Article

Boundary-Layer Meteorology

, Volume 138, Issue 2, pp 231-262

Open Access This content is freely available online to anyone, anywhere at any time.

Atmospheric Stability Effects on Wind Fields and Scalar Mixing Within and Just Above a Subalpine Forest in Sloping Terrain

  • Sean P. BurnsAffiliated withNational Center for Atmospheric ResearchDepartment of Ecology and Evolutionary Biology, University of Colorado Email author 
  • , Jielun SunAffiliated withNational Center for Atmospheric Research
  • , Donald H. LenschowAffiliated withNational Center for Atmospheric Research
  • , Steven P. OncleyAffiliated withNational Center for Atmospheric Research
  • , Britton B. StephensAffiliated withNational Center for Atmospheric Research
  • , Chuixiang YiAffiliated withSchool of Earth and Environmental Sciences, Queens College
  • , Dean E. AndersonAffiliated withU.S. Geological Survey
  • , Jia HuAffiliated withDepartment of Ecology and Evolutionary Biology, University of Colorado
  • , Russell K. MonsonAffiliated withDepartment of Ecology and Evolutionary Biology, University of Colorado

Abstract

Air temperature T a , specific humidity q, CO2 mole fraction χ c , and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri b . For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri b < 0.5. For strongly stable conditions (Ri b > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χ c have with Ri b are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO2 near the ground being 5–7 μmol mol−1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri b -binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO2 advection over heterogenous, complex terrain are discussed.

Keywords

Canopy-layer turbulence Carbon in the Mountains Experiment (CME04) Complex terrain Richardson number Scalar mixing Wind fields