scholarly journals The Possible Role of Density Current Dynamics in the Generation of Low-Level Vertical Vorticity in Supercells

2017 ◽  
Vol 74 (10) ◽  
pp. 3191-3208 ◽  
Author(s):  
Adam L. Houston
Keyword(s):  
Vertical Motion ◽  
Vertical Shear ◽  
Density Current ◽  
Vertical Vorticity ◽  
Low Level ◽  
Indirect Connection ◽  
Strong Surface ◽  
Boundary Deformation ◽  
Gust Front

Abstract A physical mechanism based on density current dynamics is proposed to explain the generation of low-level vertical vorticity in supercells. This mechanism may serve as one explanation for the associative relationship between environmental low-level vertical shear and the occurrence of significant tornadoes. The mechanism proposed herein represents an indirect connection to the generation of strong surface-based rotation: the barotropic horizontal vorticity associated with the vertical shear acts to amplify existing rotation but does not directly contribute to surface rotation. The proposed mechanism couples the likelihood of a tornado to the vertical shear through the pattern of vertical motion induced through interaction of a deformed gust front and the environmental vertical shear. Results from the experiments conducted to test the veracity of the proposed mechanism illustrate that inferred patterns of tilting and vortex line orientation are consistent with the generation of positive vertical vorticity near the axis of the existing mesocyclone and negative vertical vorticity along the rear-flank gust front. Moreover, inferred tilting is found to scale with the magnitude of the environmental vertical shear, consistent with the climatologies that motivate this work. Experiments also reveal that the proposed mechanism is capable of relating boundary deformation, mesocyclone strength, and hodograph shape to the ultimate likelihood of tornadogenesis.

Simulated Changes in Storm Morphology Associated with a Sea-Breeze Air Mass

2020 ◽  
Author(s):  
Joshua Hartigan ◽  
Robert A. Warren ◽  
Joshua S. Soderholm ◽  
Harald Richter
Keyword(s):  
Front Propagation ◽  
Sea Breeze ◽  
Wind Fields ◽  
Air Mass ◽  
Convective Storms ◽  
Low Level ◽  
Environmental Air ◽  
Homogeneous Environment ◽  
Gust Front

AbstractThe central east coast of Australia is frequently impacted by large hail and damaging winds associated with severe convective storms, with individual events recording damages exceeding AU$1 billion. These storms present a significant challenge for forecasting due to their development in seemingly marginal environments. They often have been observed to intensify upon approaching the coast, with case studies and climatological analyses indicating that interactions with the sea breeze are key to this process. The relative importance of the additional lifting and vorticity along the sea-breeze front compared to the change to a cooler, moister air mass with stronger low-level shear behind the front has yet to be investigated. Here, the role of the sea-breeze air mass is isolated using idealized numerical simulations of storms developing in a horizontally homogeneous environment. The base-state substitution (BSS) modeling technique is utilized to introduce the sea-breeze air mass following initial storm development. Compared to a simulation without BSS, the storm is longer lived and more intense, ultimately developing supercell characteristics including increased updraft rotation, deviant motion to the left of the mean wind vector, and a strong reflectivity gradient on the inflow edge. Separately simulating the changes in the thermodynamic and wind fields reveals that the enhanced storm longevity and intensity are primarily due to the latter. The change in the low-level environmental winds slows gust front propagation, allowing the storm to continue to ingest warm, potentially buoyant environmental air. At the same time, increased low-level shear promotes the development of persistent updraft rotation causing the storm to transition from a multicell to a supercell.

