Squall-Line Intensification via Hydrometeor Recirculation

2013 ◽  
Vol 70 (7) ◽  
pp. 2012-2031 ◽  
Author(s):  
Robert B. Seigel ◽  
Susan C. van den Heever
Keyword(s):  
Line Strength ◽  
Atmospheric Modeling ◽  
Squall Line ◽  
Cold Pool ◽  
Latent Heating ◽  
Moist Convection ◽  
Freezing Level ◽  
Squall Lines ◽  
Deep Moist Convection ◽  
Inflow Jet

Abstract Many studies have demonstrated the intimate connection between microphysics and deep moist convection, especially for squall lines via cold pool pathways. The present study examines four numerically simulated idealized squall lines using the Regional Atmospheric Modeling System (RAMS) and includes a control simulation that uses full two-moment microphysics and three sensitivity experiments that vary the mean diameter of the hail hydrometeor size distribution. Results suggest that a circulation centered at the freezing level supports midlevel convective updraft invigoration through increased latent heating. The circulation begins with hail hydrometeors that initiate within the convective updraft above the freezing level and are then ejected upshear because of the front-to-rear flow of the squall line. As the hail falls below the freezing level, the rear-inflow jet (RIJ) advects the hail hydrometeors downshear and into the upshear flank of the midlevel convective updraft. Because the advection occurs below the freezing level, some of the hail melts and sheds raindrops. The addition of hail and rain to the updraft increases latent heating owing to both an enhancement in riming and vapor deposition onto hail and rain. The increase in latent heating enhances buoyancy within the updraft, which leads to an increase in precipitation and cold pool intensity that promote a positive feedback on squall-line strength. The upshear-tilted simulated squall lines in this study indicate that as hail size is decreased, squall lines are invigorated through the recirculation mechanism.

scholarly journals Impacts of Stochastic Mixing in Idealized Convection-Permitting Simulations of Squall Lines

2020 ◽  
Vol 148 (12) ◽  
pp. 4971-4994
Author(s):  
McKenna W. Stanford ◽  
Hugh Morrison ◽  
Adam Varble
Keyword(s):  
Squall Line ◽  
Weather Research And Forecasting ◽  
Cold Pool ◽  
Multiplicative Factor ◽  
Mesoscale Structure ◽  
Squall Lines ◽  
Mixing Coefficients ◽  
Horizontal Mixing ◽  
Upper Level ◽  
Inflow Jet

AbstractThis study investigates impacts of altering subgrid-scale mixing in “convection-permitting” kilometer-scale horizontal-grid-spacing (Δh) simulations by applying either constant or stochastic multiplicative factors to the horizontal mixing coefficients within the Weather Research and Forecasting Model. In quasi-idealized 1-km Δh simulations of two observationally based squall-line cases, constant enhanced mixing produces larger updraft cores that are more dilute at upper levels, weakens the cold pool, rear-inflow jet, and front-to-rear flow of the squall line, and degrades the model’s effective resolution. Reducing mixing by a constant multiplicative factor has the opposite effect on all metrics. Completely turning off parameterized horizontal mixing produces bulk updraft statistics and squall-line mesoscale structure closest to an LES “benchmark” among all 1-km simulations, although the updraft cores are too undilute. The stochastic mixing scheme, which applies a multiplicative factor to the mixing coefficients that varies stochastically in time and space, is employed at 0.5-, 1-, and 2-km Δh. It generally reduces midlevel vertical velocities and enhances upper-level vertical velocities compared to simulations using the standard mixing scheme, with more substantial impacts at 1- and 2-km Δh compared to 0.5-km Δh. The stochastic scheme also increases updraft dilution to better agree with the LES for one case, but has less impact on the other case. Stochastic mixing acts to weaken the cold pool but without a significant impact on squall-line propagation. It also does not affect the model’s overall effective resolution unlike applying constant multiplicative factors to the mixing coefficients.

scholarly journals Potential Vorticity Generation by West African Squall Lines

2020 ◽  
Vol 148 (4) ◽  
pp. 1691-1715
Author(s):  
Richard H. Johnson ◽  
Paul E. Ciesielski
Keyword(s):  
Potential Vorticity ◽  
Land Surface ◽  
West African ◽  
Squall Line ◽  
Latent Heating ◽  
Moist Convection ◽  
Convective Boundary ◽  
Easterly Waves ◽  
Complex Interactions ◽  
Squall Lines

