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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| Institution: | (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 r
S 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 r
S decreases rapidly soon after rainfall.An exponential relation is found between the fraction of available energy used as evaporative flux,  , and r
S for values of r
I/rS <0.70, where r
I is the climatological resistance. On the other hand, the ratio of r
I to r
S 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 (r
S) versus r
I/rS versus ) are combined to obtain an empirical relation of the form r
I=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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