An Energy Budget Model to Calculate the Low Atmosphere Profiles of Effective Sound Speed at Night

Report No. ARL-MR-563
Authors: Arnold Tunick
Date/Pages: May 2003; 30 pages
Abstract: This report presents an alternate energy budget model to the one implemented in the Acoustic Battlefield Aid (ABFA) for generating low atmosphere profiles of effective sound speed at night. The alternate model is based on the solution of a quartic equation for surface temperature, which assumes a balance between the net (long wave) radiative flux and the turbulent fluxes of heat and moisture close to the ground. In contrast, the model in ABFA assumes that the net surface radiation is constant with time at night, reduced only in the presence of clouds, and is balanced entirely by the surface heat flux. Therefore, the alternate model may be an improved calculation for this application. However, some difficulties were encountered in the application of traditional Monin-Obukhov (M-O) profile theory for calculating nighttime turbulence structure from wind speed, temperature, and humidity difference data. Model breakdowns were problematic for a nighttime case when surface wind speeds were low, U < 4.0 ms-1, and skies were clear. During such stable and very stable conditions, the M-O scaling length too often became small?small enough to cause a computational sensitivity. Thus, we recommend that the alternate model in its present form be applied only when surface wind speeds and surface roughness lengths (i.e., canopy heights) can affect values for the M-O scaling length larger than about 13 m.
Distribution: Approved for public release
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Last Update / Reviewed: May 1, 2003