The Effect of Mesoscale Heterogeneity on the Genesis and Structure of Mesovortices within Quasi-Linear Convective Systems

Abstract This study examines the structure and evolution of quasi-linear convective systems (QLCSs) within complex mesoscale environments. Convective outflows and other mesoscale features appear to affect the rotational characteristics and associated dynamics of these systems. Thus, real-data numerical simulations of two QLCS events have been performed to (i) identify and characterize the various ambient mesoscale features that modify the structure and evolution of simulated QLCSs; and then to (ii) determine the nature of interaction of such features with the systems, with an emphasis on the genesis and evolution of low-level mesovortices. Significant low-level mesovortices develop in both simulated QLCSs as a consequence of mechanisms internal to the system—consistent with idealized numerical simulations of mesovortex-bearing QLCSs—and not as an effect of system interaction with external heterogeneity. However, meso-γ-scale (order of 10 km) heterogeneity in the form of a convective outflow boundary is sufficient to affect mesovortex strength, as air parcels populating the vortex region encounter enhanced convergence at the point of QLCS–boundary interaction. Moreover, meso-β-scale (order of 100 km) heterogeneity in the form of interacting air masses provides for along-line variations in the distributions of low- to midlevel vertical wind shear and convective available potential energy. The subsequent impact on updraft strength/tilt has implications on the vortex stretching experienced by leading-edge mesovortices.

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How does vertical wind shear influence entrainment in squall lines?

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0299.1 ◽

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

Weather and Forecasting ◽

10.1175/waf912.1 ◽

2006 ◽

Vol 21 (2) ◽

pp. 125-148 ◽

Cited By ~ 33

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.

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A 7-Year Climatology of Warm-Sector Heavy Rainfall over South China during the Pre-Summer Months

Atmosphere ◽

10.3390/atmos12070914 ◽

2021 ◽

Vol 12 (7) ◽

pp. 914

Author(s):

Tao Chen ◽

Da-Lin Zhang

Keyword(s):

South China ◽

Heavy Rainfall ◽

Large Scale ◽

Convective Available Potential Energy ◽

Potential Temperature ◽

Vertical Wind Shear ◽

Precipitable Water ◽

Atmospheric Parameter ◽

Low Level ◽

Warm Sector

In view of the limited predictability of heavy rainfall (HR) events and the limited understanding of the physical mechanisms governing the initiation and organization of the associated mesoscale convective systems (MCSs), a composite analysis of 58 HR events over the warm sector (i.e., far ahead of the surface cold front), referred to as WSHR events, over South China during the months of April to June 2008~2014 is performed in terms of precipitation, large-scale circulations, pre-storm environmental conditions, and MCS types. Results show that the large-scale circulations of the WSHR events can be categorized into pre-frontal, southwesterly warm and moist ascending airflow, and low-level vortex types, with higher frequency occurrences of the former two types. Their pre-storm environments are characterized by a deep moist layer with >50 mm column-integrated precipitable water, high convective available potential energy with the equivalent potential temperature of ≥340 K at 850 hPa, weak vertical wind shear below 400 hPa, and a low-level jet near 925 hPa with weak warm advection, based on atmospheric parameter composite. Three classes of the corresponding MCSs, exhibiting peak convective activity in the afternoon and the early morning hours, can be identified as linear-shaped, a leading convective line adjoined with trailing stratiform rainfall, and comma-shaped, respectively. It is found that many linear-shaped MCSs in coastal regions are triggered by local topography, enhanced by sea breezes, whereas the latter two classes of MCSs experience isentropic lifting in the southwesterly warm and moist flows. They all develop in large-scale environments with favorable quasi-geostrophic forcing, albeit weak. Conceptual models are finally developed to facilitate our understanding and prediction of the WSHR events over South China.

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Quasi-idealized numerical simulations of processes involved in orogenic convection initiation over the Sierras de Córdoba mountains

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-21-0007.1 ◽

2021 ◽

Author(s):

Christopher A. Davis

Keyword(s):

