scholarly journals Effects of the Low-Level Wind Profile on Outflow Position and Near-Surface Vertical Vorticity in Simulated Supercell Thunderstorms

2018 ◽  
Vol 75 (3) ◽  
pp. 731-753 ◽  
Author(s):  
Felicia Guarriello ◽  
Christopher J. Nowotarski ◽  
Craig C. Epifanio
Keyword(s):  
Shear Layer ◽  
Wind Shear ◽  
Wind Profile ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Low Level ◽  
Surface Rotation ◽  
Surface Circulation

Abstract Supercell thunderstorms are simulated using an idealized numerical model to analyze the effects of modifications to the environmental low-level wind profile on near-surface rotation. Specifically, the orientation, magnitude, and depth of the low-level vertical wind shear are modified in several suites of experiments and compared to control simulations with no vertical wind shear in the prescribed layer. The overall morphology of the simulated supercells is highly sensitive to even shallow changes in the low-level wind profile. Moreover, maximum near-surface vertical vorticity varies as the low-level wind profile is modified. The results suggest this is principally a consequence of the degree to which favorable dynamic forcing of negatively buoyant outflow is superimposed upon the near-surface circulation maximum. Simulations with easterly shear and weaker storm-relative winds over the depth of the gust front promote forward-surging outflow and smaller separation between the near-surface circulation maximum and the mesocyclone aloft compared with other hodograph shapes. This promotes near-surface vertical vorticity intensification in these simulations. Similar trends in near-surface vertical vorticity as a function of low-level shear orientation are observed for varying shear-layer depths and bulk-shear magnitudes over the shear layer. The degree to which specific hodograph shapes promote strong near-surface rotation may vary with different deep-layer wind profiles or thermodynamic environments from those simulated here; however, this study concludes that favorable positioning of the near-surface circulation maximum and mesocyclone aloft are a necessary condition for supercell tornadogenesis and this positioning may be modulated by the low-level wind profile.

scholarly journals A Numerical Simulation Study of the Effects of Anvil Shading on Quasi-Linear Convective Systems

2013 ◽  
Vol 70 (3) ◽  
pp. 767-793 ◽  
Author(s):  
Andrew J. Oberthaler ◽  
Paul M. Markowski
Keyword(s):  
Solar Radiation ◽  
Wind Shear ◽  
Wind Profile ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Convective System ◽  
Near Surface ◽  
Low Level ◽  
Convective Systems ◽  
Gust Front

Abstract Numerical simulations are used to investigate how the attenuation of solar radiation by the intervening cumulonimbus cloud, particularly its large anvil, affects the structure, intensity, and evolution of quasi-linear convective systems and the sensitivity of the effects of this “anvil shading” to the ambient wind profile. Shading of the pre-gust-front inflow environment (as opposed to shading of the cold pool) has the most important impact on the convective systems. The magnitude of the low-level cooling, associated baroclinicity, and stabilization of the pre-gust-front environment due to anvil shading generally increases as the duration of the shading increases. Thus, for a given leading anvil length, a slow-moving convective system tends to be affected more by anvil shading than does a fast-moving convective system. Differences in the forward speeds of the convective systems simulated in this study are largely attributable to differences in the mean environmental wind speed over the depth of the troposphere. Anvil shading reduces the buoyancy realized by the air parcels that ascend through the updrafts. As a result, anvil shading contributes to weaker updrafts relative to control simulations in which clouds are transparent to solar radiation. Anvil shading also affects the convective systems by modifying the low-level (nominally 0–2.5 km AGL) vertical wind shear in the pre-gust-front environment. The shear modifications affect the slope of the updraft region and system-relative rear-to-front flow, and the sign of the modifications is sensitive to the ground-relative vertical wind profile in the far-field environment. The vertical wind shear changes are brought about by baroclinic vorticity generation associated with the horizontal buoyancy gradient that develops in the shaded boundary layer (which makes the pre-gust-front, low-level vertical wind shear less westerly) and by a reduction of the vertical mixing of momentum due to the near-surface (nominally 0–300 m AGL) stabilization that accompanies the shading-induced cooling. The reduced mixing makes the pre-gust-front, low-level vertical shear more (less) westerly if the ambient, near-surface wind and wind shear are westerly (easterly).

scholarly journals Is There a “Tipping Point” between Simulated Nontornadic and Tornadic Supercells in VORTEX2 Environments?