A Numerical Study of the Impact of Vertical Shear on the Distribution of Rainfall in Hurricane Bonnie (1998)

2003 ◽  
Vol 131 (8) ◽  
pp. 1577-1599 ◽  
Author(s):  
Robert Rogers ◽  
Shuyi Chen ◽  
Joseph Tenerelli ◽  
Hugh Willoughby
Keyword(s):  
Mesoscale Model ◽  
Numerical Study ◽  
Vertical Wind Shear ◽  
Vertical Motion ◽  
Environmental Flow ◽  
Vertical Shear ◽  
Low Level ◽  
Hurricane Bonnie ◽  
Along Track ◽  
The Impact

Abstract Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem. A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm. These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow. The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm. This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall. The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate the storm between 0000 UTC 22 August and 0000 UTC 27 August 1998. During this period significant changes in the vertical shear occurred in the simulation. It changed from strong west-southwesterly, and across track, to much weaker south-southwesterly, and along track. Nearly concurrently, the azimuthal distribution of convection changed from a distinct wavenumber-1 pattern to almost azimuthally symmetric by the end of the time period. The strongest convection in the core was generally located on the downshear left side of the shear vector when the shear was strong. The azimuthal distributions and magnitudes of low-level radial inflow, reflectivity, boundary layer divergence, and low-level vertical motion all varied consistently with the evolution of the vertical shear. Additionally, the vortex showed a generally downshear tilt from the vertical. The magnitude of the tilt correlated well with changes in magnitude of the environmental shear. The accumulated rainfall was distributed symmetrically across the track of the storm when the shear was strong and across track, and it was distributed asymmetrically across the track of the storm when the shear was weak and along track.

scholarly journals A High-Resolution Simulation of Asymmetries in Severe Southern Hemisphere Tropical Cyclone Larry (2006)

2009 ◽  
Vol 137 (12) ◽  
pp. 4171-4187 ◽  
Author(s):  
Hamish A. Ramsay ◽  
Lance M. Leslie ◽  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Southern Hemisphere ◽  
Inner Core ◽  
Mesoscale Model ◽  
Deep Convection ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Low Level ◽  
Frictional Convergence

Abstract Advances in observations, theory, and modeling have revealed that inner-core asymmetries are a common feature of tropical cyclones (TCs). In this study, the inner-core asymmetries of a severe Southern Hemisphere tropical cyclone, TC Larry (2006), are investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Kepert–Wang boundary layer model. The MM5-simulated TC exhibited significant asymmetries in the inner-core region, including rainfall distribution, surface convergence, and low-level vertical motion. The near-core environment was characterized by very low environmental vertical shear and consequently the TC vortex had almost no vertical tilt. It was found that, prior to landfall, the rainfall asymmetry was very pronounced with precipitation maxima consistently to the right of the westward direction of motion. Persistent maxima in low-level convergence and vertical motion formed ahead of the translating TC, resulting in deep convection and associated hydrometeor maxima at about 500 hPa. The asymmetry in frictional convergence was mainly due to the storm motion at the eyewall, but was dominated by the proximity to land at larger radii. The displacement of about 30°–120° of azimuth between the surface and midlevel hydrometeor maxima is explained by the rapid cyclonic advection of hydrometeors by the tangential winds in the TC core. These results for TC Larry support earlier studies that show that frictional convergence in the boundary layer can play a significant role in determining the asymmetrical structures, particularly when the environmental vertical shear is weak or absent.

Tropical cyclogenesis and vertical shear in a moist Boussinesq model

2012 ◽  
Vol 706 ◽  
pp. 384-412 ◽  
Author(s):  
Qiang Deng ◽  
Leslie Smith ◽  
Andrew Majda
Keyword(s):  
Numerical Simulations ◽  
Water Vapour ◽  
Cloud Microphysics ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Structure Characteristic ◽  
Boussinesq Model ◽  
Vertical Vorticity ◽  
Low Level ◽  
Low Altitude