Numerical Simulations ◽

Mountain Range ◽

Moist Convection ◽

Low Level ◽

Physical Mechanisms ◽

Convergence Line ◽

Synoptic Conditions ◽

Deep Moist Convection ◽

Outflow Boundary ◽

Convection Initiation

Abstract The Sierras de Córdoba (SDC) mountain range in Argentina is a hotspot of deep moist convection initiation (CI). Radar climatology indicates that 44% of daytime CI events that occur near the SDC in spring and summer seasons and that are not associated with the passage of a cold front or an outflow boundary involve a northerly LLJ, and these events tend to preferentially occur over the southeast quadrant of the main ridge of the SDC. To investigate the physical mechanisms acting to cause CI, idealized convection-permitting numerical simulations with a horizontal grid spacing of 1 km were conducted using CM1. The sounding used for initializing the model featured a strong northerly LLJ, with synoptic conditions resembling those in a previously postulated conceptual model of CI over the region, making it a canonical case study. Differential heating of the mountain caused by solar insolation in conjunction with the low-level northerly flow sets up a convergence line on the eastern slopes of the SDC. The southern portion of this line experiences significant reduction in convective inhibition, and CI occurs over the SDC southeast quadrant. Thesimulated storm soon acquires supercellular characteristics, as observed. Additional simulations with varying LLJ strength also show CI over the southeast quadrant. A simulation without background flow generated convergence over the ridgeline, with widespread CI across the entire ridgeline. A simulation with mid- and upper-tropospheric westerlies removed indicates that CI is minimally influenced by gravity waves. We conclude that the low-level jet is sufficient to focus convection initiation over the southeast quadrant of the ridge.

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Precipitation Enhancement in Squall Lines Moving Over Mountainous Coastal Regions

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0222.1 ◽

2021 ◽

Author(s):

Fan Wu ◽

Kelly Lombardo

Keyword(s):

Convective Available Potential Energy ◽

Leading Edge ◽

Sea Breeze ◽

Cold Pool ◽

Pressure Perturbation ◽

Coastal Regions ◽

Low Level ◽

Squall Lines ◽

Sloping Terrain ◽

Precipitation Enhancement

AbstractA mechanism for precipitation enhancement in squall lines moving over mountainous coastal regions is quantified through idealized numerical simulations. Storm intensity and precipitation peak over the sloping terrain as storms descend from an elevated plateau toward the coastline and encounter the marine atmospheric boundary layer (MABL). Storms are most intense as they encounter the deepest MABLs. As the descending storm outflow collides with a moving MABL (sea breeze), surface and low-level air parcels initially accelerate upward, though their ultimate trajectory is governed by the magnitude of the negative non-hydrostatic inertial pressure perturbation behind the cold pool leading edge. For shallow MABLs, the baroclinic gradient across the gust front generates large horizontal vorticity, a low-level negative pressure perturbation, and thus a downward acceleration of air parcels following their initial ascent. A deep MABL reduces the baroclinically-generated vorticity, leading to a weaker pressure perturbation and minimal downward acceleration, allowing air to accelerate into a storm’s updraft.Once storms move away from the terrain base and over the full depth of the MABLs, storms over the deepest MABLs decay most rapidly, while those over the shallowest MABLs initially intensify. Though elevated ascent exists above all MABLs, the deepest MABLs substantially reduce the depth of the high-θe layer above the MABLs and limit instability. This relationship is insensitive to MABL temperature, even though surface-based ascent is present for the less cold MABLs, the MABL thermal deficit is smaller, and convective available potential energy (CAPE) is higher.

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Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

Monthly Weather Review ◽

10.1175/mwr-d-14-00036.1 ◽

2014 ◽

Vol 142 (8) ◽

pp. 2838-2859 ◽

Cited By ~ 10

Author(s):

Buo-Fu Chen ◽

Russell L. Elsberry ◽

Cheng-Shang Lee

Keyword(s):

Inner Core ◽

Outer Region ◽

Vertical Wind Shear ◽

Leading Edge ◽

Cold Pool ◽

Convective System ◽

Rain Area ◽

Characteristic Structure ◽

Low Level ◽

Mesoscale Convective

Abstract Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the inner-core rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting Model simulations. Quick Scatterometer (QuikSCAT) observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north–south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer, and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and midlevel northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (Δθ < −3 K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS.

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Characteristics and Formation of a Synoptic Situation Causing Sudden Turning of Mesoscale Convective Systems over South China

Atmosphere ◽

10.3390/atmos10040191 ◽

2019 ◽

Vol 10 (4) ◽

pp. 191

Author(s):

Zhaoming Liang ◽

Ying Liu ◽

Jinfang Yin

Keyword(s):

