Spatial representativeness of single tower measurements and the imbalance problem with eddy-covariance fluxes: results of a large-eddy simulation study

Abstract

A large-eddy simulation (LES) study is presented that investigates the spatial variability of temporal eddy covariance fluxes and the systematic underestimation of representative fluxes linked to them. It extends a prior numerical study by performing high resolution simulations that allow for virtual measurements down to 20 m in a convective boundary layer, so that conditions for small tower measurement sites can be analysed. It accounts for different convective regimes as the wind speed and the near-surface heat flux are varied. Moreover, it is the first LES imbalance study that extends to the stable boundary layer. It reveals shortcomings of single site measurements and the necessity of using horizontally-distributed observation networks. The imbalances in the convective case are attributed to a locally non-vanishing mean vertical advection due to turbulent organised structures (TOS). The strength of the TOS and thus the imbalance magnitude depends on height, the horizontal mean wind and the convection type. Contrary to the results of a prior study, TOS cannot generally be responsible for large energy imbalances: at low observation heights (corresponding to small towers and near-surface energy balance stations) the TOS related imbalances are generally about one order of magnitude smaller than those in field experiments. However, TOS may cause large imbalances at large towers not only in the case of cellular convection and low wind speeds, as found in the previous study, but also in the case of roll convection at large wind speeds.In the stably stratified boundary layer for all observation heights neither TOS nor significant imbalances are observed. Boundary-Layer Meteorol (2007) 123:77-98 Attempting to reduce imbalances in convective situations by applying the conventional linear detrending method increases the systematic flux underestimation. Thus, a new filter method is proposed.

On the Anomalous Behaviour of Scalar Flux–Variance Similarity Functions Within the Canopy Sub-layer of a Dense Alpine Forest

“…However, net radiation measurements are insensitive to such outer-layer modulations. Whether this mechanism alone may explain the lack of surface energy balance closure is subject to some debate (Steinfeld et al 2007). For example, the performed analysis showed that during a 6-min sweeping event of dry and cold air from the outer layer, the local latent heat flux actually increased (not decreased).…”

Heterogeneity-Induced Heat-Flux Patterns in the Convective Boundary Layer: Can they be Detected from Observations and is There a Blending Height?—A Large-Eddy Simulation Study for the LITFASS-2003 Experiment

“…This is because turbulent transport is not well resolved by the numerical grid in this region due to the limited eddy size. Data analysis has shown that the subgrid-scale flux contribution is below 5 % of the total flux above the third vertical grid layer (see also Steinfeld et al 2007). Therefore, the subgrid-scale flux contribution is not considered further in calculations of the turbulent flux.…”

“…Detailed descriptions of PALM have been provided by Raasch and Etling (1998) and Raasch and Schröter (2001). PALM has been successfully applied for investigations of the neutrally (Letzel et al 2008) and weakly stably stratified boundary layer (Beare et al 2007;Steinfeld et al 2007) as well as the homogeneously heated (Schröter et al 2000;Raasch and Franke 2011) and heterogeneously heated CBL (Raasch and Harbusch 2001;Letzel and Raasch 2003;Inagaki et al 2006;Steinfeld et al 2008;Maronga and Raasch 2013) and is therefore well suited for this study.…”

Energy balance closure for the LITFASS-2003 experiment

“…Also in recent studies, it was found that fluxes averaged over long time periods (Finnigan et al 2003;Mauder and Foken 2006;Sakai et al 2001) or spatially averaged fluxes (Inagaki et al 2006;Kanda et al 2004;Steinfeld et al 2007) could close the energy balance. These hypotheses were not investigated during EBEX-2000.…”