Retrospective Analysis of Micrometeorological Observations Above an Australian Wheat Crop

摘要

We apply well-established flux-gradient relationships to deduce the aerodynamic and radiative properties of a winter wheat crop, using a neglected 1971 dataset (hourly averages), only recently resurrected as part of a historical review of precision CO(2)measurements in Australia (Pearman et al. in Hist Rec Aust Sci 28:111-125, 2017). The aerodynamic roughness length (seasonal variation between 0.07 and 0.14 of the mean crop height) and broadband albedo (seasonal variation between 0.13 and 0.23) are consistent with values published in the literature over the past 50 years. Net radiation at night is found to agree with the net longwave flux only when the dry-bulb temperature exceeds 10 degrees C, probably the result of dewfall on one or both of the two instruments. During the day, the sum of the four individual radiative flux components (upwards and downwards shortwave and longwave)-the composite net radiation-exceeds the directly measured net radiation, from near zero at sunrise to approximately 100 W m(-2)at maximum net radiation approximate to 600 W m(-2), viz. an underestimate in the directly measured net radiation of close to 15. Again, this is in line with instrument comparisons made in the USA and Europe 15-25 years ago. A novel approach is used in the analysis of terms in the surface energy budget, viz., normalization of all terms by the downwelling shortwave flux. Normalization reveals, (1) near-normal frequency distributions of both the total turbulent heat flux (sensible plus latent) and the implied total storage (the residual); (2) significant diurnal variations in the total turbulent heat flux, whose standard deviations of individual values about any hourly mean during daytime are reduced significantly on those for either the sensible or latent heat flux; (3) an implied storage term with a well-defined diurnal variation, but with an overall mean value of 1 of the shortwave input. Overall, with the above results in mind, the computed momentum and heat fluxes (and also the CO(2)flux) during the daytime, at small to moderate gradient Richardson numbers, provide support for the profile approach when eddy-correlation fluxes are unavailable. Even so, possible errors due to, (1) uncertainties in the zero-plane displacement, and (2) influences of the roughness sublayer, must be borne in mind.