scholarly journals Observational Analysis of the 27 May 1997 Central Texas Tornadic Event. Part II: Tornadoes

2007 ◽  
Vol 135 (3) ◽  
pp. 727-735 ◽  
Author(s):  
Adam L. Houston ◽  
Robert B. Wilhelmson
Keyword(s):  
Boundary Layer ◽  
Observational Study ◽  
Cold Front ◽  
Convective Cell ◽  
Horizontal Shear ◽  
Low Level ◽  
Central Texas ◽  
Low Levels ◽  
Convective Cells ◽  
Gust Front

Abstract The 27 May 1997 central Texas tornadic event has been investigated in a two-part observational study. As demonstrated in Part I, the 1D environment associated with this event was unfavorable for significant (≥F2) tornadoes. Yet, the storm complex produced at least six significant tornadoes, including one rated F5 (the Jarrell, Texas, tornado). The purpose of this article is to examine the spatiotemporal interrelationships between tornadoes, preexisting boundaries, antecedent low-level mesocyclones, convective cells, and midlevel mesocyclones. It is shown that each of the six observed tornadoes that produced greater than F0 damage formed along the storm-generated gust front, not along preexisting boundaries. Half of these tornadoes formed on the distorted gust front, the portion of the storm-generated gust front whose orientation was deformed largely by the horizontal shear across the cold front. The remaining three tornadoes developed at the gust front cusp (the persistent gust front inflection located at the northeast end of the gust front distortion). Unlike the tornadoes south of the gust front cusp, these tornadoes are found to be associated with antecedent mesocyclones located in the low levels above the boundary layer. Furthermore, these mesocyclonic tornadoes are found to be larger and more destructive than the three nonmesocyclonic tornadoes. The formation of the Jarrell tornado is found to occur as a nearly stationary convective cell became collocated with a south-southwestward-moving low-level mesocyclone near the gust front cusp—a behavior that resembles the formation of nonsupercell tornadoes. It is argued that the back-building propagation/maintenance of the storm complex enabled this juxtaposition of convective cells with vorticity along the distorted gust front and may have therefore enabled tornado formation. Each of the convective cells without midlevel mesocyclones was found to remain farther from the boundaries than the mesocyclonic cells. Since the cells nearest to the boundaries were longer lived than the remaining cells, it is argued that cells near the boundaries were mesocyclonic because the boundaries yielded cells that were more likely to support temporally coherent midlevel rotation.

scholarly journals Environment and morphology of mesoscale convective systems associated with the Changma front during 9–10 July 2007

2012 ◽  
Vol 30 (8) ◽  
pp. 1235-1248 ◽  
Author(s):  
J.-H. Jeong ◽  
D.-I. Lee ◽  
C.-C. Wang ◽  
S.-M. Jang ◽  
C.-H. You ◽  
...  
Keyword(s):  
Cold Front ◽  
Low Pressure ◽  
Convective Cell ◽  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Warm Front ◽  
Synoptic Conditions ◽  
Mesoscale Convective ◽  
Convective Cells

Abstract. To understand the different environment and morphology for heavy rainfall during 9–10 July 2007, over the Korean Peninsula, mesoscale convective systems (MCSs) that accompanied the Changma front in two different regions were investigated. The sub-synoptic conditions were analysed using mesoscale analysis data (MANAL), reanalysis data, weather charts and Multi-functional Transport Satellite (MTSAT-IR) data. Dual-Doppler radar observations were used to analyse the wind fields within the precipitation systems. During both the case periods, the surface low-pressure field intensified and moved northeastward along the Changma front. A low-level warm front gradually formed with an east-west orientation, and the cold front near the low pressure was aligned from northeast to southwest. The northern convective systems (meso-α-scale) were embedded within an area of stratiform cloud north of the warm front. The development of low-level pressure resulted in horizontal and vertical wind shear due to cyclonic circulation. The wind direction was apparently different across the warm front. In addition, the southeasterly flow (below 4 km) played an important role in generating new convective cells behind the prevailing convective cell. Each isolated southern convective cell (meso-β-scale) moved along the line ahead of the cold front within the prefrontal warm sector. These convective cells developed when a strong southwesterly low-level jet (LLJ) intensified and moisture was deeply advected into the sloping frontal zone. A high equivalent potential temperature region transported warm moist air in a strong southwesterly flow, where the convectively unstable air led to updraft and downdraft with a strong reflectivity core.

