Case study of large scale atmospheric blocking and a proposed instability mechanism
Date
1988-09
Authors
Crum, Francis X., author
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Abstract
Blocking is investigated from both an observational and a theoretical viewpoint. Climatological and synoptic descriptions of blocks are summarized. A review of blocking theories is presented consisting of a detailed critique of multiple-equilibria and modon research. DeĀ spite some attractive features, both of these theories have some deficiencies as explanations of observed blocks. The main body of the work is in two parts, the first of which is a case study of blocking using isentropic analysis, with particular emphasis on the distribution and evolution of potential vorticity on isentropic surfaces. Analysis of the Montgomery streamfunction on an isentropic surface is preferable to the more traditional analysis of geopotential height on a pressure surface for describing the evolution of the horizontal structure. Vertical cross sections during the mature phase show little tilt with height, a warm core and a high tropopause. Analysis of potential vorticity indicate that the block is a broad area of uniformly low potential vorticity. An EOF analysis added confirmation to this while illustrating well the development of the block. The two components of the potential vorticity - the absolute vorticity and the static stability - evolve in the same way as the potential vorticity itself, becoming smaller or larger as the potential vorticity becomes smaller or larger. Subjective interpretation of the potential vorticity maps indicates that the low potential vorticity air characterizing the block originates from well south of the block. Isentropic trajectory analysis verifies this and notes the relationship between the trajectories and the advection ahead of an intense cyclone that formed upstream of the block as the block developed. Selected moist trajectories are also computed. The second part of the work is an examination of barotropic instability in the presence of downstream and asymmetric cross-stream variations in the basic state. Solutions are obtained with the non-divergent barotropic vorticity equation linearized about an arbitrary basic state. When downstream variation is present, the maximum amplitude of the unstable stream function is always located downstream from the location of maximum latitudinal shear. This occurs because the unstable disturbance lags in adjusting to the local instability characteristics. The instability is sensitive to the degree of downstream variation with more concentration of stream function amplitude in specific regions of the channel, smaller disturbance scales and smaller growth rates when there is strong variation that when the flow is more parallel. The smaller growth rate for the downstream variation case results from the propagating disturbance being subjected to strong shear for only a finite time before moving into regions of lesser shear where it cannot grow as fast. Asymmetric cross-stream variation has little effect on the growth rate and frequency of the most unstable mode but significantly affects the structure of the instability. Larger amplitude and more pronounced tilt opposite to the shear occur on the side of the jet with the strongest shear. This can be expected from a theoretical consideration of the energetics. Instabilities in the presence of both downstream and asymmetric cross-stream variations combine the effects of each of those individual kinds of variations. An eigenvalue method and a time integration method were used to obtain these results. They compare favorably and complement one another. A single non-linear calculation shows that as the disturbances grow to finite amplitude, they split, with high centers moving to the north and low centers to the south. The disturbances also increase in scale and stabilize the mean flow without removing all the downstream variation. Additional baroclinic and non-linear calculations are proposed to investigate the hypothesis that the development of blocking patterns may depend crucially on whether the large scale flow is conducive to an instability with blocking characteristics.
Description
September 1988.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1988.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1988.
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Subject
Dynamic meteorology
Atmosphere
Atmospheric circulation