Mechanistic Analysis of Thermo-topographic Flows in Forested Terrain

The knowledge of thermo-topographic flows in forested terrain is important in understanding land-atmosphere exchanges of mass and energy. A lot of difficulties we are facing in measuring and modeling land-atmosphere exchanges over complex terrain are due to we don′t know the mechanisms of thermo-topographic flow development. Here,we study the structure and mechanisms of terrain-induced canopy flows under stably stratified conditions by a renormalized group (RNG) k-ε turbulence model. Due to heterogeneous cooling from the canopy layer to slope surface, airflows within canopy are strongly stratified. Consequently, a primary katabatic flow is formed at the top of canopy as the air above canopy sinks from lateral sides towards the hill. However, the sinking motion is diverted following the shape of top-canopy layer as it reaches the top of canopy. The air sinking from the center hill can directly reach the hill crest and blow along the slope to form the secondary katabatic wind. Flows within canopy shift between katabatic flow layers. The shifting direction relies on the slope (H/L, Figure 1). Air sweeps horizontally from the bottom of primary katabatic layer to the secondary katabatic flow as the slope is gentle, but jumps perpendicularly from the secondary katabatic layer to slope surface to join the primary katabatic flow as the slope is steep. The generation and direction of the shifting-wind structure are primarily driven by the slope and stratification. The buoyancy force on the slope is given by g(Δθ/θ₀)sinα≈g(Δθ/θ₀)H/L, whereα is the slope angle, Δθ is the potential temperature difference between the ambient air and the colder slope flow, θ₀ is the ambient potential temperature. The buoyancy force is much larger on a steep slope than a gentle slope, leading to a stronger sinking motion above the crest that can reach the lower part of the canopy. However, the sinking motion above the crest on the gentle slope is diverted to follow the shape of slope in the upper canopy. As a result, the primary katabatic layer is deeper than the secondary katabatic layer on gentle slope, in contrast to that on steep slope. The heterogeneous cooling in the canopy layer causes two baroclinic zones in consistent with the katabatic layers: the upper canopy layer and slope surface layer. The strong baroclinity on the steep slope surface causes the wind in deep secondary katabatic layer rotating counter-clockwise. However, the rotated wind is forced to shift down when hitting the top-canopy primary katabatic layer. The wind at baroclinic zone with deep primary katabatic flow on gentle slope rotates clockwise, but shifts down slope when hitting the slope-surface secondary katabatic layer. Acknowledgement: This research was supported by NSF Grants ATM-0930015, CNS-0958379 and CNS-0855217, PSC-CUNY ENHC-42-64 and CUNY HPCC.