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Title:Observations of misovortices within the 7 January 2014 Long Lake-Axis-Parallel lake-effect snow band during the Ontario Winter Lake-effect Systems Project
Author(s):Mulholland, Jake Patrick
Advisor(s):Rauber, Robert M; Frame, Jeffrey
Department / Program:Atmospheric Sciences
Discipline:Atmospheric Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:M.S.
Genre:Thesis
Subject(s):lake-effect
lake-effect snow
misovortices
mesovortices
LLAP band
snow
Abstract:Recent lake-effect snow field projects in the Eastern Great Lakes region [e.g., the Long Lake-Axis-Parallel (LLAP; 2010-2011) Project and the Ontario Winter Lake-effect Systems (OWLeS; 2013-2014) Project] have revealed the presence of misovortices with diameters between 40 and 4000 m within LLAP bands in the vicinity of Lake Ontario. These misovortices usually develop along convergence boundaries associated with cyclonic horizontal shear zones that likely originate from solenoidally-forced secondary circulations within these warm-core bands. In most cases, the shear zone is co-located with a sharp horizontal gradient in radar reflectivity, which corresponds to the region of maximum vertical velocities. One particular band from the OWLeS Project, in which an abundance of misovortices developed, occurred on 7 January 2014. Steiger et al. (2013) postulated that the release of horizontal shearing instability (HSI) was the main mechanism for misovortexgenesis in two LLAP bands analyzed during the LLAP Project. With data from only a single mobile Doppler radar available, however, this hypothesis was not testable. In the present study, three-dimensional dual-Doppler (DD) wind syntheses reveal that two separate criteria for HSI are met along the low-level horizontal shear zone within this band. This strongly suggests that HSI was the likely cause of the misovortices within this band. Furthermore, the lack of anticyclonic-cyclonic vortex couplets throughout the event reveal that tilting of horizontal vorticity into the vertical is likely of less importance compared to the release of HSI and subsequent vortex strengthening via stretching of low-level vertical vorticity. This vortex stretching is maximized along the low-level convergence zone, where vertical velocities (generally between 1 - 3 m s-1) are greatest owing to the solenoidally-forced transverse secondary circulation of the band. A high-resolution Weather Research and Forecasting (WRF) simulation was conducted for this case study. The 333-m horizontal resolution domain depicts the presence of misovortices along the cyclonic horizontal shear zone within the band. The simulation also reveals that a linear vortex sheet originates in the vicinity of Lake Huron and Georgian Bay, and extends over Lake Ontario. This vortex sheet breaks into discrete vortices over Lake Ontario. The simulated vortices display remarkable similarities to the DD analyses in terms of intensity, depth, spacing, and size. The simulated vortices are persistent features over the length of the lake with lifetimes of well over 30 minutes. Once the vortices reach the eastern shore of Lake Ontario, however, they quickly dissipate. Competing hypotheses related to misovortexgenesis are presented herein and tested to reveal the key kinematic and dynamic properties of the misovortices within this snow band. Additionally, the origin of the vortex sheet is analyzed using the WRF simulation.
Issue Date:2016-07-19
Type:Thesis
URI:http://hdl.handle.net/2142/92653
Rights Information:Copyright 2016 Jake Mulholland
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08


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