2018 ◽  
Vol 146 (8) ◽  
pp. 2667-2693 ◽  
Author(s):  
Brice E. Coffer ◽  
Matthew D. Parker
Keyword(s):  
Wind Profile ◽  
Tipping Point ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Low Level ◽  
Surface Circulation ◽  
Wind Profiles ◽  
Tornadic Storms ◽  
Tropospheric Wind

Abstract Previous work has suggested that the lower-tropospheric wind profile may partly determine whether supercells become tornadic. If tornadogenesis within the VORTEX2 composite environments is more sensitive to the lower-tropospheric winds than to either the upper-tropospheric winds or the thermodynamic profile, then systematically varying the lower-tropospheric wind profile might reveal a “tipping point” between nontornadic and tornadic supercells. As a test, simulated supercells are initiated in environments that have been gradually interpolated between the low-level wind profiles of the nontornadic and tornadic VORTEX2 supercell composites while also interchanging the upper-tropospheric winds and thermodynamic profile. Simulated supercells become tornadic when the low-level wind profile incorporates at least 40% of the structure from the tornadic VORTEX2 composite environment. Both the nontornadic and tornadic storms have similar outflow temperatures and availability of surface vertical vorticity near their updrafts. Most distinctly, a robust low-level mesocyclone and updraft immediately overlie the intensifying near-surface circulation in each of the tornadic supercells. The nontornadic supercells have low-level updrafts that are disorganized, with pockets of descent throughout the region where surface vertical vorticity resides. The lower-tropospheric wind profile drives these distinct configurations of the low-level mesocyclone and updraft, regardless of the VORTEX2 composite upper-tropospheric wind profile or thermodynamic profile. This study therefore supports a potentially useful, robust link between the probability of supercell tornadogenesis and the lower-tropospheric wind profile, with tornadogenesis more (less) likely when the orientation of horizontal vorticity in the lowest few hundred meters is streamwise (crosswise).

scholarly journals Kinematic and Moisture Characteristics of a Nonprecipitating Cold Front Observed during IHOP. Part II: Alongfront Structures

2008 ◽  
Vol 136 (10) ◽  
pp. 3796-3821 ◽  
Author(s):  
Katja Friedrich ◽  
David E. Kingsmill ◽  
Cyrille Flamant ◽  
Hanne V. Murphey ◽  
Roger M. Wakimoto
Keyword(s):  
Wind Shear ◽  
Cold Front ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Vertical Wind ◽  
Horizontal Wind ◽  
Vertical Vorticity ◽  
Low Level ◽  
Thin Line ◽  
Early Afternoon

Kinematic and thermodynamic structures of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP) are examined with dropsondes and airborne instrumentation that includes Doppler radars, a differential absorption lidar, and in situ sensors. Intensive observations were collected along a 125-km segment of the front, with coverage of both the cold front leading edge and the post- and prefrontal areas. Whereas the first part of this two-part series of papers focused on across-front kinematic and moisture characteristics, the study herein investigates alongfront structures relevant for convection initiation. A northeast–southwest-oriented cold front moved into the observational domain from the northwest, but its motion slowed to less than 1 m s−1 in the early afternoon. In the late afternoon it was intersected by a north-northeast–south-southwest-oriented reflectivity thin line that was advected from the southwest, and another boundary that is an extension of a large-scale dryline paralleling the thin line but located farther to the east. Doppler wind synthesis suggests an increase in low-level horizontal wind shear across the cold front leading edge with the approach and intersection of the boundaries causing an increase in low-level convergence (up to ∼1 × 10−3 s−1), positive vertical vorticity (up to ∼0.5 × 10−3 s−1), and upward motion (up to ∼1 m s−1). An organized pattern of misocyclones (vertical vorticity maxima <4 km) and enhanced updrafts with a spacing of ∼5–8 km were observed at the cold front leading edge. At the same time vortex lines manifested as horizontal vorticity maxima were observed within the cold air oriented perpendicular to the cold front leading edge and on top of the vertical wind shear layer. The analysis suggests that inflection point instability was the dominant mechanism for their development. Low Richardson number (0.3–0.4), short lifetime (<2 h), horizontal wavelength of 3–6 km, and collocation with strong horizontal and vertical wind shear are characteristics that support the hypothesis that these instabilities were Kelvin–Helmholtz waves. Towering cumulus developed along the cold front forming a convective cell close to the intersection of the cold front, dryline, and reflectivity thin line.

