Using enstrophy-based diagnostics to examine the dynamic stability characteristics of anticyclones
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Blocking anticyclones are quasi-stationary atmospheric phenomena. Past research has demonstrated that blocking onset and decay periods are associated with accumulating instabilities and consequent lack of predictability. Moreover, previous results have suggested that integrated enstrophy (IRE) may be used as a measure of the instability at block onset and decay. In this dissertation, enstrophy advection (DIRE) is shown to be an indicator of instability at block onset and decay along with the IRE. More specifically, the sign of the DIRE may be used to detect the changes in instability at block onset and decay. While these enstrophy-based diagnostics are useful, a more complete understanding of them can be achieved within the framework of the two-dimensional equation of motion. Within this framework, I provide a partial answer to the question of which quantities play a role in the flow destabilization before blocking onset and decay. In particular, variations in geopotential height, relative vorticity, zonal wind, and resultant deformation tendency play a role in flow destabilization. The flow instability at block onset and decay may lead to instances in which blocking is not predicted well. Research has suggested that block decay in particular may be underpredicted. Here, the IRE and DIRE are used to evaluate the performance of the Global Ensemble Forecast System (GEFS) in predicting the onset and decay of blocking. The result is that GEFS tends to underpredict block decay. Finally, since relative maxima in the IRE have been shown to be a necessary condition for blocking onset and decay, I use the IRE here to evaluate climate model performance in simulating atmospheric blocking dynamics.
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