Coherent Structure Patterns Affect Energy Balance Closure: Evidence from Virtual Measurements for a Field Campaign

Abstract:

The energy balance closure problem is a well-known issue of eddy-covariance measurements. However, the underlying mechanisms are still under debate. Recent evidence suggests that organized low-frequency motion contributes significantly to the energy balance residual, because the associated transport cannot be captured by a point measurement.

In this study, we carry out virtual measurements using a PArallelized Large-Eddy Simulation Model (PALM). In order to represent specific measurement days of the field campaign “High definition clouds and precipitation for advancing climate prediction” (HD(CP)²), which was part of the project “High Definition Clouds and Precipitation for Advancing Climate Prediction”(HOPE) in 2013, the simulations were driven by synoptic-scale COSMO-DE reanalysis data. Planet boundary layer height, the vertical profiles of variance and skewness of vertical wind were analyzed and a comparison with Doppler-lidar observations shows good agreement. Furthermore, simulated energy imbalances were compared with real-world imbalances from two eddy-covariance stations in the model domain. Particularly poor energy balance closure was found for a day with cellular organized structures in the surface layer, while the energy balance closure was better on other days with roll-like structures. This finding might be one explanation why the energy balance closure generally tends to improve with increasing friction velocity, since roll-like structures are typically associated with higher wind speeds. In order to gain insight into the partitioning of the energy balance residual between the sensible and latent heat fluxes, we further employed a control volume method within the numerical simulation. Hence, advection and storage terms were identified as the most important causes for the lack of energy balance closure by the eddy-covariance method. The results of the virtual measurements indicate that the “missing” part of the surface energy mainly comes from the sensible heat, which means the underestimation of the sensible heat flux is the major reason for the unclosed energy balance.