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Dana Mueller
,
Bart Geerts
,
Zhien Wang
,
Min Deng
, and
Coltin Grasmick

Abstract

This study documents the evolution of an impressive, largely undular bore triggered by an MCS-generated density current on 20 June 2015, observed as part of the Plains Elevated Convection at Night (PECAN) experiment. The University of Wyoming King Air with profiling nadir- and zenith-viewing lidars sampled the south-bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s−1 and turbulence in the wave’s wake were encountered, through the early dissipative stage in which the leading wave began to lose amplitude and speed. Through most of the bore’s life cycle, its second wave had a higher or equal amplitude relative to the leading wave. Striking roll clouds formed in wave crests and wave energy was detected to about 5 km AGL. The upstream environment indicates a negative Scorer parameter region due to flow reversal at midlevels, providing a wave trapping mechanism. The observed bore strength of 2.4–2.9 and speed of 15–16 m s−1 agree well with values predicted from hydraulic theory. Surface and profiling measurements collected later in the bore’s life cycle, just after sunrise, indicate a transition to a soliton.

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Guo Lin
,
Coltin Grasmick
,
Bart Geerts
,
Zhien Wang
, and
Min Deng

Abstract

This observational study documents the consequences of a collision between two converging shallow atmospheric boundaries over the central Great Plains on the evening of 7 June 2015. This study uses data from a profiling airborne Raman lidar [the compact Raman lidar (CRL)] and other airborne and ground-based data collected during the Plains Elevated Convection at Night (PECAN) field campaign to investigate the collision between a weak cold front and the outflow from an MCS. The collision between these boundaries led to the lofting of high-CAPE, low-CIN air, resulting in deep convection, as well as an undular bore. Both boundaries behaved as density currents prior to collision. Because the MCS outflow boundary was denser and less deep than the cold-frontal air mass, the bore propagated over the latter. This bore was tracked by the CRL for about 3 h as it traveled north over the shallow cold-frontal surface and evolved into a soliton. This case study is unique by using the high temporal and spatial resolution of airborne Raman lidar measurements to describe the thermodynamic structure of interacting boundaries and a resulting bore.

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Michael J. Hosek
,
Conrad L. Ziegler
,
Michael I. Biggerstaff
,
Todd A. Murphy
, and
Zhien Wang

Abstract

This case study analyzes a tornadic supercell observed in northeast Louisiana as part of the Verification of the Origins of Rotation in Tornadoes Experiment Southeast (VORTEX-SE) on 6–7 April 2018. One mobile research radar (SR1-P), one WSR-88D equivalent (KULM), and two airborne radars (TAFT and TFOR) have sampled the storm at close proximity for ∼70 min through its mature phase, tornadogenesis at 2340 UTC, and dissipation and subsequent ingestion into a developing MCS segment. The 4D wind field and reflectivity from up to four Doppler analyses, combined with 4D diabatic Lagrangian analysis (DLA) retrievals, has enabled kinematic and thermodynamic analysis of storm-scale boundaries leading up to, during, and after the dissipation of the NWS-surveyed EF0 tornado. The kinematic and thermodynamic analyses reveal a transient current of low-level streamwise vorticity leading into the low-level supercell updraft, appearing similar to the streamwise vorticity current (SVC) that has been identified in supercell simulations and previously observed only kinematically. Vorticity dynamical calculations demonstrate that both baroclinity and horizontal stretching play significant roles in the generation and amplification of streamwise vorticity associated with this SVC. While the SVC does not directly feed streamwise vorticity to the tornado–cyclone, its development coincides with tornadogenesis and an intensification of the supercell’s main low-level updraft, although a causal relationship is unclear. Although the mesoscale environment is not high-shear/low-CAPE (HSLC), the updraft of the analyzed supercell shares some similarities to past observations and simulations of HSLC storms in the Southeast United States, most notably a pulse-like updraft that is maximized in the low- to midlevels of the storm.

Significance Statement

The purpose of this study is to analyze the airflow and thermodynamics of a highly observed tornado-producing supercell. While computer simulations can provide us with highly detailed looks at the complicated evolution of supercells, it is rare, due to the difficulty of data collection, to collect enough data to perform a highly detailed analysis on a particular supercell, especially in the Southeast United States. We identified a “current” of vorticity—rotating wind—that develops at the intersection of the supercell’s rain-cooled outflow and warm inflow, similar to previous simulations. This vorticity current develops and feeds the storm’s updraft as its tornado develops and the storm intensifies, although it does not directly enter the tornado.