The Role of Convective Outflow in the Waldo Canyon Fire

2014 ◽  
Vol 142 (9) ◽  
pp. 3061-3080 ◽  
Author(s):  
Richard H. Johnson ◽  
Russ S. Schumacher ◽  
James H. Ruppert ◽  
Daniel T. Lindsey ◽  
Julia E. Ruthford ◽  
...  
Keyword(s):  
Vertical Structure ◽  
Moist Air ◽  
Colorado Springs ◽  
San Juan Mountains ◽  
Dry Conditions ◽  
Low Levels ◽  
Upper Level ◽  
Convective Cells ◽  
Gust Front

The meteorological conditions associated with the rapid intensification and spread of the catastrophic Waldo Canyon fire on 26 June 2012 are studied. The fire caused two fatalities, destroyed 347 homes in Colorado Springs, and resulted in insurance losses of nearly $0.5 billion (U.S. dollars), making it the most economically destructive fire in Colorado’s history. While the fire was first discovered on 23 June, the paper focuses on 26 June, when it grew explosively and rapidly advanced eastward into a heavily populated area on the west side of Colorado Springs. Near-record hot and dry conditions prevailed over the Intermountain West prior to the fire, along with a persistent upper-level ridge. On 26 June, a narrow tongue of moist air aloft originating over the Southwest shifted from Utah into Colorado. Dry conditions at low levels and moisture aloft set the stage for strong microburst-producing thunderstorms to develop over Colorado. Convective cells first formed at midday over the San Juan Mountains, later consolidating into a thunderstorm complex that produced an organized convective outflow with strong, gusty winds at the surface. The leading gust front associated with the outflow moved past the Waldo Canyon fire at the hottest time of the day with recorded wind gusts up to 26 m s−1. The rapid eastward advance of the fire, as well as an onset of pyrocumulonimbus and lightning activity, was timed with the passage of the gust front. A numerical simulation, initiated one day earlier, produced mesoscale features closely resembling those observed, including the gust front passage at the fire and the vertical structure of the convective outflow.

Convective-Scale Variations in the Inner-Core Rainbands of a Tropical Cyclone

2013 ◽  
Vol 70 (2) ◽  
pp. 504-523 ◽  
Author(s):  
Anthony C. Didlake ◽  
Robert A. Houze
Keyword(s):  
Inner Core ◽  
Momentum Conservation ◽  
Vertical Shear ◽  
Lower Altitude ◽  
Low Level ◽  
Vertical Advection ◽  
Secondary Eyewall Formation ◽  
Convective Scale ◽  
Low Levels ◽  
Convective Cells

Abstract Airborne Doppler radar documented a variety of convective-scale structures within the inner-core rainbands of Hurricane Rita (2005). As predicted by past studies, wind shear determined azimuthal variations in the convection. All convective-scale circulations had radial inflow at low levels, upward motion, and outflow in the midtroposphere. Convective cells at smaller radii contained a low-level tangential jet determined largely by tangential acceleration due to angular momentum conservation (uυ/r term), while cells at larger radii contained a low-level and/or midlevel jet determined jointly by the uυ/r and vertical advection terms. The outflow was at a higher (lower) altitude for the outer (inner) cells. Radial variations in the convective cells are attributable to differences in buoyancy and vertical shear of the radial wind (∂u/∂z). More buoyant updrafts at larger radii enhance vertical advection of υ, creating local tangential jets at midlevels. At smaller radii the stronger low-level radial inflow contributes to a greater ∂u/∂z, confining convectively generated jets to low levels. The low-level tangential jet and convectively generated pressure gradients produce outward-pointing supergradient acceleration that decelerates the boundary layer inflow. Consequently, this supergradient flow will enhance convergence and convection at the radius of inner rainband cells, increasing the likelihood of secondary eyewall formation. It is hypothesized that a critical zone for secondary eyewall formation exists where sufficiently high ∂u/∂z consistently constrains the altitudes of convectively generated supergradient flow so that convection in this radial zone leads to a newly developed eyewall. Once an incipient secondary eyewall forms at a certain radius, subsidence occurring along its inner edge separates it from the primary eyewall.

scholarly journals The interaction between cold fronts and convection in the mid-latitudes

2021 ◽  
Author(s):  
George Pacey ◽  
Stephan Pfahl ◽  
Lisa Schielicke
Keyword(s):  
Diurnal Cycle ◽  
Cell Tracking ◽  
Cold Front ◽  
Heavy Rain ◽  
Convective Cell ◽  
Detection Methods ◽  
Cold Fronts ◽  
Forcing Mechanisms ◽  
Convective Cells ◽  
Frontal Boundary