Abstract The West African summer monsoon features multiple, complex interactions between African easterly waves (AEWs), moist convection, variable land surface properties, dust aerosols, and the diurnal cycle. One aspect of these interactions, the coupling between convection and AEWs, is explored using observations obtained during the 2006 African Monsoon Multidisciplinary Analyses (AMMA) field campaign. During AMMA, a research weather radar operated at Niamey, Niger, where it surveilled 28 squall-line systems characterized by leading convective lines and trailing stratiform regions. Nieto Ferreira et al. found that the squall lines were linked with the passage of AEWs and classified them into two tracks, northerly and southerly, based on the position of the African easterly jet (AEJ). Using AMMA sounding data, we create a composite of northerly squall lines that tracked on the cyclonic shear side of the AEJ. Latent heating within the trailing stratiform regions produced a midtropospheric positive potential vorticity (PV) anomaly centered at the melting level, as commonly observed in such systems. However, a unique aspect of these PV anomalies is that they combined with a 400–500-hPa positive PV anomaly extending southward from the Sahara. The latter feature is a consequence of the deep convective boundary layer over the hot Saharan Desert. Results provide evidence of a coupling and merging of two PV sources—one associated with the Saharan heat low and another with latent heating—that ends up creating a prominent midtropospheric positive PV maximum to the rear of West African squall lines.

scholarly journals Assessing the mineral dust indirect effects and radiation impacts on a simulated idealized nocturnal squall line

2012 ◽  
Vol 12 (11) ◽  
pp. 29607-29655 ◽  
Author(s):  
R. B. Seigel ◽  
S. C. van den Heever ◽  
S. M. Saleeby
Keyword(s):  
Indirect Effects ◽  
Mineral Dust ◽  
Synergistic Effects ◽  
Atmospheric Modeling ◽  
Dust Particles ◽  
Squall Line ◽  
Cold Pool ◽  
Freezing Level ◽  
Warm Rain Process ◽  
The Individual

Abstract. Mineral dust is arguably the most abundant aerosol species in the world and it plays a large role in aerosol indirect effects (AIEs). This study assesses and isolates the individual responses in a squall line that arise (1) from radiation, (2) from dust altering the microphysics, as well as (3) from the synergistic effects between (1) and (2). To accomplish these tasks, we use the Regional Atmospheric Modeling System (RAMS) set up as a cloud-resolving model (CRM). The CRM contains aerosol and microphysical schemes that allow mineral dust particles to nucleate as cloud drops and ice crystals, replenish upon evaporation and sublimation, be tracked throughout hydrometeor transition, and scavenge by precipitation and dry sedimentation. Factor separation is used on four simulations of the squall line in order to isolate the individual roles of radiation (RADIATION), microphysically active dust (DUST MICRO), and the nonlinear interactions of those factors (SYNERGY). Results indicate that RADIATION acts to increase precipitation, intensify the cold pool, and enhance the mesoscale organization of the squall line due to changes in microphysics beginning from cloud top cooling. Conversely, DUST MICRO decreases precipitation, weakens the cold pool, and weakens the mesoscale organization of the squall line due to an enhancement of the warm rain process. SYNERGY shows little impact on the squall line, except near the freezing level, where an increase in mesoscale organization takes place. The combined effect of the mineral dust AIE due to both DUST MICRO and SYNERGY is to weaken the squall line.

scholarly journals Mineral dust indirect effects and cloud radiative feedbacks of a simulated idealized nocturnal squall line

2013 ◽  
Vol 13 (8) ◽  
pp. 4467-4485 ◽  
Author(s):  
R. B. Seigel ◽  
S. C. van den Heever ◽  
S. M. Saleeby
Keyword(s):  
Indirect Effects ◽  
Mineral Dust ◽  
Synergistic Effects ◽  
Atmospheric Modeling ◽  
Dust Particles ◽  
Squall Line ◽  
Cold Pool ◽  
Freezing Level ◽  
Warm Rain Process ◽  
The Individual

