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.

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.

The Formation of Multiple Squall Lines and the Impacts of WSR-88D Radial Winds in a WRF Simulation

2010 ◽  
Vol 25 (1) ◽  
pp. 242-262 ◽  
Author(s):  
Haldun Karan ◽  
Patrick J. Fitzpatrick ◽  
Christopher M. Hill ◽  
Yongzuo Li ◽  
Qingnong Xiao ◽  
...  
Keyword(s):  
Cold Front ◽  
Gulf Coast ◽  
Inversion Layer ◽  
Wrf Model ◽  
High Water ◽  
Squall Line ◽  
Low Level ◽  
Sequence Of Events ◽  
Squall Lines ◽  
The Right

Abstract A detailed observational and Weather Research and Forecasting (WRF) model analysis utilizing Weather Surveillance Radar-1988 Doppler (WSR-88D), surface, and upper-air observations, as well as Geostationary Operational Environmental Satellite (GOES) images, shows a chain of events that leads to the formation of two prefrontal squall lines along the western Gulf coast on 29–30 April 2005. An approaching surface cold front (CF) generated an atmospheric bore that propagated along an inversion layer and excited high-frequency, low-level tropospheric gravity waves, initiating a squall line 60 km east of the cold front. This sequence of events manifested itself as low-level convergence ahead of the CF, which was detected by nearby WSR-88D radars. Two WRF model experiments were conducted in which one assimilated conventional observations (CTRL), and another included radar radial winds from nine WSR-88D locations (denoted as RADAR). Better representation of the low-level kinematics in RADAR yielded a distinct convergence line associated with the primary squall line. The RADAR experiment, as well as observations (such as an 0600 UTC Slidell, Louisiana, sounding), show that the secondary squall line formed ahead of the primary squall line due to high water vapor and warm temperature advection from the Gulf of Mexico that, when combined with a deep dry layer above the atmospheric boundary layer (ABL), destabilized the atmosphere. Concurrently, a lower-tropospheric trough, propagating faster than the surface front, enhanced lifting in the region and instigated the formation of new convection. RADAR forecasted the secondary convection not only in the right place but also at about the right time, while the CTRL experiment completely missed this secondary convection.

scholarly journals On the Dependence of Squall-Line Characteristics on Surface Conditions

2017 ◽  
Vol 74 (7) ◽  
pp. 2211-2228 ◽  
Author(s):  
Karsten Peters ◽  
Cathy Hohenegger
Keyword(s):  
Surface Temperature ◽  
Onset Time ◽  
Propagation Speed ◽  
Surface Fluxes ◽  
Squall Line ◽  
Cold Pool ◽  
Marginal Effect ◽  
Surface Conditions ◽  
Convective Systems ◽  
Squall Lines

Abstract The influence of surface conditions in the form of changing surface temperatures on fully developed mesoscale convective systems (MCSs) is investigated using a cloud-system-resolving setup of the Icosahedral Nonhydrostatic (ICON) model (1-km grid spacing). The simulated MCSs take the form of squall lines with trailing stratiform precipitation. After the squall lines have reached a quasi-steady state, secondary convection is triggered ahead of the squall line, resulting in an increase of squall-line propagation speed, also known as discrete propagation. The higher propagation speed is then maintained for the remainder of the simulations because secondary convection ahead of the squall line acts to reduce the environmental wind shear over the depth of the squall line’s cold pool. The surface conditions have only a marginal effect on the squall lines themselves. This is so because the surface fluxes cannot significantly affect the cold pool, which is continuously replenished by midtropospheric air. The midtroposphere remains similar given the use of identical initial profiles. The only effect of the surface fluxes consists in an earlier acceleration of the squall line due to earlier initiation of secondary convection with higher surface temperature. Finally, a conceptual model to estimate the change in surface temperature needed to achieve a change in onset time of prefrontal secondary convection and the associated discrete propagation events given the environmental conditions is presented.

scholarly journals Discrete Propagation in Numerically Simulated Nocturnal Squall Lines

2006 ◽  
Vol 134 (12) ◽  
pp. 3735-3752 ◽  
Author(s):  
Robert G. Fovell ◽  
Gretchen L. Mullendore ◽  
Seung-Hee Kim
Keyword(s):  
Gravity Waves ◽  
High Frequency ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Propagation Speed ◽  
Surface Fluxes ◽  
Squall Line ◽  
Radiative Processes ◽  
Squall Lines

