Continuous Wavelet Transform Analysis of Mesoscale Motions Contribution to Scalar Transport at the ICOS Sites of Lonzée and Vielsalm.

Abstract

The eddy covariance (EC) method is the standard approach for quantifying turbulent exchanges of energy and carbon dioxide (CO2) between ecosystems and the atmosphere, yet its application reveals a systematic lack of energy balance closure (EBC). A leading hypothesis for this obser- vation is the unaccounted contribution from mesoscale motions, characterized by eddies with timescales exceeding the conventional 30-minute averaging window of EC. This study investi- gates the role of such motions through the application of the wavelet transform (WT), a method that allows time–frequency analysis of single-point tower-based measurements. High-frequency measurements from the complete 2020 datasets at two ICOS sites in Belgium, a cropland in Lonzée and a mixed forest in Vielsalm, were analyzed. Fluxes of sensible heat (H), latent heat (LE), and CO2 (FC) estimated with the WT were compared to EC across different scales. At both sites, EC indicated systematic underestimation of turbulent fluxes, with annual energy balance ratios (EBR) of 0.65 in Lonzée and 0.73 in Vielsalm. Wavelet-derived fluxes tended to underestimate microscale contributions relative to EC, especially in the case of LE in Lonzée, but revealed additional mesoscale components. These contributions were negligible at Lonzée (< 2%) but averaged 5% of the microscale fluxes at Vielsalm, improving the EBR when included. In addition, the mesoscale component of FC followed the trends of the energy fluxes, although a decoupling becomes apparent under higher temperature conditions (> 16◦C). The results highlight the role of site characteristics, including measurement height, canopy height, topography, and meteorological conditions, in determining the magnitude of mesoscale transport.