of atmospheric motion cannot be captured by common eddy-covariance tower measurements. The missing flux is caused by large-scale atmospheric motion which develops under convective conditions. To sample large-scale atmospheric motion adequately, analysis of spatially-distributed measurements is required. This principle applies to observations from airborne platforms and from multi-tower set-ups. An analysis of airborne turbulence measurements on board of the NRC Twin Otter research aircraft during the BOREAS experiment is presented, which demonstrates the existence of large-scale flux contributions at a height of 30 m, and shows that their magnitude corresponds to the energy balance residual determined from tower measurements below the flight track. The analysis of multi-tower field experiments on a research farm near Ottawa, Canada, over patchy flat terrain identifies non-propagating large-scale circulations superimposed on the general wind field as dominant cause for the energy imbalance. Considering the importance of this issue, the development of innovative approaches to quantify the complete atmospheric flux of a scalar - and not only the turbulent part that can be captured using single-tower flux measurements - should be of highest priority for the micrometeorological community.