AbstractTropical cyclogenesis is studied in the context of idealized three-dimensional Boussinesq dynamics with perhaps the simplest possible model for bulk cloud physics. With low-altitude input of water vapour on realistic length and time scales, numerical simulations capture the formation of vortical hot towers. From measurements of water vapour, vertical velocity, vertical vorticity and rain, it is demonstrated that the structure, strength and lifetime of the hot towers are similar to results from models including more detailed cloud microphysics. The effects of low-altitude vertical shear are investigated by varying the initial zonal velocity profile. In the presence of weak low-level vertical shear, the hot towers retain the low-altitude monopole cyclonic structure characteristic of the zero-shear case (starting from zero velocity). Some initial velocity profiles with small vertical shear can have the effect of increasing cyclonic predominance of individual hot towers in a statistical sense, as measured by the skewness of vertical vorticity. Convergence of horizontal winds in the atmospheric boundary layer is mimicked by increasing the frequency of the moisture forcing in a horizontal subdomain. When the moisture forcing is turned off, and again for zero shear or weak low-level shear, merger of cyclonic activity results in the formation of a larger-scale cyclonic vortex. An effect of the shear is to limit the vertical extent of the resulting emergent moist vortex. For stronger low-altitude vertical shear, the individual hot towers have a low-altitude vorticity dipole rather than a cyclonic monopole. The dipoles are not conducive to the formation of larger-scale vortices, and thus sufficiently strong low-level shear prevents the vortical-hot-tower route to cyclogenesis. The results indicate that the simplest condensation and evaporation schemes are useful for exploratory numerical simulations aimed at better understanding of competing effects such as low-level moisture and vertical shear.

scholarly journals The Structure, Evolution, and Dynamics of a Nocturnal Convective System Simulated Using the WRF-ARW Model

2017 ◽  
Vol 145 (8) ◽  
pp. 3179-3201 ◽  
Author(s):  
Benjamin T. Blake ◽  
David B. Parsons ◽  
Kevin R. Haghi ◽  
Stephen G. Castleberry
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Vertical Shear ◽  
Structure Evolution ◽  
Cold Pool ◽  
Density Current ◽  
Convective System ◽  
Low Level ◽  
Arw Model ◽  
Low Level Jet

Previous studies have documented a nocturnal maximum in thunderstorm frequency during the summer across the central United States. Forecast skill for these systems remains relatively low and the explanation for this nocturnal maximum is still an area of active debate. This study utilized the WRF-ARW Model to simulate a nocturnal mesoscale convective system that occurred over the southern Great Plains on 3–4 June 2013. A low-level jet transported a narrow corridor of air above the nocturnal boundary layer with convective instability that exceeded what was observed in the daytime boundary layer. The storm was elevated and associated with bores that assisted in the maintenance of the system. Three-dimensional variations in the system’s structure were found along the cold pool, which were examined using convective system dynamics and wave theory. Shallow lifting occurred on the southern flank of the storm. Conversely, the southeastern flank had deep lifting, with favorable integrated vertical shear over the layer of maximum CAPE. The bore assisted in transporting high-CAPE air toward its LFC, and the additional lifting by the density current allowed for deep convection to occur. The bore was not coupled to the convective system and it slowly pulled away, while the convection remained in phase with the density current. These results provide a possible explanation for how convection is maintained at night in the presence of a low-level jet and a stable boundary layer, and emphasize the importance of the three-dimensionality of these systems.

scholarly journals Origins of Vorticity in a Simulated Tornadic Mesovortex Observed during PECAN on 6 July 2015

2019 ◽  
Vol 147 (1) ◽  
pp. 107-134 ◽  
Author(s):  
Matthew D. Flournoy ◽  
Michael C. Coniglio
Keyword(s):  
Convective System ◽  
Vertical Vorticity ◽  
Low Level ◽  
Convective Systems ◽  
The North ◽  
Mesoscale Convective ◽  
Forecasting System ◽  
Convection Initiation ◽  
Inflow Jet ◽  
Gust Front

To better understand and forecast nocturnal thunderstorms and their hazards, an expansive network of fixed and mobile observing systems was deployed in the summer of 2015 for the Plains Elevated Convection at Night (PECAN) field experiment to observe low-level jets, convection initiation, bores, and mesoscale convective systems. On 5–6 July 2015, mobile radars and ground-based surface and upper-air profiling systems sampled a nocturnal, quasi-linear convective system (QLCS) over South Dakota. The QLCS produced several severe wind reports and an EF-0 tornado. The QLCS and its environment leading up to the mesovortex that produced this tornado were well observed by the PECAN observing network. In this study, observations from radiosondes, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on the development of the observed tornadic mesovortex. All ensemble members simulated low-level mesovortices with one member in particular generating two mesovortices in a manner very similar to that observed. Forecasts from this member were analyzed to examine the processes increasing vertical vorticity during the development of the tornadic mesovortex. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized downdraft/rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing mesovortex.

