Wind profiles at the Boulder Tower

Abstract

Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:

• (1) The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.

• (2) The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.

• (3) The form φ_m=(1−15z/L)^-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ _m ∼ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ _m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ _m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.

• (4) With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.

• (5) Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.