Abstract. Mineral dust is arguably the most abundant aerosol species in the world and as such potentially plays a large role in aerosol indirect effects (AIEs). This study assesses and isolates the individual responses in a squall line that arise (1) from radiation, (2) from dust altering the microphysics, as well as (3) from the synergistic effects between (1) and (2). To accomplish these tasks, we use the Regional Atmospheric Modeling System (RAMS) set up as a cloud-resolving model (CRM). The CRM contains aerosol and microphysical schemes that allow mineral dust particles to nucleate as cloud drops and ice crystals, replenish upon evaporation and sublimation, be tracked throughout hydrometeor transition, and be scavenged by precipitation and dry sedimentation. Factor separation is used on four simulations of the squall line in order to isolate the individual roles of radiation (RADIATION), microphysically active dust (DUST MICRO), and the nonlinear interactions of those factors (SYNERGY). Results indicate that RADIATION acts to increase precipitation, intensify the cold pool, and enhance the mesoscale organization of the squall line due to changes in microphysics originating from cloud top cooling. Conversely, DUST MICRO decreases precipitation, weakens the cold pool, and weakens the mesoscale organization of the squall line due to an enhancement of the warm rain process. SYNERGY shows little impact on the squall line, except near the freezing level, where an increase in mesoscale organization takes place. The combined effect of the mineral dust AIE due to both DUST MICRO and SYNERGY is to weaken the squall line.

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.

Evaluating the conservation of energy variables in simulations of deep moist convection

2021 ◽  
Author(s):  
John M. Peters ◽  
Daniel R. Chavas
Keyword(s):  
Heat Capacity ◽  
Numerical Models ◽  
Deep Convection ◽  
Squall Line ◽  
Moist Convection ◽  
Conservation Of Energy ◽  
Deep Moist Convection ◽  
Static Energy ◽  
Hydrostatic Balance ◽  
Latent Heats

AbstractIt is often assumed in parcel theory calculations, numerical models, and cumulus parameterizations that moist static energy (MSE) is adiabatically conserved. However, the adiabatic conservation of MSE is only approximate because of the assumption of hydrostatic balance. Two alternative variables are evaluated here: MSE −IB and MSE +KE, wherein IB is the path integral of buoyancy (B) and KE is kinetic energy. Both of these variables relax the hydrostatic assumption and are more precisely conserved than MSE. This article quantifies the errors that result from assuming that the aforementioned variables are conserved in large eddy simulations (LES) of both disorganized and organized deep convection. Results show that both MSE −IB and MSE +KE better predict quantities along trajectories than MSE alone. MSE −IB is better conserved in isolated deep convection, whereas MSE −IB and MSE +KE perform comparably in squall line simulations. These results are explained by differences between the pressure perturbation behavior of squall lines and isolated convection. Errors in updraft B diagnoses are universally minimized when MSE−IB is assumed to be adiabatically conserved, but only when moisture dependencies of heat capacity and temperature dependency of latent heating are accounted for. When less accurate latent heat and heat capacity formulae were used, MSE−IB yielded poorer B predictions than MSE due to compensating errors. Our results suggest that various applications would benefit from using either MSE −IB or MSE +KE instead of MSE with properly formulated heat capacities and latent heats.

scholarly journals Microphysical Process Comparison of Three Microphysics Parameterization Schemes in the WRF Model for an Idealized Squall-Line Case Study

2019 ◽  
Vol 147 (9) ◽  
pp. 3093-3120 ◽  
Author(s):  
J.-W. Bao ◽  
S. A. Michelson ◽  
E. D. Grell
Keyword(s):  
Wrf Model ◽  
Propagation Speed ◽  
Front Propagation ◽  
Particle Growth ◽  
Population Characteristics ◽  
Squall Line ◽  
Microphysical Process ◽  
Process Comparison ◽  
Freezing Level ◽  
Squall Lines

