Investigating tropical squall lines with a cloud resolving model

2021 ◽  
Author(s):  
Sophie Abramian ◽  
Caroline Muller ◽  
Camille Risi
Keyword(s):  
Wind Shear ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Critical Regime ◽  
Squall Line ◽  
Cold Pool ◽  
Cold Pools ◽  
Squall Lines ◽  
Optimal Value ◽  
Cloud Resolving Model

<p>Investigating tropical squall lines with a cloud resolving model</p><p>Using a cloud resolving model, we attempt to clarify the physical processes responsible for the organization of deep clouds into squall lines in the tropics. To do so, we impose a vertical wind shear, and investigate the response of deep convection to different shear strengths in radiative convective equilibrium. As the magnitude of the shear increases, the convection becomes more and more organized into a line, perpendicular to the shear. It is due to the interaction of the low-level shear with the cold pools associated with convective downdrafts. Beyond a certain shear, called optimal shear, the line tends to orient at an angle to the shear. The existing literature suggests that this angle conserves the projection of the shear on the direction perpendicular to the squall line near the optimal value, a hypothesis that we further investigate here.</p><p>In this work, we propose a systematic method, based on image auto-correlation, to determine the angle of the squall line with respect to the shear. We highlight the existence of the sub-critical and super-critical regime, as predicted by earlier studies. In the sub-critical regime, squall lines are indeed perpendicular to the shear. Yet, angles of squall lines in the super-critical regime do not clearly correspond to the conservation of the projected component of the shear near the optimal value. In particular, squall lines often remain more perpendicular to the shear than expected.</p><p>We thus investigate the balance between shear and cold pool winds to explain this difference. Using statistical methods on extreme events, we find that this difference is due to an intensification of cold pool potential energy with shear. Cold pool intensification allows the squall line to better resist to the shear, and thus reduces its angle of orientation. This new feature leads us to conclude that two mechanisms maintain a squall line in wind shear : the orientation of clouds and the intensification of cold pools.</p>

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.

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.

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.

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.

The Interaction of Simulated Squall Lines with Idealized Mountain Ridges

2006 ◽  
Vol 134 (7) ◽  
pp. 1919-1941 ◽  
Author(s):  
Jeffrey Frame ◽  
Paul Markowski
Keyword(s):  
Squall Line ◽  
Cold Pool ◽  
Cold Air ◽  
Advanced Regional Prediction System ◽  
Topographic Features ◽  
Squall Lines ◽  
Regional Prediction ◽  
Cloud Resolving Model ◽  
Orographic Enhancement ◽  
Flow Transitions

Abstract Numerical simulations of squall lines traversing sinusoidal mountain ridges are performed using the Advanced Regional Prediction System cloud-resolving model. Precipitation and updraft strength are enhanced through orographic ascent as a squall line approaches a ridge. The simulated squall line then weakens as it descends the ridge because some of the cold pool is blocked by the terrain, resulting in less lift along the gust front and weaker convective cells. The flow within the cold pool accelerates slightly and the depth of the cold air decreases owing to upstream blocking, transitioning the flow in the cold pool head from subcritical to supercritical, then back to subcritical at the bottom of the ridge. A hydraulic jump forms when the flow transitions the second time, enabling the development of a new convective line downwind of the mountain. These new updrafts grow and eventually replace the older updrafts that weakened during descent. This process results in the discrete propagation of a squall line just downstream of a ridge, resulting in the formation of rain shadows downstream from topographic features. Discrete propagation only occurs if a ridge is of sufficient height, however. This replacement process repeats itself if a squall line encounters multiple ridges. The risk of damaging winds from a squall line is greater on the lee side of ridges and on the top of high ridges. These terrain-forced intensity fluctuations increase with mountain height, because the higher terrain permits even less cold air to flow over it. A wider ridge results in a more gradual orographic enhancement and downslope-induced weakening.

On the persistence of tropical shallow convective squall lines - the role of cold pools

2020 ◽  
Author(s):  
Ludovic Touzé-Peiffer ◽  
Nicolas Rochetin ◽  
Raphaela Vogel
Keyword(s):  
Wind Shear ◽  
Leading Edge ◽  
Critical Factor ◽  
Trade Wind ◽  
Squall Line ◽  
Cold Pools ◽  
Squall Lines ◽  
German Model ◽  
Convective Cells

<p>A considerable amount of literature has been devoted to the study of strong convective squall line. In particular, many studies have noted the role of cold pools on the persistence of these squall lines. Observations and simulations have shown that squall lines are often associated with pools of air cooled by partial rain evaporation. Such cold pools spread at the surface and may initiate new convective cells at their edges, thus contributing to the maintenance of a squall line. Under which environmental conditions the lifting at the edges of cold pools is most efficient has been subject to many debates. Yet, it is generally acknowledged that the environmental wind shear is a critical factor in this process. </p><p>Recent observations and realistic simulations over the trade-wind region have revealed persistent structures of shallow cumuli associated with surface cold pools. We will call these structures shallow convective squall lines, due to their similarity with strong convective squall lines. Based on simulations from the German model ICON and on recent observations from the field campaign EUREC4A, we will study the characteristics of these shallow convective squall lines and their lifecycle. Similarly to strong convective squall lines, shallow convective squall lines organized around a leading edge composed by many updrafts and downdrafts feeding the surface cold pools. We will see that the environmental wind shear plays a key role in the persistence of these shallow convective squall line, and we will compare our findings with classical theories for strong convective squall lines.</p>

scholarly journals Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

2015 ◽  
Vol 15 (4) ◽  
pp. 1745-1768 ◽  
Author(s):  
J. S. Reid ◽  
N. D. Lagrosas ◽  
H. H. Jonsson ◽  
E. A. Reid ◽  
W. R. Sessions ◽  
...  
Keyword(s):  
Boundary Layer ◽  
South China Sea ◽  
Wind Shear ◽  
Large Scale ◽  
Aerosol Particles ◽  
Vertical Wind Shear ◽  
Sea Salt ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Squall Lines

