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Kevin M. Grise
,
Sean M. Davis
,
Isla R. Simpson
,
Darryn W. Waugh
,
Qiang Fu
,
Robert J. Allen
,
Karen H. Rosenlof
,
Caroline C. Ummenhofer
,
Kristopher B. Karnauskas
,
Amanda C. Maycock
,
Xiao-Wei Quan
,
Thomas Birner
, and
Paul W. Staten

Abstract

Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.

Full access
Pedro L. Fernández-Cabán
,
A. Addison Alford
,
Martin J. Bell
,
Michael I. Biggerstaff
,
Gordon D. Carrie
,
Brian Hirth
,
Karen Kosiba
,
Brian M. Phillips
,
John L. Schroeder
,
Sean M. Waugh
,
Eric Williford
,
Joshua Wurman
, and
Forrest J. Masters

Abstract

While Hurricane Harvey will best be remembered for record rainfall that led to widespread flooding in southeastern Texas and western Louisiana, the storm also produced some of the most extreme wind speeds ever to be captured by an adaptive mesonet at landfall. This paper describes the unique tools and the strategy used by the Digital Hurricane Consortium (DHC), an ad hoc group of atmospheric scientists and wind engineers, to intercept and collect high-resolution measurements of Harvey’s inner core and eyewall as it passed over Aransas Bay into mainland Texas. The DHC successfully deployed more than 25 observational assets, leading to an unprecedented view of the boundary layer and winds aloft in the eyewall of a major hurricane at landfall. Analysis of anemometric measurements and mobile radar data during heavy convection shows the kinematic structure of the hurricane at landfall and the suspected influence of circulations aloft on surface winds and extreme surface gusts. Evidence of mesoscale vortices in the interior of the eyewall is also presented. Finally, the paper reports on an atmospheric sounding in the inner eyewall that produced an exceptionally large and potentially record value of precipitable water content for observed soundings in the continental United States.

Full access
Paul W. Staten
,
Kevin M. Grise
,
Sean M. Davis
,
Kristopher B. Karnauskas
,
Darryn W. Waugh
,
Amanda C. Maycock
,
Qiang Fu
,
Kerry Cook
,
Ori Adam
,
Isla R. Simpson
,
Robert J Allen
,
Karen Rosenlof
,
Gang Chen
,
Caroline C. Ummenhofer
,
Xiao-Wei Quan
,
James P. Kossin
,
Nicholas A. Davis
, and
Seok-Woo Son

Abstract

Over the past 15 years, numerous studies have suggested that the sinking branches of Earth’s Hadley circulation and the associated subtropical dry zones have shifted poleward over the late twentieth century and early twenty-first century. Early estimates of this tropical widening from satellite observations and reanalyses varied from 0.25° to 3° latitude per decade, while estimates from global climate models show widening at the lower end of the observed range. In 2016, two working groups, the U.S. Climate Variability and Predictability (CLIVAR) working group on the Changing Width of the Tropical Belt and the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project, were formed to synthesize current understanding of the magnitude, causes, and impacts of the recent tropical widening evident in observations. These working groups concluded that the large rates of observed tropical widening noted by earlier studies resulted from their use of metrics that poorly capture changes in the Hadley circulation, or from the use of reanalyses that contained spurious trends. Accounting for these issues reduces the range of observed expansion rates to 0.25°–0.5° latitude decade‒1—within the range from model simulations. Models indicate that most of the recent Northern Hemisphere tropical widening is consistent with natural variability, whereas increasing greenhouse gases and decreasing stratospheric ozone likely played an important role in Southern Hemisphere widening. Whatever the cause or rate of expansion, understanding the regional impacts of tropical widening requires additional work, as different forcings can produce different regional patterns of widening.

Free access
Paul W. Staten
,
Kevin M. Grise
,
Sean M. Davis
,
Kristopher B. Karnauskas
,
Darryn W. Waugh
,
Amanda C. Maycock
,
Qiang Fu
,
Kerry Cook
,
Ori Adam
,
Isla R. Simpson
,
Robert J Allen
,
Karen Rosenlof
,
Gang Chen
,
Caroline C. Ummenhofer
,
Xiao-Wei Quan
,
James P. Kossin
,
Nicholas A. Davis
, and
Seok-Woo Son
Full access
Gijs de Boer
,
Constantin Diehl
,
Jamey Jacob
,
Adam Houston
,
Suzanne W. Smith
,
Phillip Chilson
,
David G. Schmale III
,
Janet Intrieri
,
James Pinto
,
Jack Elston
,
David Brus
,
Osku Kemppinen
,
Alex Clark
,
Dale Lawrence
,
Sean C. C. Bailey
,
Michael P. Sama
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Amy Frazier
,
Christopher Crick
,
Victoria Natalie
,
Elizabeth Pillar-Little
,
Petra Klein
,
Sean Waugh
,
Julie K. Lundquist
,
Lindsay Barbieri
,
Stephan T. Kral
,
Anders A. Jensen
,
Cory Dixon
,
Steven Borenstein
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Daniel Hesselius
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Kathleen Human
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Philip Hall
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Brian Argrow
,
Troy Thornberry
,
Randy Wright
, and
Jason T. Kelly

