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.

A Squall-Line-Like Principal Rainband in Typhoon Hagupit (2008) Observed by Airborne Doppler Radar

2014 ◽  
Vol 71 (7) ◽  
pp. 2733-2746 ◽  
Author(s):  
Xiaowen Tang ◽  
Wen-Chau Lee ◽  
Michael Bell
Keyword(s):  
Wind Shear ◽  
Doppler Radar ◽  
Structural Characteristics ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Squall Lines ◽  
Convective Cells

Abstract This study examines the structure and dynamics of Typhoon Hagupit’s (2008) principal rainband using airborne radar and dropsonde observations. The convection in Hagupit’s principal rainband was organized into a well-defined line with trailing stratiform precipitation on the inner side. Individual convective cells had intense updrafts and downdrafts and were aligned in a wavelike pattern along the line. The line-averaged vertical cross section possessed a slightly inward-tilting convective core and two branches of low-level inflow feeding the convection. The result of a thermodynamic retrieval showed a pronounced cold pool behind the convective line. The horizontal and vertical structures of this principal rainband show characteristics that are different than the existing conceptual model and are more similar to squall lines and outer rainbands. The unique convective structure of Hagupit’s principal rainband was associated with veering low-level vertical wind shear and large convective instability in the environment. A quantitative assessment of the cold pool strength showed that it was quasi balanced with that of the low-level vertical wind shear. The balanced state and the structural characteristics of convection in Hagupit’s principal rainband were dynamically consistent with the theory of cold pool dynamics widely applied to strong and long-lived squall lines. The analyses suggest that cold pool dynamics played a role in determining the principal rainband structure in addition to storm-scale vortex dynamics.

Observations of a Squall Line and Its Near Environment Using High-Frequency Rawinsonde Launches during VORTEX2

2010 ◽  
Vol 138 (11) ◽  
pp. 4076-4097 ◽  
Author(s):  
George H. Bryan ◽  
Matthew D. Parker
Keyword(s):  
Wind Shear ◽  
Deep Layer ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Stratiform Region ◽  
Gust Front

Abstract Rawinsonde data were collected before and during passage of a squall line in Oklahoma on 15 May 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Nine soundings were released within 3 h, allowing for unprecedented analysis of the squall line’s internal structure and nearby environment. Four soundings were released in the prestorm environment and they document the following features: low-level cooling associated with the reduction of solar isolation by a cirrus anvil; abrupt warming (1.5 K in 30 min) above the boundary layer, which is probably attributable to a gravity wave; increases in both low-level and deep-layer vertical wind shear within 100 km of the squall line; and evidence of ascent extending at least 75 km ahead of the squall line. The next sounding was released ∼5 km ahead of the squall line’s gust front; it documented a moist absolutely unstable layer within a 2-km-deep layer of ascent, with vertical air velocity of approximately 6 m s−1. Another sounding was released after the gust front passed but before precipitation began; this sounding showed the cold pool to be ∼4 km deep, with a cold pool intensity C ≈ 35 m s−1, even though this sounding was located only 8 km behind the surface gust front. The final three soundings were released in the trailing stratiform region of the squall line, and they showed typical features such as: “onion”-shaped soundings, nearly uniform equivalent potential temperature over a deep layer, and an elevated rear inflow jet. The cold pool was 4.7 km deep in the trailing stratiform region, and extended ∼1 km above the melting level, suggesting that sublimation was a contributor to cold pool development. A mesoscale analysis of the sounding data shows an upshear tilt to the squall line, which is consistent with the cold pool intensity C being much larger than a measure of environmental vertical wind shear ΔU. This dataset should be useful for evaluating cloud-scale numerical model simulations and analytic theory, but the authors argue that additional observations of this type should be collected in future field projects.

A Multimodel Assessment of RKW Theory’s Relevance to Squall-Line Characteristics

2006 ◽  
Vol 134 (10) ◽  
pp. 2772-2792 ◽  
Author(s):  
George H. Bryan ◽  
Jason C. Knievel ◽  
Matthew D. Parker
Keyword(s):  
Wind Shear ◽  
Numerical Models ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
System Structure ◽  
Squall Line ◽  
Cold Pool ◽  
Density Currents ◽  
Optimal State ◽  
Squall Lines

Abstract The authors evaluate whether the structure and intensity of simulated squall lines can be explained by “RKW theory,” which most specifically addresses how density currents evolve in sheared environments. In contrast to earlier studies, this study compares output from four numerical models, rather than from just one. All of the authors’ simulations support the qualitative application of RKW theory, whereby squall-line structure is primarily governed by two effects: the intensity of the squall line’s surface-based cold pool, and the low- to midlevel environmental vertical wind shear. The simulations using newly developed models generally support the theory’s quantitative application, whereby an optimal state for system structure also optimizes system intensity. However, there are significant systematic differences between the newer numerical models and the older model that was originally used to develop RKW theory. Two systematic differences are analyzed in detail, and causes for these differences are proposed.

