The Measurement and Investigation of Atmospheric Flows in Complex Terrain using an Airborne Doppler Lidar

Godwin, Kevin S., Department of Environmental Sciences, University of Virginia
De Wekker, Stephan, Department of Environmental Sciences, University of Virginia

The measurement of flows in complex terrain is difficult. In this thesis, an attempt is made to measure these flows using airborne Doppler lidar data obtained on 12 November 2007 over the Salinas Valley in central California. Uncertainties and challenges related to the retrieval of winds from airborne Doppler lidar are also investigated.
The measurements show evidence of multiple atmospheric flows and aerosol
layers within the valley. In the presence of northwesterly synoptic winds a shallow sea breeze transitions into an up-valley flow that accelerates along the valley floor. At the same time, a northerly cross-valley flow at ridge height combined with thermally driven flows over the sunlit slopes creates a cross-valley circulation resulting in an asymmetric distribution of aerosols within the valley volume.
Two of the challenges in the retrieval of near-surface winds from airborne
Doppler lidar are (1) the retrieval of near surface winds in the presence of intense ground backscatter and (2) the uncertainty of 3-dimensional wind profiles caused by wind heterogeneity often found in complex terrain. Using a two-step approach that involves tracking aerosol radial velocities beyond ground interference (threading) and applying a raw data ground intensity filter, additional winds can now be resolved 50-100 m closer to the surface than previously possible. These winds reveal a low level jet that was not
observed using conventional processing techniques. The calculation of three-dimensional wind profiles from airborne Doppler lidar data makes a significant assumption that winds within the scanning volume are homogeneous. Over complex terrain this assumption is typically violated. Approaches are discussed to reduce this uncertainty. including the use of contiguous segments of a conical scan, the design of specific scanning patterns to reduce the scanning volume, and the identification of particular radial velocity patterns in an environment characterized by wind shear.

MA (Master of Arts)
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