ABSTRACT

Because unmanned aircraft systems (UAS) offer new perspectives on the atmosphere, their use in atmospheric science is expanding rapidly. In support of this growth, the International Society for Atmospheric Research Using Remotely-Piloted Aircraft (ISARRA) has been developed and has convened annual meetings and “flight weeks.” The 2018 flight week, dubbed the Lower Atmospheric Profiling Studies at Elevation–A Remotely-Piloted Aircraft Team Experiment (LAPSE-RATE), involved a 1-week deployment to Colorado’s San Luis Valley. Between 14 and 20 July 2018 over 100 students, scientists, engineers, pilots, and outreach coordinators conducted an intensive field operation using unmanned aircraft and ground-based assets to develop datasets, community, and capabilities. In addition to a coordinated “Community Day” which offered a chance for groups to share their aircraft and science with the San Luis Valley community, LAPSE-RATE participants conducted nearly 1,300 research flights totaling over 250 flight hours. The measurements collected have been used to advance capabilities (instrumentation, platforms, sampling techniques, and modeling tools), conduct a detailed system intercomparison study, develop new collaborations, and foster community support for the use of UAS in atmospheric science.

Free access
Karen A. Kosiba
,
Anthony W. Lyza
,
Robert J. Trapp
,
Erik N. Rasmussen
,
Matthew Parker
,
Michael I. Biggerstaff
,
Stephen W. Nesbitt
,
Christopher C. Weiss
,
Joshua Wurman
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Kevin R. Knupp
,
Brice Coffer
,
Vanna C. Chmielewski
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Daniel T. Dawson
,
Eric Bruning
,
Tyler M. Bell
,
Michael C. Coniglio
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Todd A. Murphy
,
Michael French
,
Leanne Blind-Doskocil
,
Anthony E. Reinhart
,
Edward Wolff
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Morgan E. Schneider
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Miranda Silcott
,
Elizabeth Smith
,
Joshua Aikins
,
Melissa Wagner
,
Paul Robinson
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James M. Wilczak
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Trevor White
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David Bodine
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Matthew R. Kumjian
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Sean M. Waugh
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A. Addison Alford
,
Kim Elmore
,
Pavlos Kollias
, and
David D. Turner

Abstract

Quasi-linear convective systems (QLCSs) are responsible for approximately a quarter of all tornado events in the U.S., but no field campaigns have focused specifically on collecting data to understand QLCS tornadogenesis. The Propagation, Evolution, and Rotation in Linear System (PERiLS) project was the first observational study of tornadoes associated with QLCSs ever undertaken. Participants were drawn from more than 10 universities, laboratories, and institutes, with over 100 students participating in field activities. The PERiLS field phases spanned two years, late winters and early springs of 2022 and 2023, to increase the probability of intercepting significant tornadic QLCS events in a range of large-scale and local environments. The field phases of PERiLS collected data in nine tornadic and nontornadic QLCSs with unprecedented detail and diversity of measurements. The design and execution of the PERiLS field phase and preliminary data and ongoing analyses are shown.

Open access
Greg M. McFarquhar
,
Elizabeth Smith
,
Elizabeth A. Pillar-Little
,
Keith Brewster
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Phillip B. Chilson
,
Temple R. Lee
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Sean Waugh
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Nusrat Yussouf
,
Xuguang Wang
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Ming Xue
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Gijs de Boer
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Jeremy A. Gibbs
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Chris Fiebrich
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Bruce Baker
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Jerry Brotzge
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Frederick Carr
,
Hui Christophersen
,
Martin Fengler
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Philip Hall
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Terry Hock
,
Adam Houston
,
Robert Huck
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Jamey Jacob
,
Robert Palmer
,
Patricia K. Quinn
,
Melissa Wagner
,
Yan (Rockee) Zhang
, and
Darren Hawk
Free access