Calculation of sensible and latent heat fluxes,and surface resistance from profile data |
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Authors: | Adrian DeHeer-Amissah Ulf Högström Ann-Sofi Smedman-Högström |
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Affiliation: | (1) Department of Meteorology, University of Uppsala, Uppsala, Sweden |
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Abstract: | A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.It is shown that surface resistance rS exhibits a diurnal trend: values are smallest (ca. 150 s m-1) a few hours before noon and increase to as much as 800 s m-1 towards dusk. The minimum values tend to be higher during dry periods when the soil moisture is low. There is also some indication that rS decreases rapidly soon after rainfall.An exponential relation is found between the fraction of available energy used as evaporative flux, , and rS for values of rI/rS <0.70, where rI is the climatological resistance. On the other hand, the ratio of rI to rS is linearly correlated with , implying that an equilibrium state is established between the grass surface and the atmosphere, at least from mid-morning to mid-afternoon when the leaves are dry. Near-noon values calculated by Stewart and Thom for Thetford Forest also follow a linear trend.The above two regression results (In (rS) versus rI/rS versus ) are combined to obtain an empirical relation of the form rI=m exp (a-b) which is used to estimate evaporative flux. The estimates are found to be within 20% of calculated values.Guest Scientist from Department of Physics, University of Cape Coast, Cape Coast, Ghana. |
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