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Abstract
The kinematics and thermodynamics of wake lows have been extensively examined in the literature. However, there has been relatively little focus on the widespread, sometimes very strong winds associated with wake lows. Some wake lows are, essentially, severe local storms, producing widespread and sometimes intense damage, similar to that of a derecho, but they occur in environments supporting elevated convection, a phenomenon not often perceived as a significant wind damage threat. Three significant wake lows that affected Alabama and/or Georgia, producing widespread (25 000–50 000 km2) wind damage, and local wind gusts near 25 m s−1, are reviewed in detail. The environments and morphology of the wake lows are addressed, using radar, surface, and upper-air data.
Abstract
The kinematics and thermodynamics of wake lows have been extensively examined in the literature. However, there has been relatively little focus on the widespread, sometimes very strong winds associated with wake lows. Some wake lows are, essentially, severe local storms, producing widespread and sometimes intense damage, similar to that of a derecho, but they occur in environments supporting elevated convection, a phenomenon not often perceived as a significant wind damage threat. Three significant wake lows that affected Alabama and/or Georgia, producing widespread (25 000–50 000 km2) wind damage, and local wind gusts near 25 m s−1, are reviewed in detail. The environments and morphology of the wake lows are addressed, using radar, surface, and upper-air data.
Abstract
The “impedance relation” between the wind perturbation within an ageostrophic atmospheric disturbance and its pressure perturbation and intrinsic propagation speed has been in use for decades. The correlation between wind and pressure perturbation was established through this relation. However, a simple Lagrangian model of an air parcel traversing an idealized sinusoidal wave in the pressure field indicates that the impedance relation produces significant errors. Examination of the nonlinearized horizontal momentum equation with a sinusoidal disturbance in pressure reveals an additional nonlinear term in the impedance relation, not previously included.
In this paper, the impedance relation is rederived, with the solution being the original equation with the addition of the nonlinear term. The new equation is then evaluated against the Lagrangian model of an air parcel traversing an idealized gravity wave, as well as three observed cases. It is shown that the new impedance relation is indeed more accurate in predicting wind perturbations in disturbances based on pressure perturbations and intrinsic speed than the accepted equation. Implications for determination of the intrinsic phase speed of a disturbance when pressure and wind perturbations are known (another widely used application of the impedance relation) are also discussed.
Abstract
The “impedance relation” between the wind perturbation within an ageostrophic atmospheric disturbance and its pressure perturbation and intrinsic propagation speed has been in use for decades. The correlation between wind and pressure perturbation was established through this relation. However, a simple Lagrangian model of an air parcel traversing an idealized sinusoidal wave in the pressure field indicates that the impedance relation produces significant errors. Examination of the nonlinearized horizontal momentum equation with a sinusoidal disturbance in pressure reveals an additional nonlinear term in the impedance relation, not previously included.
In this paper, the impedance relation is rederived, with the solution being the original equation with the addition of the nonlinear term. The new equation is then evaluated against the Lagrangian model of an air parcel traversing an idealized gravity wave, as well as three observed cases. It is shown that the new impedance relation is indeed more accurate in predicting wind perturbations in disturbances based on pressure perturbations and intrinsic speed than the accepted equation. Implications for determination of the intrinsic phase speed of a disturbance when pressure and wind perturbations are known (another widely used application of the impedance relation) are also discussed.
Abstract
Detailed observations of boundary layer structure were acquired on 14 September 2001, prior to and during the landfall of Tropical Storm Gabrielle. The Mobile Integrated Profiling System (MIPS) and the Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) were collocated at the western Florida coastline near Venice, very close to the wind center at landfall. Prior to landfall, the boundary layer was rendered weakly stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall. The cool air mass was expansive, with an area within the 23°C surface isotherm of about 50 000 km2. East-northeasterly surface flow transported this cool air off the west coast of Florida, toward the convergent warm core of the Gabrielle, and promoted the development of shallow warm and cold fronts that were prominent during the landfall phase.
