Wavelet analysis of urban and rural surface flux

During the CAPITOUL (CAnopy and Particules Interaction in TOulouse Urban Layer) experiment, the one-year instrumental network involved a downtown station with a 30m high telescopic tower installed on a terrace-roof, and a 10m high tower installed on an agricultural parcel, 40km from the city centre. On both stations, momentum, sensible and latent heat fluxes were measured continuously, whereas the urban station also measured the CO2 turbulent flux at the top of the mast, and the momentum and sensible heat flux at an intermediate level (12m below the top of the mast). A time-frequency calculation using the Morlet's wavelet combined with conditional analysis was performed on the different high-rate signals. Using that conditional wavelet technique, we show how the various events contributing to the flux can be identified, according to the site location (rural vs. urban), to the measurement height (at the urban site), and to the meteorological conditions (stability conditions, upwind site characteristics, etc.). The differences observed between rural and urban areas are highlighted and related to the peculiarities of the urban energy budget (low evaporation; considerable anthropogenic heat source, mainly during wintertime; neutral stratification during night-time; and high values of the roughness length).