Near-Neutral Surface Layer Turbulence at the Boulder Atmospheric Observatory Tower: Evidence of Increasing Vertical Turbulence with Height

Brent M. Bowen Atmospheric Sciences Division, Lawrence Livermore National Laboratory, Livermore, California

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Abstract

Wind and turbulence profiles were analyzed during breezy, near-neutral conditions at the Boulder Atmospheric Observatory tower to quantify the effects of an abrupt 20- to 30-m increase in terrain located 3–5 km west of the tower. Results indicate that regional roughness length z0 is greatest and local z0 is smallest in sectors downwind of a long, shallow bluff, suggesting that distant, more complex, upwind terrain enhances horizontal turbulence but has little effect on local wind speed profiles. Turbulence parameter profiles are nearly constant up to 200 m above ground level (AGL), and friction velocity u∗ is nearly constant up to 150–200 m for simple fetch with z0 equal to 4 cm and near-surface wind speed approximately equal to 7 m s−1 at 10 m AGL. Horizontal turbulence parameters, however, increase by 50% to nearly 100% at all tower levels downwind of the distant terrain bluff when compared with those with simple fetch. The effect of the bluff on vertical turbulence increases with height. Although the effect on the 10-m standard deviation of vertical wind speed σw is negligible, the 200-m σw increases by about 0.5 m s−1, or about twice the 10-m value. The u∗ also increases by 16% to nearly 100% between the 10- and 200-m heights downwind of the bluff. Primarily because of the enhanced σw downwind of the bluff, atmospheric dispersion could be underestimated by factors of about 2.5, 2.0, and 1.3 at the 200-, 100-, and 50-m levels, respectively, if the vertical and transverse standard deviations of the wind angle fluctuations are estimated from 10-m values.

Corresponding author address: Brent M. Bowen, Lawrence Livermore National Laboratory, National Atmospheric Release Advisory Capability (NARAC), L-103, P.O. Box 808, Livermore, CA 94551.

bbowen@llnl.gov

Abstract

Wind and turbulence profiles were analyzed during breezy, near-neutral conditions at the Boulder Atmospheric Observatory tower to quantify the effects of an abrupt 20- to 30-m increase in terrain located 3–5 km west of the tower. Results indicate that regional roughness length z0 is greatest and local z0 is smallest in sectors downwind of a long, shallow bluff, suggesting that distant, more complex, upwind terrain enhances horizontal turbulence but has little effect on local wind speed profiles. Turbulence parameter profiles are nearly constant up to 200 m above ground level (AGL), and friction velocity u∗ is nearly constant up to 150–200 m for simple fetch with z0 equal to 4 cm and near-surface wind speed approximately equal to 7 m s−1 at 10 m AGL. Horizontal turbulence parameters, however, increase by 50% to nearly 100% at all tower levels downwind of the distant terrain bluff when compared with those with simple fetch. The effect of the bluff on vertical turbulence increases with height. Although the effect on the 10-m standard deviation of vertical wind speed σw is negligible, the 200-m σw increases by about 0.5 m s−1, or about twice the 10-m value. The u∗ also increases by 16% to nearly 100% between the 10- and 200-m heights downwind of the bluff. Primarily because of the enhanced σw downwind of the bluff, atmospheric dispersion could be underestimated by factors of about 2.5, 2.0, and 1.3 at the 200-, 100-, and 50-m levels, respectively, if the vertical and transverse standard deviations of the wind angle fluctuations are estimated from 10-m values.

Corresponding author address: Brent M. Bowen, Lawrence Livermore National Laboratory, National Atmospheric Release Advisory Capability (NARAC), L-103, P.O. Box 808, Livermore, CA 94551.

bbowen@llnl.gov

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