Airflow properties of the boundary layer around the coastal zone are examined using the MIPS and SMART-R data. Wind profiles exhibited considerable temporal variability throughout the period of observations. The stable offshore flow within stratiform precipitation exhibited a modest jet that descended from about 600 to 300 m within the 20-km zone centered on the coastline. In contrast, the onshore flow on the western side of the wind center produced a more turbulent boundary layer that exhibited a well-defined top varying between 400 and 1000 m MSL. The horizontal variability of each boundary layer is examined using high-resolution Doppler radar scans at locations up to 15 km on either side of the coastline, along the mean flow direction of the boundary layer. These analyses reveal that transitions in boundary layer structure for both the stable and unstable regimes were most substantial within 5 km of the coastline.
Abstract
Detailed observations of boundary layer structure were acquired on 14 September 2001, prior to and during the landfall of Tropical Storm Gabrielle. The Mobile Integrated Profiling System (MIPS) and the Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) were collocated at the western Florida coastline near Venice, very close to the wind center at landfall. Prior to landfall, the boundary layer was rendered weakly stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall. The cool air mass was expansive, with an area within the 23°C surface isotherm of about 50 000 km2. East-northeasterly surface flow transported this cool air off the west coast of Florida, toward the convergent warm core of the Gabrielle, and promoted the development of shallow warm and cold fronts that were prominent during the landfall phase.
Airflow properties of the boundary layer around the coastal zone are examined using the MIPS and SMART-R data. Wind profiles exhibited considerable temporal variability throughout the period of observations. The stable offshore flow within stratiform precipitation exhibited a modest jet that descended from about 600 to 300 m within the 20-km zone centered on the coastline. In contrast, the onshore flow on the western side of the wind center produced a more turbulent boundary layer that exhibited a well-defined top varying between 400 and 1000 m MSL. The horizontal variability of each boundary layer is examined using high-resolution Doppler radar scans at locations up to 15 km on either side of the coastline, along the mean flow direction of the boundary layer. These analyses reveal that transitions in boundary layer structure for both the stable and unstable regimes were most substantial within 5 km of the coastline.
Abstract
The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.
Abstract
The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.
Abstract
This paper utilizes experimental data from a multiple Doppler radar and surface mesoscale network to describe the evolution and structure of a small, isolated, mesoscale convective system over the South Park region of central Colorado. This system evolved from a cluster of convective clouds which eventually transformed to a mature system possessing both stratiform and convective components. The structure of individual precipitating convective clouds comprising the mature system depended on their location (upshear or downshear) relative to the system. Unsteady upshear convective components formed discretely and propagated upshear. In contrast, downshear convective components occupied a greater area, exhibited more steadiness, and propagated downshear.
Doppler analyses indicate that mesoscale updrafts within anvils flanking the convective cores existed relatively early, about 1.5 h after first cloud formation. Mesoscale downdrafts did not appear until ∼3 h after precipitation initiation. The appearance of a mesoscale downdraft was temporally correlated with intensification of the upshear convective region. The analyses suggest a close dependence between upshear convection and the stratiform region in this case. Upshear convection supplied condensate to the stratiform region, while the stratiform region produced mesoscale downdrafts whose outflow boundary helped maintain the upshear convection.
Abstract
This paper utilizes experimental data from a multiple Doppler radar and surface mesoscale network to describe the evolution and structure of a small, isolated, mesoscale convective system over the South Park region of central Colorado. This system evolved from a cluster of convective clouds which eventually transformed to a mature system possessing both stratiform and convective components. The structure of individual precipitating convective clouds comprising the mature system depended on their location (upshear or downshear) relative to the system. Unsteady upshear convective components formed discretely and propagated upshear. In contrast, downshear convective components occupied a greater area, exhibited more steadiness, and propagated downshear.
