Boundary-Layer Meteorology (2019) 171:79–99 https://doi.org/10.1007/s10546-018-0412-0 RESEARCH ARTICLE An Assessment of Eddy-Covariance-Based Surface Fluxes Above an Evaporating Heated Surface Under Fair-Weather Daytime Conditions Song-Lak Kang 1 Received: 19 March 2018 / Accepted: 15 November 2018 / Published online: 13 December 2018 © Springer Nature B.V. 2018 Abstract 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 τ c at which the crossover from the domain of the three positive cospectra to a mixed-sign domain occurs. The τ c 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 τ 30 is as large (or small) as 1.09 (or 0.78) times that using τ c . However, on 14 (or 13) days, the vertical heat (or moisture) fluxes using the period τ c are explained by those calculated with the period τ 30 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 τ c 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. Keywords Averaging period · Composite midday fluxes · Eddy covariance · Multi-resolution technique · Positive cospectral domain 1 Introduction The surface vertical fluxes of heat and moisture are the critical parameters needed to estimate the amount of energy and water vapour transferred between the Earth’s surface and the atmosphere. For atmospheric modelling, the surface fluxes are needed as input parameters, B Song-Lak Kang slkang@gwnu.ac.kr 1 Department of Atmospheric and Environmental Sciences, Gangneung-Wonju National University, 7 Jukheon-gil, Gangneung-si, Gangwon-do 25457, Republic of Korea 123