Mesoscale Eddies Affect Near-Surface Turbulent Exchange: Evidence from Lidar and Tower Measurements

Abstract

The eddy-covariance technique tends to underestimate turbulent heat fluxes, which results in nonclosure of the surface energy balance. This study shows experimental evidence that mesoscale turbulent organized structures, which are inherently not captured by the standard eddy-covariance technique, can affect near-surface turbulent exchange. By using a combined setup of three Doppler wind lidars above a cropland-dominated area in Germany, low-frequency turbulent structures were detected in the surface layer down to a few meters above ground. In addition, data from two micrometeorological stations in the study area were analyzed with respect to energy balance closure. In accordance with several previous studies, the data confirm a strong friction velocity dependence of the energy balance residual. At both stations, the energy balance residual was found to be positively correlated with the vertical moisture gradient in the lower atmospheric boundary layer, but at only one station was it correlated with the temperature gradient. This result indicates that mesoscale transport probably contributes more to the latent heat flux than to the sensible heat flux, but this conclusion depends largely on the measurement site. Moreover, flow distortion due to tower mountings and measurement devices affects the energy balance closure considerably for certain wind directions.

The influence of idealized surface heterogeneity on virtual turbulent flux measurements

“…The same correlation has also been noticed in a recent year-long large-eddy simulation (LES) by Schalkwijk et al (2016) and in an idealized LES study by Inagaki et al (2006). In addition, the study of Eder et al (2015b) could relate the energy balance residual to the mean gradients in the lower boundary-layer, thereby providing more evidence for the connection between the energy imbalance and the presence of quasi-stationary structures in the boundary layer. These circulations typically arise in heterogeneous terrain but may also develop over a completely homogeneous surface to a lesser extent, depending on the atmospheric stability regime, due to self-organization.…”

“…Moreover, Stoy et al (2013) also found a good correlation between the friction velocity (u * ) and the energy balance closure. This result was reproduced by Eder et al (2015b) by means of a study combining Doppler wind lidar and EC tower data. The same correlation has also been noticed in a recent year-long large-eddy simulation (LES) by Schalkwijk et al (2016) and in an idealized LES study by Inagaki et al (2006).…”

The HD(CP)<sup>2</sup> Observational Prototype Experiment (HOPE) – an overview

“…Thus, a characteriza-tion of coherence patterns in the otherwise turbulent boundary layer requires extensive spatiotemporal averaging. Eder et al (2015) investigated the complete surface energy budget and tested the hypothesis of whether so-called turbulent organized structures (TOS), low-frequency structures that fill the entire ABL, are a major cause for the frequently unclosed surface energy balances as they contribute to the vertical energy fluxes. In fact, by means of data from horizontally and vertically scanning Doppler lidars the authors could show that TOS with timescales larger than 30 min extend deep into the surface layer.…”

Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

“…Apart from the different laser transmitters, the HYB has similar system settings as the WTX. The Doppler lidar at Selhausen, the third site, was a Stream Line system manufactured by HALO Photonics Ltd. (Pearson et al, 2009, hereafter called HALO), which measures with a range-gate length of 18 m (Eder et al, 2015). In contrast to the WindTracer systems having a laser pulse of high energy, the HALO and the WLS200 operate in a "low-pulse energy/high-pulse rate mode" and they can resolve the lowest hundreds of m a.g.l.…”