scholarly journals Observations of the 9 June 2009 Dissipating Supercell from VORTEX2

2015 ◽  
Vol 30 (2) ◽  
pp. 368-388 ◽  
Author(s):  
Casey E. Davenport ◽  
Matthew D. Parker
Keyword(s):  
Wind Shear ◽  
Wind Profile ◽  
Vertical Wind Shear ◽  
Vertical Vorticity ◽  
Low Level ◽  
Stationary Boundary ◽  
Convective Inhibition ◽  
Rare Opportunity ◽  
Over Time ◽  
Unique Dataset

Abstract On 9 June 2009, the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) captured a unique dataset of dense observations throughout the lifetime of an isolated supercell, including its demise. This event provides a rare opportunity to explore the conditions and processes associated with supercell demise, as well as add to our understanding of supercell maintenance within the context of storm–environment interactions. The target storm on 9 June formed just to the cool side of a quasi-stationary boundary and initially exhibited strong low-level rotation. Over time, however, the storm moved deeper into the cool air and completely dissipated. Three near-inflow soundings launched over the lifetime of the supercell illustrated an increase in low-level convective inhibition (CIN) over time. However, an elevated layer containing sufficient instability and modest inhibition was also present, suggesting an unrealized potential for elevated convection. The near-storm environment also demonstrated a notable decrease in bulk vertical wind shear and storm-relative helicity over the lifetime of the storm. Although the likely impact of an increasingly stable near-storm environment is seemingly straightforward, the extent to which the evolving wind profile influenced storm dissipation is less certain. Dual-Doppler wind syntheses suggest decreases in the production of updraft vertical vorticity via tilting and stretching, indicating that the storm demise may have resulted from a complicated interplay between stabilization and changing environmental shear.

scholarly journals Impacts of Increasing Low-Level Shear on Supercells during the Early Evening Transition*

2015 ◽  
Vol 143 (5) ◽  
pp. 1945-1969 ◽  
Author(s):  
Brice E. Coffer ◽  
Matthew D. Parker
Keyword(s):  
Wind Shear ◽  
Dynamic Pressure ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Dynamical Response ◽  
Vertical Vorticity ◽  
Low Level ◽  
Evening Transition ◽  
Early Evening ◽  
Early Evening Transition

Abstract The dynamical response of simulated supercells to temporally increasing lower-tropospheric vertical wind shear is investigated using idealized simulations. These simulations are based upon observed soundings from two cases that underwent an early evening transition during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower-tropospheric shear. The primary effect of the increase in low-level shear is to establish larger values of vertical vorticity at lower altitudes in the storm’s updraft. In turn, this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm’s mesocyclone. This is particularly important at low levels, where it increases the storm's ability to lift cool surface air (including outflow). Trajectories launched in developing vortices show that, despite comparable buoyant accelerations, parcels experience greater vertical velocity and stretching of vertical vorticity due to increased dynamic accelerations when the low-level shear is increased. Thus, even as low-level stability gradually increases in the early evening, the supercells’ low-level updraft intensity and surface vorticity production can increase. These results are consistent with climatological observations of a supercell’s likelihood of tornadogenesis during the early evening hours.

scholarly journals Development of an Intense, Warm-Core Mesoscale Vortex Associated with the 8 May 2009 “Super Derecho” Convective Event*