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.

scholarly journals The Kinematic and Microphysical Characteristics and Associated Precipitation Efficiency of Subtropical Convection during SoWMEX/TiMREX

2015 ◽  
Vol 143 (1) ◽  
pp. 317-340 ◽  
Author(s):  
Wei-Yu Chang ◽  
Wen-Chau Lee ◽  
Yu-Chieng Liou
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Convective Precipitation ◽  
Squall Line ◽  
Convective System ◽  
Low Level ◽  
Precipitation Efficiency

Abstract Dual-Doppler, polarimetric radar observations and precipitation efficiency (PE) calculations are used to analyze subtropical heavy rainfall events that occurred in southern Taiwan from 14 to 17 June 2008 during the Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) field campaign. Two different periods of distinct precipitation systems with diverse kinematic and microphysical characteristics were investigated: 1) prefrontal squall line (PFSL) and 2) southwesterly monsoon mesoscale convective system (SWMCS). The PFSL was accompanied by a low-level front-to-rear inflow and pronounced vertical wind shear. In contrast, the SWMCS had a low-level southwesterly rear-to-front flow with a uniform vertical wind field. The PFSL (SWMCS) contained high (low) lightning frequency associated with strong (moderate) updrafts and intense graupel–rain/graupel–small hail mixing (more snow and less graupel water content) above the freezing level. It is postulated that the reduced vertical wind shear and enhanced accretional growth of rain by high liquid water content at low levels in the SWMCS helped produce rainfall more efficiently (53.1%). On the contrary, the deeper convection of the PFSL had lower PE (45.0%) associated with the evaporative loss of rain and the upstream transport of liquid water to form larger stratiform regions. By studying these two events, the dependence of PE on the environmental and microphysical factors of subtropical heavy precipitation systems are investigated by observational data for the first time. Overall, the PE of the convective precipitation region (47.9%) from 14 to 17 June is similar to past studies of convective precipitation in tropical regions.

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 (19) ◽  
pp. 11097-11115 ◽  
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 the typhoon center to the northwest of Taiwan, the westerly LLJ occurred as a result 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 (p'd) are induced with positive (negative) p'd 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 merging 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 p'd 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.

Processes Controlling the Structure and Longevity of Two Quasi-Linear Convective Systems Crossing the Southern New England Coast

2013 ◽  
Vol 141 (11) ◽  
pp. 3710-3734 ◽  
Author(s):  
Kelly Lombardo ◽  
Brian A. Colle
Keyword(s):  
New York ◽  
New York City ◽  
York City ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Vertical Wind ◽  
Cold Pool ◽  
Convective Systems ◽  
Atlantic Coastal

Abstract This paper explores the structural evolution and physical processes that explain the modification of two quasi-linear convective systems (QLCSs) that encountered the densely populated New York City–Atlantic coastal region. One QLCS on 31 May 2002 traversed the Atlantic coastal boundary with little change in its intensity, producing widespread severe wind damage across New York City and Long Island. During this event, warm air advection at 925 hPa helped destabilize the layer above this level over the coastal zone, while the marine boundary layer deepened below this level. The 0–3-km line-perpendicular vertical wind shear was relatively strong, which supported ascent along the leading edge of the diabatically generated cold pool. The surface-based convective system became slightly elevated as it moved over the marine waters. In contrast, the 23 July 2002 QLCS decayed upon encountering the Atlantic coastline, despite its coincidence with a surface cold front. The most unstable CAPE values during this decaying event were 400–800 J kg−1 greater than the sustaining 31 May event, though the 0–3-km vertical wind shear was approximately half. Weaker shear likely contributed to limited ascent along the leading edge of the surface based cold pool, and ultimately the demise of the convective line. Sensitivity tests highlight the importance of the relationship between the cold pool and vertical shear during these two events, and illustrate the limited role of the marine layer in modifying the evolution of these two convective systems.

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 Subrainband Structure and Dynamic Characteristics in the Principal Rainband of Typhoon Hagupit (2008)

2018 ◽  
Vol 146 (1) ◽  
pp. 157-173 ◽  
Author(s):  
Xiaowen Tang ◽  
Wen-Chau Lee ◽  
Michael Bell
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Local Environment ◽  
Vertical Wind ◽  
Cold Pool ◽  
Precipitation Region ◽  
Squall Lines ◽  
Convective Scale ◽  
Radial Outflow ◽  
Positive Impacts

Abstract The principal rainband in tropical cyclones is currently depicted as a solitary and continuous precipitation region. However, the airborne radar observations of the principal rainband in Typhoon Hagupit (2008) reveal multiple subrainband structures. These subbands possess many characteristics of the squall lines with trailing stratiform in the midlatitudes and are different from those documented in previous principal rainband studies. The updraft and reflectivity cores are upright and elevated. The updraft is fed by a low-level radial outflow from the inner side. The tangential wind speed shows a clear midlevel jet on the inner side of the reflectivity core. Except for the structural similarities, the dynamics of the subbands is also similar to the squall lines. The local environment near the subbands shows little convective inhibition, modest instability, and vertical wind shear. The temperature retrieval shows a cold pool structure in the stratiform precipitation region. The estimated vertical wind shear induced by the cold pool is close to that of the local environment. The structural and dynamic similarities to the squall lines imply that the variation of principal rainbands is subjected to convective-scale dynamics related to the local environment in addition to storm-scale dynamics. The subbands show positive impacts to the vortex intensity in terms of potential vorticity redistribution and absolute angular momentum advection. The positive impacts are closely related to specific structural characteristics of the subbands, which suggests the importance of understanding the convective-scale structure and dynamics of the principal rainband.

Relationship between System Slope and Updraft Intensity in Squall Lines

2010 ◽  
Vol 138 (9) ◽  
pp. 3572-3578 ◽  
Author(s):  
Matthew D. Parker
Keyword(s):  
Pressure Gradient ◽  
Line Intensity ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Squall Line ◽  
Cold Pool ◽  
Optimal State ◽  
Squall Lines ◽  
Simple Demonstration ◽  
Perturbation Pressure

Abstract In recent years there has been debate about whether squall lines have an “optimal state.” It has been repeatedly demonstrated that the slope of a squall line’s convective region is related to the comparative magnitudes of the squall line’s cold pool and the base-state vertical wind shear. The present work addresses a related assertion, that squall-line intensity ought to be maximized for an upright updraft zone. A simple demonstration shows that upright systems realize more of their buoyancy because their attendant downward-directed perturbation pressure gradient accelerations are weaker.

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