Rapid Scan Views of Convectively Generated Mesovortices in Sheared Tropical Cyclone Gustav (2002)

2006 ◽  
Vol 21 (6) ◽  
pp. 1041-1050 ◽  
Author(s):  
Eric A. Hendricks ◽  
Michael T. Montgomery
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Level Structure ◽  
Vertical Wind Shear ◽  
Vertical Shear ◽  
Low Level ◽  
Rapid Scan ◽  
Mesoscale Processes ◽  
Vertical Shearing

Abstract On 9–10 September 2002, multiple mesovortices were captured in great detail by rapid scan visible satellite imagery in subtropical, then later, Tropical Storm Gustav. These mesovortices were observed as low-level cloud swirls while the low-level structure of the storm was exposed due to vertical shearing. They are shown to form most plausibly via vortex tube stretching associated with deep convection; they become decoupled from the convective towers by vertical shear; they are advected with the low-level circulation; finally they initiate new hot towers on their boundaries. Partial evidence of an axisymmetrizing mesovortex and its hypothesized role in the parent vortex spinup is presented. Observations from the mesoscale and synoptic scale are synthesized to provide a multiscale perspective of the intensification of Gustav that occurred on 10 September. The most important large-scale factors were the concurrent relaxation of the 850–200-hPa-deep layer vertical wind shear from 10–15 to 5–10 m s−1 and movement over pockets of very warm sea surface temperatures (approximately 29.5°–30.5°C). The mesoscale observations are not sufficient alone to determine the precise role of the deep convection and mesovortices in the intensification. However, qualitative comparisons are made between the mesoscale processes observed in Gustav and recent full-physics and idealized numerical simulations to obtain additional insight.

The Role of Vertical Wind Shear in Modulating Maximum Supercell Updraft Velocities

2019 ◽  
Vol 76 (10) ◽  
pp. 3169-3189 ◽  
Author(s):  
John M. Peters ◽  
Christopher J. Nowotarski ◽  
Hugh Morrison
Keyword(s):  
Pressure Gradient ◽  
Wind Shear ◽  
Dynamic Pressure ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Strong Constraint ◽  
Low Level ◽  
Relative Flow

Abstract Observed supercell updrafts consistently produce the fastest mid- to upper-tropospheric vertical velocities among all modes of convection. Two hypotheses for this feature are investigated. In the dynamic hypothesis, upward, largely rotationally driven pressure gradient accelerations enhance supercell updrafts relative to other forms of convection. In the thermodynamic hypothesis, supercell updrafts have more low-level inflow than ordinary updrafts because of the large vertical wind shear in supercell environments. This large inflow makes supercell updrafts wider than that of ordinary convection and less susceptible to the deleterious effects of entrainment-driven updraft core dilution on buoyancy. These hypotheses are tested using a large suite of idealized supercell simulations, wherein vertical shear, CAPE, and moisture are systematically varied. Consistent with the thermodynamic hypothesis, storms with the largest storm-relative flow have larger inflow, are wider, have larger buoyancy, and have faster updrafts. Analyses of the vertical momentum forcing along trajectories shows that maximum vertical velocities are often enhanced by dynamic pressure accelerations, but this enhancement is accompanied by larger downward buoyant pressure accelerations than in ordinary convection. Integrated buoyancy along parcel paths is therefore a strong constraint on maximum updraft speeds. Thus, through a combination of processes consistent with the dynamic and thermodynamic hypotheses, supercell updrafts are able to realize a larger percentage of CAPE than ordinary updrafts.