Doppler analyses indicate that mesoscale updrafts within anvils flanking the convective cores existed relatively early, about 1.5 h after first cloud formation. Mesoscale downdrafts did not appear until ∼3 h after precipitation initiation. The appearance of a mesoscale downdraft was temporally correlated with intensification of the upshear convective region. The analyses suggest a close dependence between upshear convection and the stratiform region in this case. Upshear convection supplied condensate to the stratiform region, while the stratiform region produced mesoscale downdrafts whose outflow boundary helped maintain the upshear convection.
Abstract
Previous work has shown that vorticity mixing in the tropical cyclone (TC) inner core can promote mesovortex (MV) formation and impact storm intensity. Observations of MVs have largely been serendipitous but are necessary to improve understanding of these features and their role in TC dynamics. This study presents nearly 10 h of ground-based dual-Doppler analysis of MVs in the eyewall of Hurricane Ike (2008) near and during landfall. Derived 3D winds, vertical vorticity, horizontal divergence, and perturbation pressures are analyzed. Results indicate persistent kinematic field arrangements and evolving vertical structures. Perturbation pressure retrievals suggest local pressure minima associated with the MVs. Preferential updraft locations appear to transition cyclonically about the local vorticity maximum as the MVs progress around the eye. Based on published observational datasets, the dual-Doppler updraft magnitudes in Ike’s MVs are within the top 5%–10% of TC vertical velocities. The MVs are marked by peak vorticity in the lowest 2 km and contain vertically coherent vorticity structures extending to 8 km AGL. After prolonged land interaction, the MV structures deteriorate. First, the vertical extent of localized vorticity diminishes, followed by a deterioration in the prelandfall characteristic kinematic arrangements. This supports the notion that the replenishment of a high vorticity annulus contributes to MV production and maintenance, and when the elevated vorticity aloft is not maintained, MV kinematic patterns become less consistent. It is unclear whether the decay of the vertically coherent vorticity structures occurs in response to land interaction, TC inner core processes, or some combination of both.
Abstract
Previous work has shown that vorticity mixing in the tropical cyclone (TC) inner core can promote mesovortex (MV) formation and impact storm intensity. Observations of MVs have largely been serendipitous but are necessary to improve understanding of these features and their role in TC dynamics. This study presents nearly 10 h of ground-based dual-Doppler analysis of MVs in the eyewall of Hurricane Ike (2008) near and during landfall. Derived 3D winds, vertical vorticity, horizontal divergence, and perturbation pressures are analyzed. Results indicate persistent kinematic field arrangements and evolving vertical structures. Perturbation pressure retrievals suggest local pressure minima associated with the MVs. Preferential updraft locations appear to transition cyclonically about the local vorticity maximum as the MVs progress around the eye. Based on published observational datasets, the dual-Doppler updraft magnitudes in Ike’s MVs are within the top 5%–10% of TC vertical velocities. The MVs are marked by peak vorticity in the lowest 2 km and contain vertically coherent vorticity structures extending to 8 km AGL. After prolonged land interaction, the MV structures deteriorate. First, the vertical extent of localized vorticity diminishes, followed by a deterioration in the prelandfall characteristic kinematic arrangements. This supports the notion that the replenishment of a high vorticity annulus contributes to MV production and maintenance, and when the elevated vorticity aloft is not maintained, MV kinematic patterns become less consistent. It is unclear whether the decay of the vertically coherent vorticity structures occurs in response to land interaction, TC inner core processes, or some combination of both.
