7 Why Canxt SBL Models Reproduce the Near-Zero Heat Flux at High Wind Speeds?

Monday, 20 June 2016
Alta-Deer Valley (Sheraton Salt Lake City Hotel)
Rafael Maroneze, UFSM, Santa Maria, RS, Brazil; and J. Sun, F. D. Costa, and O. C. Acevedo
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Handout (800.4 kB)

Many observations taken at different surfaces show that heat fluxes approach zero for high wind speeds, as the temperature gradients tend to be destroyed in such situations. However, it does not happen in numerical models used to reproduce the nocturnal boundary layer flow. In these models, it is usually possible to discern a maximum of the absolute heat flux at intermediate stabilities, but not the flux vanishing at the neutral limit. The main reason is that, differently from what happens in the real world, the model atmosphere near the surface never becomes truly neutral, because a stable thermal gradient is always present, regardless of wind speed. In the present study, this characteristic of numerical models is explored, considering different ways of parameterizing the surface fluxes. The modelsx inability to totally destroy the thermal gradient is related to the way the soil and surface properties are parameterized. It is shown that if an alternative scheme using a layer of air as lower boundary condition is used, fluxes and thermal gradients are removed under intense winds. Furthermore, the relationship between turbulence and wind speed in such idealized scheme reproduces the double-regime behavior observed by Sun et al. (2012)
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