scholarly journals Diagnosing Mesoscale Vertical Motion from Horizontal Velocity and Density Data

2005 ◽  
Vol 35 (10) ◽  
pp. 1744-1762 ◽  
Author(s):  
Enric Pallàs Sanz ◽  
Álvaro Viúdez
Keyword(s):  
Vertical Velocity ◽  
Extreme Values ◽  
Vertical Motion ◽  
Shear Velocity ◽  
Horizontal Velocity ◽  
Vertical Shear ◽  
Coriolis Parameter ◽  
Vertical Vorticity ◽  
Omega Equation ◽  
Q Vector

Abstract The mesoscale vertical velocity is obtained by solving a generalized omega equation (ω equation) using density and horizontal velocity data from three consecutive quasi-synoptic high-resolution surveys in the Alboran Sea. The Atlantic Jet (AJ) and the northern part of the Western Alboran Gyre (WAG) were observed as a large density anticyclonic front extending down to 200–230 m. The horizontal velocity uh in the AJ reached maxima of 1.2 m s−1 for the three surveys, with extreme Rossby numbers of ζ/f ≈ −0.9 in the WAG and +0.9 in the AJ (where ζ is the vertical vorticity and f is the Coriolis parameter). The generalized ω equation includes the ageostrophic horizontal flow. It is found that the most important “forcing” term in this equation is ( fζph + ∇hϱ) · ∇2huh, where ζph is the horizontal (pseudo) vorticity and ϱ is the buoyancy. This term is related to the horizontal advection of vertical vorticity by the vertical shear velocity, uhz · ∇hζ. Extreme values of the diagnosed vertical velocity w were located at 80–100 m with max{w} ⊂ [34, 45] and min{w} ⊂ [−64, −34] m day−1. Comparison with the quasigeostrophic (QG) ω equation shows that, because of the large Rossby numbers, non-QG terms are important. The differences between w and the QG vertical velocity are mainly related to the divergence of the ageostrophic part of the total Q vector (Qh ≡ ∇huh · ∇hϱ) in the ω equation.

Respective and combined impacts of regional SST anomalies on tropical cyclogenesis in different sectors of the western North Pacific

2021 ◽  
Author(s):  
Renguang Wu
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Environmental Variables ◽  
Western North Pacific ◽  
Environmental Changes ◽  
North Atlantic Ocean ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Low Level ◽  
Sst Anomalies

<p>Tropical cyclone activity over the western North Pacific (WNP) is subjected to impacts of sea surface temperature (SST) anomalies in the three tropical oceans. In this talk, the interannual variations in the tropical cyclone (TC) over the WNP and the influences of regional SST anomalies are documented by separating the WNP into four quadrants considering SST-induced non-uniform environmental changes. It will be shown that the TC variations in the northwest and southeast quadrants are related to both equatorial central-eastern Pacific (EPO) and tropical Indian Ocean (TIO) SST anomalies. The TC variation in the northeast quadrant is mainly related to tropical North Atlantic Ocean (TNA) SST anomalies. The main environmental variables differ for the TC variations in the four quadrants. Low-level (850-hPa) vorticity is important for the TC variations in the northwest, southwest and southeast quadrants. Mid-level (700-hPa) humidity contributes to the TC variations in the northwest, northeast and southeast quadrants. The vertical shear has a supplementary contribution to the TC variation in the southeast quadrant. The potential intensity negatively affects the TC variations in the southwest and southeast quadrants. The remote SST anomalies modulate different environmental variables over the WNP. The TIO SST influence is manifested in the low-level vorticity and vertical motion. The TNA SST impact occurs through the low-level vorticity change. The EPO SST effect occurs via changing the low-level vorticity and vertical motion as well as the mid-level moisture and vertical shear. The environmental variables experience more prominent changes when SST anomalies coexist in two remote regions. Numerical experiments confirm the EPO and TIO SST anomaly impacts on the environmental conditions affecting the WNP TC variations.</p>

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