Estimation of Mesoscale Contributions to Surface Flux Exchanges from Flux Tower Data Using the Continuous Wavelet Transform

Mesoscale organized atmospheric structures have been proposed as the main source of the persistent lack of energy balance closure (EBC) observed by the eddy covariance (EC) technique. The principal hypothesis is that these secondary circulations, which occur at scales not fully captured by single-tower EC, transport significant amounts of energy.
To try to detect and characterize mesoscale structures contributing to this energy transport, we applied the continuous wavelet transform (CWT) to high-frequency micrometeorological data collected at two contrasted ICOS sites characterized by energy balance ratios ranging from 0.7 to 0.8. The first is a crop site located in a flat patchy agricultural landscape (BE-Lon), the second is a forest site with larger fetches located in a hilly landscape (BE-Vie). Unlike classical spectral methods, the CWT provides a time-frequency representation of the signal (Fig 1), allowing us to capture nonstationary, scale-dependent features such as low-frequency fluctuations associated with propagating mesoscale motions. Our analysis focused specifically on naturally convective days when mesoscale circulations are more likely to emerge.