Eastern China ◽

Vertical Wind Shear ◽

Northwestern Pacific ◽

Synoptic Situation ◽

The Tibetan Plateau ◽

Northwestern Pacific Ocean ◽

Low Level ◽

Convective Systems ◽

Mesoscale Convective ◽

Turning Motion

The characteristics and formation of a synoptic situation that causes a sudden turning motion of warm-sector mesoscale convective systems (MCSs) over South China are described, based on the collection and investigation of associated cases during April–June 2011–2017 using high-resolution observational data and ERA (ECMWF Re-Analysis)-Interim data. The results show that the blocking of a marked low-level high over eastern China (eastern high) on a strengthening low-level trough over southwestern China (southwestern trough) results in significant enhancement of southerly winds ahead of the trough, which produces a strong southeastward vertical wind shear at low levels near western Guangdong province. This low-level vertical wind shear results in sudden southeastward turning motion for the warm-sector MCSs entering into Guangdong province from Guangxi province. The formation of the eastern high is mainly attributable to the strong cyclonic wind anomaly over the northwestern Pacific Ocean, which continuously brings cold air from higher latitudes to eastern China, where high synoptic-scale transient anomaly of geopotential height (SSTA-GH) forms. This cyclonic wind anomaly is induced by a low SSTA-GH, which travels from the north and south sides of the Tibetan Plateau to the northwestern Pacific Ocean and develops significantly as a result of a strong upper-level low SSTA-GH coupling with it or approaching it. On the other hand, the high SSTA-GH over eastern China blocks the eastward extension of the low SSTA-GH originating from the Tibetan Plateau. Consequently, this low SSTA-GH turns to extend or move southeastward/southward to southwestern China, leading to intensification of the southwestern trough.

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Rapid Mesoscale Environmental Changes Accompanying Genesis of an Unusual Tornado

Weather and Forecasting ◽

10.1175/waf-d-15-0105.1 ◽

2016 ◽

Vol 31 (3) ◽

pp. 763-786 ◽

Cited By ~ 4

Author(s):

Steven E. Koch ◽

Randolph Ware ◽

Hongli Jiang ◽

Yuanfu Xie

Keyword(s):

Environmental Changes ◽

Rapid Development ◽

Microwave Radiometer ◽

Convective Available Potential Energy ◽

Potential Temperature ◽

Vertical Wind Shear ◽

Low Level ◽

Short Period ◽

Stretching Deformation ◽

Upper Level Jet

Abstract This study documents a very rapid increase in convective instability, vertical wind shear, and mesoscale forcing for ascent leading to the formation of a highly unusual tornado as detected by a ground-based microwave radiometer and wind profiler, and in 1-km resolution mesoanalyses. Mesoscale forcing for the rapid development of severe convection began with the arrival of a strong upper-level jet streak with pronounced divergence in its left exit region and associated intensification of the low-level flow to the south of a pronounced warm front. The resultant increase in stretching deformation along the front occurred in association with warming immediately to its south as low-level clouds dissipated. This created a narrow ribbon of intense frontogenesis and a rapid increase in convective available potential energy (CAPE) within 75 min of tornadogenesis. The Windsor, Colorado, storm formed at the juncture of this warm frontogenesis zone and a developing dryline. Storm-relative helicity suddenly increased to large values during this pretornadic period as a midtropospheric layer of strong southeasterly winds descended to low levels. The following events also occurred simultaneously within this short period of time: a pronounced decrease in midtropospheric equivalent potential temperature θe accompanying the descending jet, an increase in low-level θe associated with the surface sensible heating, and elimination of the capping inversion and convective inhibition. The simultaneous nature of these rapid changes over such a short period of time, not fully captured in Storm Prediction Center mesoanalyses, was likely critical in generating this unusual tornadic event.

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

Monthly Weather Review ◽

10.1175/mwr-d-13-00356.1 ◽

2014 ◽

Vol 142 (12) ◽

pp. 4791-4822 ◽

Cited By ~ 8

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.

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Ambient Conditions Associated with the Maintenance and Decay of Quasi-Linear Convective Systems Crossing the Northeastern U.S. Coast

Monthly Weather Review ◽

10.1175/mwr-d-12-00050.1 ◽

2012 ◽

Vol 140 (12) ◽

pp. 3805-3819 ◽

Cited By ~ 12

Author(s):

Kelly A. Lombardo ◽

Brian A. Colle

Keyword(s):

Coastal Waters ◽

Systems Approach ◽

Atlantic Coast ◽

Vertical Wind Shear ◽

Sea Surface ◽

Vertical Shear ◽

Cold Pool ◽

Ambient Conditions ◽

Low Level ◽

Convective Systems

Abstract Quasi-linear convective systems (QLCSs) crossing the Atlantic coastline over the northeastern United States were classified into three categories based on their evolution upon encountering the coast. Composite analyses show that convective lines that decay near the Atlantic coast or slowly decay over the coastal waters are associated with 900–800-hPa frontogenesis, with greater ambient 0–3-km vertical wind shear for the slowly decaying lines. Systems that maintain their intensity over the coastal ocean are associated with 900-hPa warm air advection, but with little low-level frontogenetical forcing. Neither sea surface temperature nor ambient instability was a clear delimiter between the three evolutions. Sustaining convective lines have the strongest environmental 0–3-km shear of the three types, and this shear increases as these systems approach the coast. In contrast, the low-level shear decreases as decaying and slowly decaying convective lines move toward the Atlantic coastline. There was also a weaker mean surface cold pool for the sustaining systems than the two types of decaying QLCSs, which may favor a more long-lived system if the horizontal vorticity from this cold pool is more balanced by low-level vertical shear.

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