A field study of the turbulent fluxes of heat, water vapour and momentum at a ‘typical’ agricultural site

Abstract

AbstractAn extensive set of measurements of turbulent fluxes of sensible heat, water vapour and momentum together with mean profiles of temperature, water vapour and wind speed at a ‘non‐ideal’ agricultural site in southern Sweden have been analysed in terms of the Monin‐Obukhov similarity theory. The fluxes of sensible heat, H, and water vapour, E, have been determined with the eddy correlation method, the accuracy of H + ELv being c. Φ 12% according to comparison with net radiation minus ground heat flux. The momentum flux has been determined from low level wind measurements, with a stability dependent ‘skin friction method’.During stationary conditions with winds from ‘undisturbed’ directions (extensive forest areas more than 3 km distant) and unstable stratification, the data for dimensionless wind and temperature gradients, Φm and ϕh, respectively, are well described by the expressions put forward by Businger et al. (1971), with von Kàrmàn's constant, k = 0.35. The corresponding dimensionless water vapour gradient, Φe, is shown also to follow similarity during these conditions (although exhibiting larger scatter than the two other plots), coinciding with Φh for strong instability but approaching Φm= 1.35 Φh, at neutrality.For stable stratification the dimensionless profiles do not follow similarity in this study, nor do cases with winds from a 700 m distant forest nor non‐stationary cases, as might be expected.A combined energy balance and gradient approach is shown to give good results not only for the ‘ideal’, unstable case but also for non‐stationary conditions and for conditions with winds from the ‘disturbed’ direction. With temperature and humidity differences measured between 0.5 and 1.14 m above ground and with Φe/Φh = 1.35 there is no systematic difference between measured and calculated fluxes, the accuracy in an individual calculated flux being better than 20%. For the stable case it is shown that the flux of sensible heat can be determined with good accuracy with the flux‐gradient method, using data from the 0.5 ‐ 1.14 m layer. The water vapour flux is obtained with good accuracy as a residual term from the energy balance equation.