2014 ◽  
Vol 71 (3) ◽  
pp. 1218-1240 ◽  
Author(s):  
Clark Evans ◽  
Morris L. Weisman ◽  
Lance F. Bosart
Keyword(s):  
Wind Shear ◽  
Vortex Formation ◽  
Vertical Wind Shear ◽  
Cyclonic Circulation ◽  
Vertical Wind ◽  
Streamwise Vorticity ◽  
Vertical Vorticity ◽  
Low Level ◽  
Warm Core ◽  
Low Level Jet

Abstract In this study, the dynamical processes contributing to warm-core meso-β-scale vortex formation associated with the 8 May 2009 “super derecho” are examined utilizing two complementary quasi-Lagrangian approaches—a circulation budget and backward trajectory analyses—applied to a fortuitous numerical simulation of the event. Warm-core meso-β-scale vortex formation occurs in a deeply moist, potentially stable environment that is conducive to the development of near-surface rotation and is somewhat atypical compared to known derecho-supporting environments. Air parcels in the vicinity of the developing vortex primarily originate near the surface in the streamwise vorticity-rich environment, associated with the vertical wind shear of the low-level jet, immediately to the east of the eastward-moving system. Cyclonic vertical vorticity is generated along inflowing air parcels primarily by the ascent-induced tilting of streamwise vorticity and amplified primarily by ascent-induced vortex tube stretching. Descent-induced tilting of crosswise vorticity contributes to cyclonic vertical vorticity generation for the small population of air parcels in the vicinity of the developing vortex that originate to its north and west. No consistent source of preexisting vertical vorticity is present within the environment. Cyclonic circulation on the scale of the warm-core meso-β-scale vortex increases in the lower troposphere in response to the mean vortex-scale convergence of cyclonic absolute vorticity and the local expulsion of eddy anticyclonic vertical vorticity generated within the system’s cold pool. Increased cyclonic circulation is partially offset by the system-scale tilting of horizontal vorticity associated with the low-level jet, rear-inflow jet, environmental vertical wind shear, and rotational flow of the warm-core vortex itself.

scholarly journals An Observational Study of Mesoscale Convective Systems with Heavy Rainfall over the Korean Peninsula

2006 ◽  
Vol 21 (2) ◽  
pp. 125-148 ◽  
Author(s):  
Hyung Woo Kim ◽  
Dong Kyou Lee
Keyword(s):  
Observational Study ◽  
Korean Peninsula ◽  
Wind Shear ◽  
Heavy Rainfall ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract A heavy rainfall event induced by mesoscale convective systems (MCSs) occurred over the middle Korean Peninsula from 25 to 27 July 1996. This heavy rainfall caused a large loss of life and property damage as a result of flash floods and landslides. An observational study was conducted using Weather Surveillance Radar-1988 Doppler (WSR-88D) data from 0930 UTC 26 July to 0303 UTC 27 July 1996. Dominant synoptic features in this case had many similarities to those in previous studies, such as the presence of a quasi-stationary frontal system, a weak upper-level trough, sufficient moisture transportation by a low-level jet from a tropical storm landfall, strong potential and convective instability, and strong vertical wind shear. The thermodynamic characteristics and wind shear presented favorable conditions for a heavy rainfall occurrence. The early convective cells in the MCSs initiated over the coastal area, facilitated by the mesoscale boundaries of the land–sea contrast, rain–no rain regions, saturated–unsaturated soils, and steep horizontal pressure and thermal gradients. Two MCSs passed through the heavy rainfall regions during the investigation period. The first MCS initiated at 1000 UTC 26 July and had the characteristics of a supercell storm with small amounts of precipitation, the appearance of a mesocyclone with tilting storm, a rear-inflow jet at the midlevel of the storm, and fast forward propagation. The second MCS initiated over the upstream area of the first MCS at 1800 UTC 26 July and had the characteristics of a multicell storm, such as a broken areal-type squall line, slow or quasi-stationary backward propagation, heavy rainfall in a concentrated area due to the merging of the convective storms, and a stagnated cluster system. These systems merged and stagnated because their movement was blocked by the Taebaek Mountain Range, and they continued to develop because of the vertical wind shear resulting from a low-level easterly inflow.

