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Rezaul Mahmood
,
Megan Schargorodski
,
Eric Rappin
,
Melissa Griffin
,
Patrick Collins
,
Kevin Knupp
,
Andrew Quilligan
,
Ryan Wade
, and
Kevin Cary

Abstract

A total solar eclipse traversed the continental United States on 21 August 2017. It was the first such event in 99 years and provided a rare opportunity to observe the atmospheric response from a variety of instrumented observational platforms. This paper discusses the high-quality observations collected by the Kentucky Mesonet (www.kymesonet.org), a research-grade meteorological and climatological observation network consisting of 72 stations and measuring air temperature, precipitation, relative humidity, solar radiation, wind speed, and wind direction. The network samples the atmosphere, for most variables, every 3 s and then calculates and records observations every 5 min. During the total solar eclipse, these observations were complemented by observations collected from three atmospheric profiling systems positioned in the path of the eclipse and operated by the University of Alabama in Huntsville (UAH). Observational data demonstrate that solar radiation at the surface dropped from >800 to 0 W m‒2, the air temperature decreased by about 4.5°C, and, most interestingly, a land-breeze–sea-breeze-type wind developed. In addition, due to the high density of observations, the network recorded a detailed representation of the spatial variation of surface meteorology. The UAH profiling system captured collapse and reformation of the planetary boundary layer and related changes during the total solar eclipse.

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Rezaul Mahmood
,
Megan Schargorodski
,
Eric Rappin
,
Melissa Griffin
,
Patrick Collins
,
Kevin Knupp
,
Andrew Quilligan
,
Ryan Wade
,
Kevin Cary
, and
Stuart Foster
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John W. Nielsen-Gammon
,
Christina L. Powell
,
M. J. Mahoney
,
Wayne M. Angevine
,
Christoph Senff
,
Allen White
,
Carl Berkowitz
,
Christopher Doran
, and
Kevin Knupp

Abstract

An airborne microwave temperature profiler (MTP) was deployed during the Texas 2000 Air Quality Study (TexAQS-2000) to make measurements of boundary layer thermal structure. An objective technique was developed and tested for estimating the mixed layer (ML) height from the MTP vertical temperature profiles. The technique identifies the ML height as a threshold increase of potential temperature from its minimum value within the boundary layer. To calibrate the technique and evaluate the usefulness of this approach, coincident estimates from radiosondes, radar wind profilers, an aerosol backscatter lidar, and in situ aircraft measurements were compared with each other and with the MTP. Relative biases among all instruments were generally less than 50 m, and the agreement between MTP ML height estimates and other estimates was at least as good as the agreement among the other estimates. The ML height estimates from the MTP and other instruments are utilized to determine the spatial and temporal evolution of ML height in the Houston, Texas, area on 1 September 2000. An elevated temperature inversion was present, so ML growth was inhibited until early afternoon. In the afternoon, large spatial variations in ML height developed across the Houston area. The highest ML heights, well over 2 km, were observed to the north of Houston, while downwind of Galveston Bay and within the late afternoon sea breeze ML heights were much lower. The spatial variations that were found away from the immediate influence of coastal circulations were unexpected, and multiple independent ML height estimates were essential for documenting this feature.

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Marcia Cronce
,
Robert M. Rauber
,
Kevin R. Knupp
,
Brian F. Jewett
,
Justin T. Walters
, and
Dustin Phillips

