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Title: Diagnosis of mid-latitude synoptic development : a review, and a case study of a developing cyclone
Author: Graham, Richard John
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1991
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Part I of this thesis is a review of methods for diagnosing synoptic development. Emphasis is placed on the use of the quasi-geostrophic (QG) ω-equation, and on a physical interpretation of the forcing of vertical motion in the QG system. Both the Q-vector formulation (Hoskins,, 1978) and the formulation based on the vertical differential of vorticity advection and horizontal Laplacian of thermal advection (referred to as the VT form), are discussed and compared. Forms of the ω-equation, based on more general balance considerations are also reviewed; specifically the form due to Krishnamurti (1968a), and the geostrophic coordinate semi-geostrophic (SG) form of Hoskins and Draghici (1977). The role of diabatic heating in forcing vertical motion is considered, and a brief discussion of moist symmetric instability is given. The review is placed within the underlying theoretical framework of linear baroclinic instability theory, in the form developed by Eady (1949). Finally, the key concepts underlying the potential vorticity (PV) description of cyclogenesis, reviewed and extended by Hoskins (1985) are summarised. In part II a case study of the development of the 'hurricane Charley' storm which affected southern England in August 1986 is presented. It is found that the storm developed as a secondary vorticity maximum, which formed on the forward (eastern) side of the old hurricane, was intensified by the action of an upper level trough. The formation and growth of the secondary anomaly, prior to the involvement of the upper trough, is singled out for study. It was found that Sutcliffe techniques were of greater use than PV methods in diagnosing the forward development - with the low-level Q divergence giving the clearest indication of a developing secondary. Separate consideration of the confluent and shear components of the Q-vector forcing is shown to give additional insight. A diagnostic analysis, using ECMWF initialised analyses, revealed that 45-50% of the vorticity growth occurred through forcing associated with large-scale latent heat release. Moreover, latent heat release acts to transfer the locus of development away from the parent low to a position which favours growth of the secondary, and thus controls the character of the development in addition to its intensity. Latent heating is shown to enhance the baroclinic forcing and to influence the onset of 'phase locking' between the secondary and the upper trough. The effects of convection were found to be unimportant in the intensification. The diagnostic analysis is used to show that failure to correctly analyse the structure of the warm moist air mass ahead of the old hurricane circulation was a probable cause of poor 48hr forecasts of the storm.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available