Assessing the influence of complex terrain on severe convective environments in northeastern Alabama

2021 ◽  
Author(s):  
Branden Katona ◽  
Paul Markowski
Keyword(s):  
Boundary Layer ◽  
Complex Terrain ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Spatial Distributions ◽  
Convective Storm ◽  
Self Organizing Maps ◽  
Low Level ◽  
Wind Regimes

AbstractStorms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective stormenvironments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

scholarly journals A numerical study of convection in rainbands of Typhoon Morakot (2009) with extreme rainfall: roles of pressure perturbations with low-level wind maxima

2015 ◽  
Vol 15 (6) ◽  
pp. 8479-8523
Author(s):  
C.-C. Wang ◽  
H.-C. Kuo ◽  
R. H. Johnson ◽  
C.-Y. Lee ◽  
S.-Y. Huang ◽  
...  
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Extreme Rainfall ◽  
Vertical Wind ◽  
Cold Pool ◽  
Rear Side ◽  
Gradient Force ◽  
Typhoon Morakot ◽  
Low Level ◽  
Pressure Perturbations

Abstract. This paper investigates the formation and evolution of deep convection inside the east–west oriented rainbands associated with a low-level jet (LLJ) in Typhoon Morakot (2009). With typhoon center to the northwest of Taiwan, the westerly LLJ was resulted from the interaction of typhoon circulation with the southwest monsoon flow, which supplied the water vapor for the extreme rainfall (of ~1000 mm) over southwestern Taiwan. The Cloud-Resolving Storm Simulator with 1 km grid spacing was used to simulate the event, and it successfully reproduced the slow-moving rainbands, the embedded cells, and the dynamics of merger and back-building (BB) on 8 August as observed. Our model results suggest that the intense convection interacted strongly with the westerly LLJ that provided reversed vertical wind shear below and above the jet core. Inside mature cells, significant dynamical pressure perturbations (pd') are induced with positive (negative) pd' at the western (eastern) flank of the updraft near the surface and a reversed pattern aloft (>2 km). This configuration produced an upward directed pressure gradient force (PGF) to the rear side and favors new development to the west, which further leads to cell merger as the mature cells slowdown in eastward propagation. The strong updrafts also acted to elevate the jet and enhance the local vertical wind shear at the rear flank. Additional analysis reveals that the upward PGF there is resulted mainly by the shearing effect but also by the extension of upward acceleration at low levels. In the horizontal, the upstream-directed PGF induced by the rear-side positive pd' near the surface is much smaller, but can provide additional convergence for BB development upstream. Finally, the cold-pool mechanism for BB appears to be not important in the Morakot case, as the conditions for strong evaporation in downdrafts do not exist.

How does vertical wind shear influence entrainment in squall lines?

2021 ◽  
Author(s):  
Jake P. Mulholland ◽  
John M. Peters ◽  
Hugh Morrison
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Boundary Shear ◽  
Squall Lines ◽  
Outflow Boundary

AbstractThe influence of vertical wind shear on updraft entrainment in squall lines is not well understood. To address this knowledge gap, a suite of high-resolution idealized numerical model simulations of squall lines were run in various vertical wind shear (hereafter “shear”) environments to study the effects of shear on entrainment in deep convective updrafts. Low-level horizontal mass flux into the leading edge of the cold pool was strongest in the simulations with the strongest low-level shear. These simulations consequently displayed wider updrafts, less entrainment-driven dilution, and larger buoyancy than the simulations with comparatively weak low-level shear. An analysis of vertical accelerations along trajectories that passed through updrafts showed larger net accelerations from buoyancy in the simulations with stronger low-level shear, which demonstrates how less entrainment-driven dilution equated to stronger updrafts. The effects of upper-level shear on entrainment and updraft vertical velocities were generally less pronounced than the effects of low-level shear. We argue that in addition to the outflow boundary-shear interactions and their effect on updraft tilt established by previous authors, decreased entrainment-driven dilution is yet another beneficial effect of strong low-level shear on squall line updraft intensity.

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