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Guo Lin
,
Bart Geerts
,
Zhien Wang
,
Coltin Grasmick
,
Xiaoqin Jing
, and
Jing Yang

Abstract

Small-scale variations within the low-level outflow and inflow of an MCS can either support or deter the upscale growth and maintenance of the MCS. However, these small-scale variations, in particular in the thermodynamics (temperature and humidity), remain poorly understood, due to a lack of detailed measurements. The compact Raman lidar (CRL) deployed on the University of Wyoming King Air aircraft directly sampled temperature and water vapor profiles at unprecedented vertical and along-track resolutions along the southern margin of a series of mature nocturnal MCSs traveling along a frontal boundary on 1 July 2015 during the Plains Elevated Convection at Night (PECAN) campaign. Here, the capability of the airborne CRL to document interactions between the MCS inflow and outflow currents is illustrated. The CRL reveals the well-defined boundary of a cooler current. This is interpreted as the frontal boundary sharpened by convectively induced cold pools, in particular by the outflow boundary of the downstream MCS. In one CRL transect, the frontal/outflow boundary appeared as a distinct two-layer structure of moisture and aerosols formed by moist stable boundary layer air advected above the boundary. The second transect, one hour later, reveals a single sloping boundary. In both cases, the lofting of the moist stably stratified air over the boundary favors MCS maintenance, through enhanced elevated CAPE and reduced CIN. The CRL data are sufficiently resolved to reveal Kelvin–Helmholtz (KH) billows and the vertical structure of the outflow boundary, which in this case behaved as a density current rather than an undular bore.

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Philip T. Bergmaier
,
Bart Geerts
,
Zhien Wang
,
Bo Liu
, and
Patrick C. Campbell

Abstract

Part I of this study describes the mesoscale structure of a dryline over southeastern Wyoming. This dryline formed just east of the western rim of the high plains on 22 June 2010 and became more defined as it progressed eastward during the afternoon. Part I also describes the numerically simulated structure and evolution of this dryline and the observed initiation of deep convection in the vicinity of the dryline.

An instrumented aircraft, the University of Wyoming King Air, repeatedly flew across this dryline, mostly low enough to penetrate the moist-air wedge east of the dryline. Flight-level in situ data along these low-level penetrations indicate relatively high values of convective available potential energy (CAPE; >1500 J kg−1), yet low convective inhibition, within a few kilometers of the dryline. Water vapor transects obtained from a compact nadir-pointing Raman lidar aboard the aircraft reveal an extremely sharp humidity gradient below flight level along the dryline, coinciding with the fineline seen in operational weather radar base reflectivity imagery. They also reveal several plumes of higher specific humidity within the dry elevated mixed layer above the moist-air wedge, possibly precursors of cumulus clouds. The vertical structure of the dryline revealed by Raman lidar and the flight-level data correspond well to that in the high-resolution numerical simulation.

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Coltin Grasmick
,
Bart Geerts
,
David D. Turner
,
Zhien Wang
, and
T. M. Weckwerth

Abstract

The vertical structures of a leading outflow boundary ahead of a continental nocturnal MCS and of the upstream environment are examined in order to answer the question of whether this vertical structure affects new cell formation and thus MCS maintenance. The MCS in question, observed on 15 July 2015 as part of the Plains Elevated Convection at Night (PECAN) experiment, formed near sunset as a surface-based, density current–driven system. As the night progressed and a stable boundary layer developed, convection became elevated, multiple fine lines became apparent (indicative of an undular bore), and convection increasingly lagged the outflow boundary. Bore-like boundaries became most apparent where the outflow boundary was oriented more perpendicular to the low-level jet, and the lower troposphere was more susceptible to wave trapping. This case study uses a rich array of radiosonde data, as well as airborne Raman lidar and ground-based interferometer data, to profile the temperature and humidity in the lower troposphere. In all soundings, the lifting of air in the residual mixed layer over a depth corresponding to the Raman lidar observed vertical displacement reduced CIN to near zero and enabled deep convection, even though most unstable CAPE steadily decreased during the evolution of this MCS. Both types of outflow boundaries (density currents and bores) initiated convection that helped maintain the MCS. In the case of density currents, cold pool depth and wind shear determined new cell formation and thus MCS maintenance. For bore-like boundaries, bore transformation and propagation were additional factors that determined whether convection initiated and whether it contributed to the MCS or remained separated.

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Belay Demoz
,
Cyrille Flamant
,
Tammy Weckwerth
,
David Whiteman
,
Keith Evans
,
Frédéric Fabry
,
Paolo Di Girolamo
,
David Miller
,
Bart Geerts
,
William Brown
,
Geary Schwemmer
,
Bruce Gentry
,
Wayne Feltz
, and
Zhien Wang

Abstract

A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment.

Moisture and temperature jumps of about 3 g kg−1 and 1–2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1–2 g kg−1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s−1 and variable horizontal widths of 2–5 km, although some were as wide as 7–8 km. These updrafts were up to 1.5 g kg−1 moister than the surrounding environment.

Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg−1), were detrimental to CI in this case.

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