Methods to Estimate Surface Fluxes of Momentum and Heat from Routine Weather Observations for Dispersion Applications under Stable Stratification |
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| Authors: | Ashok K Luhar Ken N Rayner |
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| Institution: | (1) Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, PMB 1, Aspendale, VIC, 3195, Australia;(2) Department of Environment and Conservation Western Australia, 181-205 Davy Street, Booragoon, WA, 6154, Australia |
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| Abstract: | We examine the efficacy of two methods commonly used to estimate the vertical turbulent fluxes of momentum and sensible heat
from routinely observed mean quantities in the surface layer under stable stratification. The single-level method uses mean
wind speed and temperature measurements at a single height, whereas the two-level method uses mean wind speed measurements
at a single height and mean temperature measurements at two heights. These methods are used in popular meteorological processors
such as the U.S. Environmental Protection Agency approved AERMET and CALMET to generate inputs for dispersion simulations.
We use data from a flux station of the U.K. Met Office at Cardington for comparison. It is found that the single-level method
does not describe the flux variation in the weakly stable regime at all, because of its assumption that the temperature scale,
i.e. the ratio of the kinematic sensible heat flux to the friction velocity, is constant, which is plausible only under strongly
stable conditions. On the other hand, the two-level method provides a physically realistic variation of the fluxes with stability,
but the required temperature measurements at two levels are usually not available on a routine basis. If measurements of the
standard deviation of temperature are also available, in addition to the mean temperature at a single level, then they can
be usefully employed in a third (single-level) method, with the consequence that the computed fluxes are very similar to those
obtained from the two-level method. An improvement to the original single-level method is considered, and flux calculations
under low wind conditions are also discussed. |
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| Keywords: | AERMET CALMET Cardington data Flux– gradient relationships Low winds Monin– Obukhov similarity theory Surface layer |
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