Abstract

The University of Alabama in Huntsville Mobile Integrated Profiling System 915-MHz profiler was deployed in January and February of 2004 to measure vertical air velocities in finescale precipitation bands in winter cyclones. The profiler was placed to sample the “wraparound” quadrant of three winter cyclones in the central and southern United States, and it obtained high-resolution measurements of the vertical structure of a series of bands in each storm. The data revealed bands that were up to 6 km deep, 10–50 km wide, and spaced about 5–20 km apart. Measurements of vertical air motion w within these bands were retrieved from the Doppler spectra using the lower-bound method, adapted to account for the effects of spectral broadening caused by the horizontal wind, wind shear, and turbulence. Derived vertical air motions ranged from −4.3 to 6.7 m s−1, with an uncertainty of about ±0.6 m s−1. Approximately 29% of the 1515 total derived values were negative, 35% exceeded 1 m s−1, and 9% exceeded 2.0 m s−1. These values are consistent with studies in the Pacific Northwest, except that more extreme values were observed in one band than have been previously reported. There was a high correlation between values of signal-to-noise ratio (SNR) and w within each band (0.60 ≤ r ≤ 0.85), in the composite of bands from each cyclone (0.59 ≤ r ≤ 0.79), and in the overall analysis (r = 0.68). The strongest updrafts were typically between 2.0 and 4.0 m s−1 and were located near the center of each band in regions of high SNR. Regions of downdrafts within the bands had maximum values between −1.0 and −4.3 m s−1 and were typically located along the edges of the bands in regions of low SNR. These results are consistent with snow growth and sublimation processes. The magnitudes of the vertical velocities in the core of the bands were comparable to theoretical predictions for moist symmetric instability (MSI) under inviscid conditions but would appear to be somewhat larger than expected for MSI when turbulent mixing is considered, suggesting that other instabilities, such as potential instability, may have contributed to the band development in these storms.

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Joseph A. Finlon
,
Greg M. McFarquhar
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Robert M. Rauber
,
David M. Plummer
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Brian F. Jewett
,
David Leon
, and
Kevin R. Knupp

Abstract

Since the advent of dual-polarization radar, methods of classifying hydrometeors by type from measured polarization variables have been developed. The deterministic approach of existing hydrometeor classification algorithms of assigning only one dominant habit to each radar sample volume does not properly consider the distribution of habits present in that volume, however. During the Profiling of Winter Storms field campaign, the “NSF/NCAR C-130” aircraft, equipped with in situ microphysical probes, made multiple passes through the comma heads of two cyclones as the Mobile Alabama X-band dual-polarization radar performed range–height indicator scans in the same plane as the C-130 flight track. On 14–15 February and 21–22 February 2010, 579 and 202 coincident data points, respectively, were identified when the plane was within 10 s (~1 km) of a radar gate. For all particles that occurred for times within different binned intervals of radar reflectivity Z HH and of differential reflectivity Z DR, the reflectivity-weighted contribution of each habit and the frequency distributions of axis ratio and sphericity were determined. This permitted the determination of habits that dominate particular Z HH and Z DR intervals; only 40% of the Z HHZ DR bins were found to have a habit that contributes over 50% to the reflectivity in that bin. Of these bins, only 12% had a habit that contributes over 75% to the reflectivity. These findings show the general lack of dominance of a given habit for a particular Z HH and Z DR and suggest that determining the probability of specific habits in radar volumes may be more suitable than the deterministic methods currently used.

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Anthony W. Lyza
,
Todd A. Murphy
,
Barrett T. Goudeau
,
Preston T. Pangle
,
Kevin R. Knupp
, and
Ryan A. Wade

Abstract

The Sand Mountain and Lookout Mountain Plateaus in northeastern Alabama have been established as a regional relative maximum in tornadogenesis reports within the southeastern United States. Investigation of long-term surface datasets has revealed (i) stronger and more backed winds atop Sand Mountain than over the Tennessee Valley, and (ii) measured cloud-base heights are lower to the surface atop Sand Mountain than over the Tennessee Valley. These observations suggest that low-level wind shear and lifting condensation level (LCL) height changes may lead to conditions more favorable for tornadogenesis atop the plateaus than over the Tennessee Valley. However, prior to fall 2016, no intensive observations had been made to further investigate low-level flow or thermodynamic changes in the topography of northeastern Alabama. This paper provides detailed analysis of observations gathered during VORTEX-SE field campaign cases from fall 2016 through spring 2019. These observations indicate that downslope winds form along the northwest edge of Sand Mountain in at least some severe storm environments in northeastern Alabama. Wind profiles gathered across northeastern Alabama indicate that low-level helicity changes can be substantial over small distances across different areas of the topographic system. LCL height changes often scale to changes in land elevation, which can be on the order of 200–300 m across northeastern Alabama.

