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Title: Boundary layer frictional processes in mid-latitude cyclones
Author: Adamson, Daniel Stephen
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2001
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This Thesis develops understanding of the physical mechanisms responsible for the frictional effects observed in cyclone development. To do this, a first-order closure mixing-length boundary layer scheme has been added to a baroclinic life cycle model to accurately represent the frictional processes occurring in cyclone development. Life cycles simulated with the model consist of normal mode baroclinic growth with cyclone development followed by barotropic decay. By considering life cycles where friction is the only diabatic process, it is found that surface drag reduces rates of baroclinic growth and barotropic decay by 40%. The classical description of frictional effects in rotating geophysical flows involves the Ekman spin-down of a barotropic vortex. This mechanism is studied by considering the quasi-geostrophic w-equation with a frictional term. However, these barotropic vortex ideas do not account for the baroclinic processes occurring, especially within the frontal regions. To address these shortcomings, a potential vorticity (PV) approach is adopted. Large frictionally generated positive PV anomalies form close to developing warm and cold fronts, due to the relative alignment of surface and thermal wind vectors. These PV anomalies are advected upwards and polewards along the warm conveyor belt and then westwards. This results in a band of positive PV associated with high static stability in the lower troposphere above the surface low centre. Using Rossby edge wave theory, a mechanism is proposed to explain the reduced baroclinic development observed in terms of this positive PV anomaly. Hence the baroclinic dynamics are shown to play a crucial role in the frictional modification of cyclone development. The classical notion of Ekman spin-down is shown to be of secondary importance. This mechanism by which frictional processes reduce cyclone development is found also to be valid in the presence of sensible and latent heat fluxes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available