<p>Cold fronts provide an environment favourable for convective initiation in the mid-latitudes. Some studies also show the presence of a cold front can increase the chance of certain convective hazards, such as hail and heavy rain. Convection initiates in three locations in respect to cold fronts: <em>ahead</em> of the cold front in the warm sector of the cyclone, directly <em>at</em> the cold frontal boundary and also <em>behind</em> the cold front. Previous literature has typically focused on each initiation location independently, thus a comprehensive study investigating the link between cold fronts and convection is currently lacking from literature. This study seeks to better understand the climatology, scale interactions and forcing mechanisms of convection at each initiation location relative to the front (i.e., behind, at, ahead).</p><p>Automatic front detection methods are applied to reanalysis data and a convective cell-tracking dataset from the German Weather Service is used to build a climatology of cold fronts and convection between April–September. Convective cells are found to initiate most commonly 200–300km ahead of the cold front during late afternoon. Cells behind the front primarily initiate in north-western Germany and exhibit a strong diurnal cycle. On the contrary, cells at and ahead of the front initiate most frequently in southern Germany and exhibit a less prominent diurnal cycle, especially for cells at the frontal boundary. Lightning probability decreases with closing proximity to the cold front and the average number of cell initiations per day is significantly higher on days with cold fronts opposed to days without. The next stages of research will investigate the relative importance of various forcing mechanisms on the development of convective cells at different cell-front positions.</p>

scholarly journals A Comparison of Inner and Outer Spiral Rainbands in a Numerically Simulated Tropical Cyclone

2012 ◽  
Vol 140 (9) ◽  
pp. 2782-2805 ◽  
Author(s):  
Qingqing Li ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Horizontal Wind ◽  
Cold Pool ◽  
Wind Maximum ◽  
Low Level ◽  
Convective Systems ◽  
Tangential Wind ◽  
Spiral Rainbands ◽  
Convective Cells

Abstract The simulated inner and outer spiral rainbands in a tropical cyclone are compared in this study. The inner rainbands are generally active immediately outside the eyewall in the rapid filamentation zone, while the outer rainbands are active in regions outside about 3 times the radius of maximum wind. The inner rainbands are characterized by the convectively coupled vortex Rossby waves. The movement of the outer rainbands follows the low-level vector winds associated with the azimuthally averaged low-level flow and the radially outward cross-band flow caused by the downdraft-induced cold pool in the boundary layer. Convective cells in outer rainbands are typical of convective systems and move cyclonically and radially outward (inward) at large (small) radii. Net upward vertical mass transports (VMTs) appear throughout the depth of the troposphere in the whole inner-rainband region, while net downward VMTs are found below 4-km height in the outer-rainband region. In the whole inner-rainband region, only a very shallow layer with net horizontal convergence appears below 2-km height, while a deep layer with net convergence is found below 7.5-km height with net divergence aloft in the outer-rainband region. The inner rainband shows two tangential wind maxima, respectively, located near the top of the inflow boundary layer and immediately below the upper-tropospheric outflow layer. A secondary horizontal wind maximum occurs at about 4-km height on the inner edge of the outer rainband. Distinct features of the upwind, middle, and downwind sectors of the outer rainband are also discussed.

Convective Contribution to the Genesis of Hurricane Ophelia (2005)

2009 ◽  
Vol 137 (9) ◽  
pp. 2778-2800 ◽  
Author(s):  
Robert A. Houze ◽  
Wen-Chau Lee ◽  
Michael M. Bell
Keyword(s):  
Latent Heat ◽  
Doppler Radar ◽  
Potential Temperature ◽  
Convective Cell ◽  
Cyclonic Vorticity ◽  
Stratiform Region ◽  
Unstable Layer ◽  
Tropical Depressions ◽  
Convective Cells ◽  
Gust Front

Abstract The convection occurring in the tropical depression that became Hurricane Ophelia (2005) was investigated just prior to tropical storm formation. Doppler radar showed a deep, wide, intense convective cell of a type that has been previously thought to occur in intensifying tropical depressions but has not heretofore been documented in detail. The updraft of the cell was 10 km wide, 17 km deep, had updrafts of 10–20 m s−1 throughout its mid- to upper levels, and contained a cyclonic vorticity maximum. The massive convective updraft was maintained by strong positive buoyancy, which was maximum at about the 10-km level, probably aided by latent heat of freezing. Evaporative cooling and precipitation drag occurred in the rain shower of the cell but were insufficient to produce a strong downdraft or gust front outflow to force the updraft. The convective updraft was fed by a layer of strong inflow that was several kilometers deep. Wind-induced turbulence, just above the ocean surface, enriched the equivalent potential temperature of the boundary layer of the inflow air, thus creating an unstable layer with little convective inhibition. This air was raised to its level of free convection when it encountered the denser air in the rainy core of the convection. The updraft motion and latent heat release in the intense cell generated potential vorticity throughout the low to midlevels, and contained a cyclonic vortex at the midlevels. Vorticity generated throughout the depth of the low to midtroposphere within convective updraft cells was subsequently incorporated into a stratiform region attached to the region of active convective cells. The vorticity perturbations at the low to midlevels in convective cells and their attached stratiform regions were available to be axisymmetrized into the larger-scale intensifying depression vortex.