Free access
Kevin R. Knupp
,
Todd A. Murphy
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Timothy A. Coleman
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Ryan A. Wade
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Stephanie A. Mullins
,
Christopher J. Schultz
,
Elise V. Schultz
,
Lawrence Carey
,
Adam Sherrer
,
Eugene W. McCaul Jr.
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Brian Carcione
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Stephen Latimer
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Andy Kula
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Kevin Laws
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Patrick T. Marsh
, and
Kim Klockow

By many metrics, the tornado outbreak on 27 April 2011 was the most significant tornado outbreak since 1950, exceeding the super outbreak of 3–4 April 1974. The number of tornadoes over a 24-h period (midnight to midnight) was 199; the tornado fatalities and injuries were 316 and more than 2,700, respectively; and the insurable loss exceeded $4 billion (U.S. dollars). In this paper, we provide a meteorological overview of this outbreak and illustrate that the event was composed of three mesoscale events: a large early morning quasi-linear convective system (QLCS), a midday QLCS, and numerous afternoon supercell storms. The main data sources include NWS and research radars, profilers, surface measurements, and photos and videos of the tornadoes. The primary motivation for this preliminary research is to document the diverse characteristics (e.g., tornado characteristics and mesoscale organization of deep convection) of this outbreak and summarize preliminary analyses that are worthy of additional research on this case.

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Robert M. Rauber
,
Joseph Wegman
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David M. Plummer
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Andrew A. Rosenow
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Melissa Peterson
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Greg M. McFarquhar
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Brian F. Jewett
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David Leon
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Patrick S. Market
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Kevin R. Knupp
,
Jason M. Keeler
, and
Steven M. Battaglia

Abstract

This paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral width within the temperature range from −10° to −20°C, while microphysical measurements showed the simultaneous presence of graupel, ice particles, and supercooled water at the same temperatures, together supporting noninductive charging as an important charging mechanism in these storms.

A climatology of lightning flashes across the comma head of 16 winter cyclones shows that lightning flashes commonly occur on the southern side of the comma head where dry-slot air is more likely to overrun lower-level moist air. Over 90% of the cloud-to-ground flashes had negative polarity, suggesting the cells were not strongly sheared aloft. About 55% of the flashes were associated with cloud-to-ground flashes while 45% were in-cloud flashes.

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David A. R. Kristovich
,
Richard D. Clark
,
Jeffrey Frame
,
Bart Geerts
,
Kevin R. Knupp
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Karen A. Kosiba
,
Neil F. Laird
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Nicholas D. Metz
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Justin R. Minder
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Todd D. Sikora
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W. James Steenburgh
,
Scott M. Steiger
,
Joshua Wurman
, and
George S. Young

Abstract

Intense lake-effect snowstorms regularly develop over the eastern Great Lakes, resulting in extreme winter weather conditions with snowfalls sometimes exceeding 1 m. The Ontario Winter Lake-effect Systems (OWLeS) field campaign sought to obtain unprecedented observations of these highly complex winter storms.

OWLeS employed an extensive and diverse array of instrumentation, including the University of Wyoming King Air research aircraft, five university-owned upper-air sounding systems, three Center for Severe Weather Research Doppler on Wheels radars, a wind profiler, profiling cloud and precipitation radars, an airborne lidar, mobile mesonets, deployable weather Pods, and snowfall and particle measuring systems. Close collaborations with National Weather Service Forecast Offices during and following OWLeS have provided a direct pathway for results of observational and numerical modeling analyses to improve the prediction of severe lake-effect snowstorm evolution. The roles of atmospheric boundary layer processes over heterogeneous surfaces (water, ice, and land), mixed-phase microphysics within shallow convection, topography, and mesoscale convective structures are being explored.

More than 75 students representing nine institutions participated in a wide variety of data collection efforts, including the operation of radars, radiosonde systems, mobile mesonets, and snow observation equipment in challenging and severe winter weather environments.

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Christopher Davis
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Nolan Atkins
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Diana Bartels
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Lance Bosart
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Michael Coniglio
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George Bryan
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William Cotton
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David Dowell
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Brian Jewett
,
Robert Johns
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David Jorgensen
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Jason Knievel
,
Kevin Knupp
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Wen-Chau Lee
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Gregory McFarquhar
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James Moore
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Ron Przybylinski
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Robert Rauber
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Bradley Smull
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Robert Trapp
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Stanley Trier
,
Roger Wakimoto
,
Morris Weisman
, and
Conrad Ziegler

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sounding coverage and documented the thermodynamic structure of the PBL, both within MCSs and in their environment. The present article reviews the scientific goals of the study and the facility deployment strategy, summarizes the cases observed, and highlights the forthcoming significant research directions and opportunities.

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