Abstract Three bulk microphysics schemes with different complexities in the Weather Research and Forecasting Model are compared in terms of the individual microphysical process terms of the hydrometeor mass and number mixing ratio tendency equations in an idealized 2D squall-line case. Through evaluation of these process terms and of hydrometeor size distributions, it is shown that the differences in the simulated population characteristics of snow, graupel, and rainwater are the prominent factors contributing to the differences in the development of the simulated squall lines using these schemes. In this particular case, the gust front propagation speed produced by the Thompson scheme is faster than in the other two schemes during the first 2 h of the simulation because it has a larger dominant graupel size. After 2 h into the simulation, the initially less intense squall lines in the runs using the WSM6 and Morrison schemes start to catch up in intensity and development to the run using the Thompson scheme. Because the dominant size of graupel particles in the runs using the WSM6 and Morrison schemes is smaller, these particles take more time to fall below the freezing level and enhance the rainwater production and its evaporative cooling. In the run using the Thompson scheme, the graupel production slows down at later times while the snow particle growth increases, leading to more snow falling below the freezing level to melt and surpass graupel particle melting in the production of rainwater.

scholarly journals Numerical Simulations of Bow Echo Formation Following a Squall Line–Supercell Merger

2014 ◽  
Vol 142 (12) ◽  
pp. 4791-4822 ◽  
Author(s):  
Adam J. French ◽  
Matthew D. Parker
Keyword(s):  
Numerical Simulations ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Direct Result ◽  
Squall Line ◽  
Cold Pool ◽  
Surface Winds ◽  
Sensitivity Tests ◽  
Bow Echo ◽  
Inflow Jet

Abstract Output from idealized numerical simulations is used to investigate the storm-scale processes responsible for squall-line evolution following a merger with an isolated supercell. A simulation including a squall line–supercell merger is compared to one using the same initial squall line and background environment without the merger. These simulations reveal that while bow echo formation is favored by the strongly sheared background environment, the merger produces a more compact bowing structure owing to a locally enhanced rear-inflow jet. The merger also represents a favored location for severe weather production relative to other portions of the squall line, with surface winds, vertical vorticity, and rainfall all being maximized in the vicinity of the merger. An analysis of storm-scale processes reveals that the premerger squall line weakens as it encounters outflow from the preline supercell, and the supercell becomes the leading edge of the merged system. Subsequent localized strengthening of the cold pool and rear-inflow jet produce a compact, intense bow echo local to the merger, with a descending rear-inflow jet creating a broad swath of damaging surface winds. These features, common to severe bow echoes, are shown to be a direct result of the merger in the present simulations, and are diminished or absent in the no-merger simulation. Sensitivity tests reveal that mergers in a weaker vertical wind shear environment do not produce an enhanced bow echo structure, and only produce a localized region of marginally enhanced surface winds. Additional tests demonstrate that the details of postmerger evolution vary with merger location along the line.

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.

scholarly journals Atmospheric CO2 budget over the Amazon basin: the role of convective systems

2011 ◽  
Vol 26 (4) ◽  
pp. 529-540
Author(s):  
Valdir Herrmann ◽  
Saulo Ribeiro de Freitas
Keyword(s):  
Amazon Basin ◽  
Transport Model ◽  
Conservation Equation ◽  
Atmospheric Modeling ◽  
Convective Transport ◽  
Atmospheric Co2 ◽  
Moist Convection ◽  
Convective Systems ◽  
Deep Moist Convection

This work studies the atmospheric CO2 budget in the Amazon basin, focusing on the role of shallow and deep convective systems. The vertical redistribution of CO2 is numerically simulated using an Eulerian transport model coupled to the Brazilian developments on the Regional Atmospheric Modeling System (BRAMS). The transport model includes grid-scale advection, diffusion in the PBL (Planetary Boundary Layer) and convective transport by sub-grid shallow and deep moist convection. In the simulation, the mass conservation equation is solved for six tracers, including or not the shallow and deep moist convection terms. The rectifier effect is also showed through simulation of the transport to the free troposphere of PBL air masses with low CO2 concentrations due to assimilation by vegetation during the afternoon, when both CO2 fixation and convection are at their peak. The model is applied to simulate July 2001 with a 30 km grid resolution covering the northwest part of South America. We compare the model results with airborne CO2 observations collected in the Amazon basin during the 2001 CLAIRE field campaign.

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