Vertical Structure of Turbulence in the Lower Atmospheric Boundary Layer above a Deciduous Forest in Complex Terrain

Abstract

It is well known that parameterizations developed using observations from flat terrain have difficulty over complex terrain, which motivates a better understanding of turbulence exchanges occurring in these areas. In this work we addressed the question of how the vertical variability of turbulence features evolves over the lowest few hundred meters of the convective and nocturnal boundary layer above a forested ridge as a function of cloud cover and mean wind. We used one year of observations obtained from a WindCube V2.1 lidar installed in eastern Tennessee in the Southeast U.S. coupled with observations from a 60-m micrometeorological tower. The wind lidar has 20-m range gates spanning from 40 m to 300 m above ground. We used the lidar’s high-frequency observations to derive turbulent kinetic energy (TKE), vertical velocity variance (${\sigma }_w^2$), vertical velocity skewness (S), and kurtosis (K). We observed the largest decrease in the diurnal wind speed on clear, windy days. Under clear sky conditions, increasing TKE and ${\sigma }_w^2$ yielded positive S throughout the lower convective boundary layer. Under cloudy regimes, the distribution of TKE was height-independent and corresponded with smaller ${\sigma }_w^2$ and near-zero S. Our results provide insights into turbulence processes over forested complex terrain and support the refinement of turbulence parameterizations used in weather forecast models.