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

2006 ◽  
Vol 21 (2) ◽  
pp. 125-148 ◽  
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.

Frontogenesis and frontal arrest of a dense filament in the oceanic surface boundary layer

2017 ◽  
Vol 837 ◽  
pp. 341-380 ◽  
Author(s):  
Peter P. Sullivan ◽  
James C. McWilliams
Keyword(s):  
Boundary Layer ◽  
Turbulent Mixing ◽  
Shear Instability ◽  
Upper Ocean ◽  
Surface Boundary ◽  
Horizontal Shear ◽  
Boundary Layer Turbulence ◽  
Filament Axis ◽  
Small Width ◽  
The Cross

The evolution of upper ocean currents involves a set of complex, poorly understood interactions between submesoscale turbulence (e.g. density fronts and filaments and coherent vortices) and smaller-scale boundary-layer turbulence. Here we simulate the lifecycle of a cold (dense) filament undergoing frontogenesis in the presence of turbulence generated by surface stress and/or buoyancy loss. This phenomenon is examined in large-eddy simulations with resolved turbulent motions in large horizontal domains using${\sim}10^{10}$grid points. Steady winds are oriented in directions perpendicular or parallel to the filament axis. Due to turbulent vertical momentum mixing, cold filaments generate a potent two-celled secondary circulation in the cross-filament plane that is frontogenetic, sharpens the cross-filament buoyancy and horizontal velocity gradients and blocks Ekman buoyancy flux across the cold filament core towards the warm filament edge. Within less than a day, the frontogenesis is arrested at a small width,${\approx}100~\text{m}$, primarily by an enhancement of the turbulence through a small submesoscale, horizontal shear instability of the sharpened filament, followed by a subsequent slow decay of the filament by further turbulent mixing. The boundary-layer turbulence is inhomogeneous and non-stationary in relation to the evolving submesoscale currents and density stratification. The occurrence of frontogenesis and arrest are qualitatively similar with varying stress direction or with convective cooling, but the detailed evolution and flow structure differ among the cases. Thus submesoscale filament frontogenesis caused by boundary-layer turbulence, frontal arrest by frontal instability and frontal decay by forward energy cascade, and turbulent mixing are generic processes in the upper ocean.

scholarly journals A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans

2015 ◽  
Vol 28 (17) ◽  
pp. 6743-6762 ◽  
Author(s):  
Catherine M. Naud ◽  
Derek J. Posselt ◽  
Susan C. van den Heever
Keyword(s):  
Large Scale ◽  
Cold Front ◽  
Conveyor Belt ◽  
Low Level ◽  
Scale Parameters ◽  
Convective Clouds ◽  
Cold Fronts ◽  
Surface Front ◽  
High Level ◽  
The Impact

Abstract The distribution of cloud and precipitation properties across oceanic extratropical cyclone cold fronts is examined using four years of combined CloudSat radar and CALIPSO lidar retrievals. The global annual mean cloud and precipitation distributions show that low-level clouds are ubiquitous in the postfrontal zone while higher-level cloud frequency and precipitation peak in the warm sector along the surface front. Increases in temperature and moisture within the cold front region are associated with larger high-level but lower mid-/low-level cloud frequencies and precipitation decreases in the cold sector. This behavior seems to be related to a shift from stratiform to convective clouds and precipitation. Stronger ascent in the warm conveyor belt tends to enhance cloudiness and precipitation across the cold front. A strong temperature contrast between the warm and cold sectors also encourages greater post-cold-frontal cloud occurrence. While the seasonal contrasts in environmental temperature, moisture, and ascent strength are enough to explain most of the variations in cloud and precipitation across cold fronts in both hemispheres, they do not fully explain the differences between Northern and Southern Hemisphere cold fronts. These differences are better explained when the impact of the contrast in temperature across the cold front is also considered. In addition, these large-scale parameters do not explain the relatively large frequency in springtime postfrontal precipitation.

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