Energy balance closure at a variety of ecosystems in Central Europe with contrasting topographies

A long-standing problem in micrometeorology is that at most eddy covariance sites around the world, the sum of the sensible and latent heat flux measurements is less than the available energy, resulting in the so-called energy balance closure problem. This study utilised the national network of eddy covariance towers in the Czech Republic to examine the degree of energy balance closure at sites covering a wide variety of vegetation types and terrain complexities. The degree of energy balance closure at each site varied depending on the method used to calculate the closure fraction. When the closure was computed using linear regressions of half-hourly sums of turbulent heat fluxes against half-hourly available energy values, closure ranged from 0.68 (beech forest) to 0.81 (spruce forest). However, when closure was computed using the bulk energy balance ratio method, values ranged from 0.61 to 0.73. Highest closure occurred in moderately unstable atmospheric conditions, while closure also increased with increases in the correlation coefficients for vertical wind velocity and water vapour, and vertical wind velocity and sonic temperature. Lowest closure was found at a beech forest in the Carpathian Mountains, where evidence suggested that the complex topography to the south of the eddy covariance tower was influencing the airflow and resulting in poor energy balance closure results. Energy balance closure was also particularly low at a rapeseed cropland, and this was attributed to the low frequency of moderately unstable to strongly unstable conditions at the site.