On the Contrasting Roles of Foliated versus Nonfoliated Forest Canopies Governing Clear Night Turbulence over Complex Terrain

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Abstract
The absence of routine micrometeorological observations over complex terrain has resulted in knowledge gaps on how heat, momentum, and mass fluxes are modified by different canopies and stabilities. For weather forecast and air quality models, however, it is essential to represent nocturnal boundary layer (NBL) processes across different foliation and stability regimes. Oftentimes, the turbulence-exchange processes are overly simplified which can cause incorrectly modeled near-surface thermodynamics and kinematics. We investigated differences in turbulence characteristics within and above foliated versus nonfoliated canopies under low and high stability regimes on clear nights. We used observations from a wind lidar, which samples from 40 to 300 m above ground level, and two micrometeorological towers installed along a forested ridge in eastern Tennessee in the Southeast United States. We used the tower-derived observations to obtain below- and above-canopy horizontal wind ( U ), turbulent kinetic energy (TKE), and friction velocity ( ) and the bulk Richardson number (Ri b ) to determine stability. Irrespective of stability or season, negative skewness in the lidar-derived vertical velocity fields was associated with the downward transport of turbulence, resulting in above-canopy TKE and increases and sporadic increases in below-canopy TKE and . Overall, the smallest TKE and corresponded with the most stable conditions. These findings illustrate how lidar measurements coupled with micrometeorological tower observations can advance knowledge of NBL processes that are critical to better represent in weather forecasting models. Significance Statement: Physical processes happening near Earth’s surface during the nighttime are poorly represented within weather forecast models, particularly areas with forests and ridges. We examined nighttime turbulence characteristics using observations from surface-based instrumentation and a new lidar capable of providing fine-scale details about lower-atmospheric structure. We found occasional large nighttime increases in above-canopy wind speeds and turbulence due to the downward transport of air from aloft. When the atmosphere remained well mixed during the nighttime in the cool season, there was also an increase in those quantities within the lower forest canopy. The findings underscore how new observing systems provide insights on meteorological processes occurring during the nighttime to better enable their representation in weather forecasting models.

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