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Title:Observed microphysical characteristics and inferred thermodynamic processes contributing to the structure, evolution, and maintenance of nocturnal elevated mesoscale convective systems
Author(s):Stechman, Daniel M.
Director of Research:Rauber, Robert M.
Doctoral Committee Chair(s):Rauber, Robert M.; McFarquhar, Greg M.
Doctoral Committee Member(s):Jewett, Brian F.; Bell, Michael M.
Department / Program:Atmospheric Sciences
Discipline:Atmospheric Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:During the 2015 Plains Elevated Convection at Night (PECAN) project, airborne radar and optical array probe data were collected within 42 spiral ascents/descents of the NOAA P-3 aircraft across various regions of six mesoscale convective systems (MCSs) at multiple stages of system development. These spirals sampled MCSs corresponding to various archetypes, including several conforming to the classic leading-line/trailing-stratiform mode, and others conforming to the parallel stratiform, leading stratiform, and nonlinear system modes. In one case observations were made in the stratiform region trailing a frontal squall line. Multiple-Doppler syntheses of the wind and reflectivity fields within the 20 June 2015 MCS were used to understand the microphysical and thermodynamic characteristics observed in the spirals in the context of the MCS reflectivity and kinematic structure. A second component of this study was a statistical analysis of the microphysical and thermodynamic structures observed in 37 PECAN spirals. The thermodynamic and microphysical structure of the MCSs were analyzed in the context of three primary MCS regions, namely the transition zone (TZ), enhanced stratiform rain region (ESR), and the anvil region (AR). The 20 June MCS analysis showed that within the transition zone coincident with the rear inflow notch, cooling by sublimation of particles combined with enhanced descent within the rear inflow jet (RIJ) allowed ice particles to survive to temperatures as warm as +6.8°C. In addition, mesoscale descent associated with the RIJ allowed for subsaturated air to persist above and within the melting layer (ML) despite sustained precipitation in the observed regions. Moistening associated with sublimation occurred at a greater rate in the subsaturated air above the ML than the rate of evaporative moistening below the ML due to changes in particle concentrations accompanying increases in particle fall velocity with the phase change across the ML. The environment above the ML moistened between subsequent RIJ spirals, concurrent with maturation of the MCS and closer proximities to the convective line (CL). The effects of aggregation on the temperature dependent microphysical characteristics increased with increasing relative humidity, while the impacts of sublimation on the effects of aggregation and particle shapes, sizes and concentrations became less important. The statistical analysis of 37 PECAN spirals showed that aggregation was common within each of the three MCS regions, with its impacts on the temperature dependent microphysical characteristics the greatest in the enhanced stratiform rain region (ESR), where predominantly ice saturated conditions were found. Progressively smaller changes in particle sizes, shapes, and concentrations due to aggregation were coincident with decreases in the average relative humidity above the ML in the transition zone (TZ) and anvil region (AR). The reverse is true of the impacts of sublimation on the microphysical characteristics of these regions, such that sublimation was most dominant in the AR, where subsaturated conditions persisted to temperatures of −11°C on average and particle number and mass concentrations decreased rapidly with increasing temperature, consistent with a reduced importance of aggregation. Sublimation similarly limited the effectiveness of aggregation within the TZ. Minimal changes with respect to temperature were observed in the microphysical characteristics within the predominantly ice saturated environment of the ESR. The latent cooling imparted with sublimation is thus expected to have been the greatest within the AR, where the descent of the RIJ (if present) would likely initiate. Mesoscale ascent in the stratiform region of a trailing frontal squall line likely contributed to the notably different microphysical characteristics observed within two spirals. These exhibited increasing total number and mass concentrations with increasing temperature along with a high incidence of pristine ice crystals, characteristics absent within all other PECAN spirals.
Issue Date:2018-09-04
Rights Information:Copyright 2018 Daniel Stechman
Date Available in IDEALS:2019-02-07
Date Deposited:2018-12

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