An Assessment of Eddy-Covariance-Based Surface Fluxes Above an Evaporating Heated Surface Under Fair-Weather Daytime Conditions
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Above an evaporating heated surface under fair-weather daytime conditions, the cospectra between the vertical velocity component, temperature, and water-vapour mixing ratio should be positive. We have applied a multi-resolution technique to a 3.64-h long, 10-Hz time series centred at midday for 16 fair-weather days at a mid-latitude site during spring to measure the averaging period τc at which the crossover from the domain of the three positive cospectra to a mixed-sign domain occurs. The τc values broadly range from 9 to 42 min, with 13 of the 16 days having values less than 30 min. When mesoscale circulations induced by surface heterogeneity are likely to be present, the vertical heat (or moisture) flux computed with the conventional averaging period of 30 min τ30 is as large (or small) as 1.09 (or 0.78) times that using τc. However, on 14 (or 13) days, the vertical heat (or moisture) fluxes using the period τc are explained by those calculated with the period τ30 within a difference range of ± 1%. The insignificant difference is due to the insensitivity of the fluxes to the averaging period at scales larger than approximately 7 min. Therefore, despite a broad range of τc values, the 30-min-averaged surface fluxes can be treated as the required turbulent fluxes. Although this finding is not robust, given that data were collected at one location over 16 days, it supports the use of 30-min-averaged surfaces fluxes, particularly for the composite midday fluxes on fair-weather days.
KeywordsAveraging period Composite midday fluxes Eddy covariance Multi-resolution technique Positive cospectral domain
This work was supported by the National Research Foundation of Korea (NRF) grand funded by the Korea government Ministry of Science and ICT (MSIT) (No. NRF-2018R1A2B6008631). The author also thanks those who collected the surface dataset at the Boulder Atmospheric Observatory (BAO) site during the XPIA field campaign.
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