The study of boundary layer flow through a forest canopy on complex terrain has, until recently, been limited to modelling and laboratory studies. This thesis presents a unique set of field measurements from within and above a canopy situated on a ridge. A climatological study of the observed dataset is presented to identify the significant fea- tures of these flows that differentiate them from air flows above and within a homogeneous canopy on flat terrain. The ridge is found to impact on the flows in the following ways. On the summit the velocity profile resembles that of a canopy profile on flat terrain with little variation in wind.speed below the canopy and an obvious inflection point at the canopy top. On the windward slope, the inflection point disappears. Significant amounts of -u'w' at the canopy top indicates that turbulent mixing acts strongly to transport higher mo- mentum air down into the canopy, which smooths the layer of shear. The profile on the lee slope is dependent on the size of a separation region that can develop on the lee slope of the forested ridge. The direction of the mean wind within the canopy on the lee slope is dependent on the hill-induced pressure gradient, which tends to drive a reversed flow up the lee slope, and on the turbulent mixing which tends to drive the flow down-slope through the mixing of higher momentum air from above the canopy. If the hill slope is sufficiently large (so the pressure gradient is large), or the canopy is sufficiently deep (so that turbulence is unable to mix the higher momentum air all the way to the bottom), then flow separation can occur. Case studies are presented to investigate the formation and development of the separation region on the lee slope of the forested ridge. The presence of a flow separation region is observed to extend the width of the dynamic pressure profile such that, as the separation region expands up the lee slope towards the summit, the minimum is forced back to the upwind edge of the separation region. Large scale separation is observed on the ridge, whereby the separation region extends beyond the top of the canopy. Within the separation region, there is little variation in wind speed or vertical momentum flux with height as the inflection point is elevated to the top of the separation region. Comparisons between the observed case studies and model simulations are made to quan- tify the success of the model at simulating canopy air flows over complex terrain. The model is found to successfully capture the main features the these flows. Areas where the model was less successful are attributed to the inhomogeneous nature of the canopy and the terrain at the field site, and to the low resolution of the model.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:550799 |
| Date | January 2011 |
| Creators | Grant, Eleanor Rose |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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