Abstract Simulations of a typical midlatitude squall line were used to investigate a mechanism for discrete propagation, defined as convective initiation ahead of an existing squall line leading to a faster propagation speed for the storm complex. Radar imagery often shows new cells appearing in advance of squall lines, suggesting a causal relationship and prompting the search for an “action-at-a-distance” mechanism to explain the phenomenon. In the simulations presented, the identified mechanism involves gravity waves of both low and high frequency generated in response to the latent heating, which subsequently propagate out ahead of the storm. The net result of the low-frequency response, combined with surface fluxes and radiative processes, was a cooler and more moist lower troposphere, establishing a shallow cloud deck extending ahead of the storm. High-frequency gravity waves, excited in response to fluctuations in convective activity in the main storm, were subsequently ducted by the storm’s own upper-tropospheric forward anvil outflow. These waves helped positively buoyant cumulus clouds to occasionally form in the deck. A fraction of these clouds persisted long enough to merge with the main line, invigorating the parent storm. Discrete propagation occurred when clouds developed into deep convection prior to merger, weakening the parent storm. The ducting conditions, as diagnosed with the Scorer parameter, are shown to be sensitive to vertical wind shear and radiation, but not to the microphysical parameterization or simulation geometry.

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.

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.

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 Analysis of Dual-Polarimetric Radar Variables and Quantitative Precipitation Estimators for Landfall Typhoons and Squall Lines Based on Disdrometer Data in Southern China

Atmosphere ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 30 ◽  
Author(s):  
Yonghua Zhang ◽  
Liping Liu ◽  
Shuoben Bi ◽  
Zhifang Wu ◽  
Ping Shen ◽  
...  
Keyword(s):  
Drop Size ◽  
Southern China ◽  
Average Diameter ◽  
Squall Line ◽  
Polarimetric Radar ◽  
Quantitative Precipitation Estimation ◽  
Squall Lines ◽  
Fitting Method ◽  
Precipitation Estimation ◽  
Differential Reflectivity

Typhoon rainstorms often cause disasters in southern China. Quantitative precipitation estimation (QPE) with the use of polarimetric radar can improve the accuracy of precipitation estimation and enhance typhoon defense ability. On the basis of the observed drop size distribution (DSD) of raindrops, a comparison is conducted among the DSD parameters and the polarimetric radar observation retrieved from DSD in five typhoon and three squall line events that occurred in southern China from 2016 to 2017. A new piecewise fitting method (PFM) is used to develop the QPE estimators for landfall typhoons and squall lines. The performance of QPE is evaluated by two fitting methods for two precipitation types using DSD data collected. Findings indicate that the number concentration of raindrops in typhoon precipitation is large and the average diameter is small, while the raindrops in squall line rain have opposite characteristics. The differential reflectivity (ZDR) and specific differential phase (KDP) in these two precipitation types increase slowly with the reflectivity factor (ZH), whereas the two precipitation types have different ZDR and KDP in the same ZH. Thus, it is critical to fit the rainfall estimator for different precipitation types. Enhanced estimation can be obtained using the estimators for specific precipitation types, whether the estimators are derived from the conventional fitting method (CFM) or PFM, and the estimators fitted using the PFM can produce better results. The estimators for the developed polarimetric radar can be used in operational QPE and quantitative precipitation foresting, and they can improve disaster defense against typhoons and heavy rains.

Centrifugal Wave Front Propagation Speed for Localizing the Origin of Ventricular Arrhythmias

2018 ◽  
Vol 4 (3) ◽  
pp. 355-363 ◽  
Author(s):  
Masaharu Masuda ◽  
Mitsutoshi Asai ◽  
Osamu Iida ◽  
Shin Okamoto ◽  
Takayuki Ishihara ◽  
...  
Keyword(s):  
Wave Front ◽  
Ventricular Arrhythmias ◽  
Propagation Speed ◽  
Front Propagation ◽  
Wave Front Propagation

A Synoptic Study on the Formation of Squall Lines in the North Central United States

1955 ◽  
Vol 36 (8) ◽  
pp. 390-396 ◽  
Author(s):  
John M. Porter ◽  
L. L. Means ◽  
J. E. Hovde ◽  
W. B. Chappell
Keyword(s):  
United States ◽  
Squall Line ◽  
Line Formation ◽  
Check List ◽  
North Central ◽  
The North ◽  
Squall Lines ◽  
Synoptic Conditions ◽  
Central United States

The aim of this study was to discover synoptic conditions which favored squall-line formation in the north-central United States. The squall lines over approximately a three-year period were classified into three types and studied using charts from the surface to 200 mb. Although the various parameters were not reduced to a strictly objective technique, a check list of significant parameters was prepared for the use and guidance of forecasters. It is believed that this check sheet will help the forecaster in forecasting squall-line development in the north-central United States.

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