Abstract. In a joint NRL/Manila Observatory mission, as part of the Seven SouthEast Asian Studies program (7-SEAS), a 2-week, late September 2011 research cruise in the northern Palawan archipelago was undertaken to observe the nature of southwest monsoonal aerosol particles in the South China Sea/East Sea (SCS/ES) and Sulu Sea region. Previous analyses suggested this region as a receptor for biomass burning from Borneo and Sumatra for boundary layer air entering the monsoonal trough. Anthropogenic pollution and biofuel emissions are also ubiquitous, as is heavy shipping traffic. Here, we provide an overview of the regional environment during the cruise, a time series of key aerosol and meteorological parameters, and their interrelationships. Overall, this cruise provides a narrative of the processes that control regional aerosol loadings and their possible feedbacks with clouds and precipitation. While 2011 was a moderate El Niño–Southern Oscillation (ENSO) La Niña year, higher burning activity and lower precipitation was more typical of neutral conditions. The large-scale aerosol environment was modulated by the Madden–Julian Oscillation (MJO) and its associated tropical cyclone (TC) activity in a manner consistent with the conceptual analysis performed by Reid et al. (2012). Advancement of the MJO from phase 3 to 6 with accompanying cyclogenesis during the cruise period strengthened flow patterns in the SCS/ES that modulated aerosol life cycle. TC inflow arms of significant convection sometimes span from Sumatra to Luzon, resulting in very low particle concentrations (minimum condensation nuclei CN < 150 cm−3, non-sea-salt PM2.5 < 1 μg m−3). However, elevated carbon monoxide levels were occasionally observed suggesting passage of polluted air masses whose aerosol particles had been rained out. Conversely, two drier periods occurred with higher aerosol particle concentrations originating from Borneo and Southern Sumatra (CN > 3000 cm−3 and non-sea-salt PM2.5 10–25 μg m−3). These cases corresponded with two different mechanisms of convection suppression: lower free-tropospheric dry-air intrusion from the Indian Ocean, and large-scale TC-induced subsidence. Veering vertical wind shear also resulted in aerosol transport into this region being mainly in the marine boundary layer (MBL), although lower free troposphere transport was possible on the western sides of Sumatra and Borneo. At the hourly time scale, particle concentrations were observed to be modulated by integer factors through convection and associated cold pools. Geostationary satellite observations suggest that convection often takes the form of squall lines, which are bowed up to 500 km across the monsoonal flow and 50 km wide. These squall lines, initiated by cold pools from large thunderstorms and likely sustained by a veering vertical wind shear and aforementioned mid-troposphere dry layers, propagated over 1500 km across the entirety of the SCS/ES, effectively cutting large swaths of MBL aerosol particles out of the region. Our conclusion is that while large-scale flow patterns are very important in modulating convection, and hence in allowing long-range transport of smoke and pollution, more short-lived phenomena can modulate cloud condensation nuclei (CCN) concentrations in the region, resulting in pockets of clean and polluted MBL air. This will no doubt complicate large scale comparisons of aerosol–cloud interaction.

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.

scholarly journals The Behavior of Squall Lines in Horizontally Heterogeneous Coastal Environments

2018 ◽  
Vol 75 (4) ◽  
pp. 1243-1269 ◽  
Author(s):  
Kelly Lombardo ◽  
Tristan Kading
Keyword(s):  
Deep Convection ◽  
Internal Gravity Wave ◽  
Sea Breeze ◽  
Squall Line ◽  
Cold Pool ◽  
Stable Layer ◽  
Squall Lines ◽  
Unstable Layer ◽  
Storm Characteristics ◽  
Marine Layer

Abstract Inland squall lines respond to the stable marine atmospheric boundary layer (MABL) as they move toward a coastline and offshore. As a storm’s cold pool collides with the marine layer, characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics. Idealized numerical experiments exploring a parameter space of MABL characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the MABL. When the outflow is less buoyant, storms are forced by a cold pool within the marine environment. When the buoyancies are equivalent, a hybrid cold pool–internal gravity wave develops after the collision. The collision between a cold pool and less buoyant MABL initiates internal waves along the stable layer, regardless of MABL depth. These waves are inefficient at lifting air into the storm, and ascent from the trailing cold pool is needed to support deep convection. Storm intensity decreases with deeper and less buoyant MABLs, in part due to the reduction in elevated instability. Precipitation is enhanced just prior to the collision between a storm and the deepest marine layers. Storms modify their environment downstream, leading to the development of a moist adiabatic unstable layer and a lowering of the level of free convection (LFC) to below the top of the deepest marine layer. An MABL moving as a sea breeze into the storm-modified air successfully lifts parcels to the new LFC, generating convective towers ahead of the squall line. This mechanism may contribute to increased coastal flash flooding risks during observed events.

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