Publication Date

2023

Document Type

Dissertation/Thesis

First Advisor

Gensini, Vittorio A.

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geographic and Atmospheric Sciences

Abstract

Elevated mixed layers (EMLs) are an important influence on the severe convective storm climatology in the contiguous United States (CONUS), playing a role in storm generation, sustenance, and suppression. A function of the topography in the western CONUS and northern Mexico, EMLs are elevated layers of nearly dry adiabatic lapse rates and high potential temperature, typically with a capping inversion at their base. Although it is well-established that EMLs are primarily a warm-season phenomenon most frequent in the Great Plains, no research to date has examined their variability in-depth, or whether they have changed through time. This study creates an updated, high-resolution climatology of the EML to analyze EML variability and changes in EML occurrence and characteristics over the last four decades. An objective algorithm is applied to ECMWF Reanalysis Version 5 (ERA5) to detect EMLs, defined in part as layers of steep lapse rates (≥8.0 ℃∙km^(-1)) at least 200 mb thick, in the CONUS and northern Mexico from 1979 to 2021. The interannual, intra-annual, and seasonal variability of EMLs are investigated, as are the typical values and ranges of EML attributes including lapse rates, potential temperature, and convective inhibition (CIN). Long-term trends in these attributes and EML days are calculated and assessed for significance. Additionally, practically perfect hindcasts of hail and tornadoes are used to assess whether severe thunderstorms favor certain regions relative to the EML center. Results reveal that EMLs are most frequent over the Great Plains in spring and summer, with a standard deviation of 4–10 EML days per year highlighting sizable interannual variability. Mean CIN associated with the EML’s capping inversion suggests many EMLs prohibit convection, although, like nearly all EML characteristics, there is considerable spread and notable seasonal variability. In the western Great Plains, statistically significant increases in EML days (four to five more days per decade) coincide with warmer EML bases and steeper EML lapse rates, driven by warming and drying in the low levels of the western CONUS during the study period. Additionally, increases in EML base temperatures result in significantly more EML-related CIN over the Great Plains, which may continue to have implications for storm frequency, intensity, severe perils, and precipitation if this trend persists into the future.

Extent

109 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

Media Type

Text

Available for download on Friday, March 08, 2024

Included in

Meteorology Commons

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