Abstract
Apparent interactions between ducted gravity waves and preexisting mesocyclones are investigated. Preliminary analyses of Weather Surveillance Radar-1988 Doppler (WSR-88D) observations from several cases reveal that the intersection of fine lines, whose propagation speed is consistent with that of gravity waves, and existing mesocyclones leads to an increase in the rotational velocity of the mesocyclone. Utilizing simplified ducted wave kinematics and the vorticity equation, changes in vorticity associated with convergence–divergence and perturbation wind shear within the gravity wave are examined. Convergence ahead of wave ridges may be significant, causing mesocyclone intensification through vorticity stretching. It will also be shown that a wave may significantly change the vertical wind shear and streamwise vorticity in storm inflow. A simple one-dimensional model is presented, which shows that vorticity decreases temporarily ahead of the wave ridge, then increases rapidly behind the ridge as positive tilting and stretching act together. The mesocyclone vorticity reaches a peak just ahead of the wave ridge, then begins to rapidly decrease behind the ridge. Model results compared very well to actual measurements in a sample case in which a mesocyclone interacted with two gravity waves of different amplitudes.
Abstract
Apparent interactions between ducted gravity waves and preexisting mesocyclones are investigated. Preliminary analyses of Weather Surveillance Radar-1988 Doppler (WSR-88D) observations from several cases reveal that the intersection of fine lines, whose propagation speed is consistent with that of gravity waves, and existing mesocyclones leads to an increase in the rotational velocity of the mesocyclone. Utilizing simplified ducted wave kinematics and the vorticity equation, changes in vorticity associated with convergence–divergence and perturbation wind shear within the gravity wave are examined. Convergence ahead of wave ridges may be significant, causing mesocyclone intensification through vorticity stretching. It will also be shown that a wave may significantly change the vertical wind shear and streamwise vorticity in storm inflow. A simple one-dimensional model is presented, which shows that vorticity decreases temporarily ahead of the wave ridge, then increases rapidly behind the ridge as positive tilting and stretching act together. The mesocyclone vorticity reaches a peak just ahead of the wave ridge, then begins to rapidly decrease behind the ridge. Model results compared very well to actual measurements in a sample case in which a mesocyclone interacted with two gravity waves of different amplitudes.
Abstract
The contrasting behavior of cloud-to-ground (CG) lightning associated with three adjacent supercell thunderstorms observed on 18 May 1995 is examined. Thunderstorm characteristics and anvil interactions are related to north–south variations in CG lightning properties. While tornadic activity was not consistently related to variations in CG properties, radar reflectivity factor area greater than 65 dBZ was generally inversely related to CG frequency. It is hypothesized that suppression of CG activity was produced by reduction of large number concentrations of precipitation-sized particles (i.e., presence of large hail) in the particle interaction mixed phase region. It is further hypothesized that seeding from upstream storm anvil ice was associated with nearly coincident enhancement of CG activity in downstream storms. Likewise, it is hypothesized that the reduction in 65 dBZ echo area (inferred suppression of hail) is related to this inferred anvil seeding process.
Abstract
The contrasting behavior of cloud-to-ground (CG) lightning associated with three adjacent supercell thunderstorms observed on 18 May 1995 is examined. Thunderstorm characteristics and anvil interactions are related to north–south variations in CG lightning properties. While tornadic activity was not consistently related to variations in CG properties, radar reflectivity factor area greater than 65 dBZ was generally inversely related to CG frequency. It is hypothesized that suppression of CG activity was produced by reduction of large number concentrations of precipitation-sized particles (i.e., presence of large hail) in the particle interaction mixed phase region. It is further hypothesized that seeding from upstream storm anvil ice was associated with nearly coincident enhancement of CG activity in downstream storms. Likewise, it is hypothesized that the reduction in 65 dBZ echo area (inferred suppression of hail) is related to this inferred anvil seeding process.
Abstract
An analysis of an intense, quasi-steady thunderstorm which developed over mountainous terrain is presented. This storm, extensively analyzed using multiple Doppler radar and surface mesonet data, formed within an environment having strong low-level wind shear. The evolution and characteristics of the mesoscale systems prior to storm formation are presented in Part I (Cotton et al., 1982). Such an environment was responsible for several unique storm features, including a quasi-steady primary updraft circulation and movement 50° to the left of the cloud layer (2–8 km AGL) environmental winds.
Several interactions were observed or inferred near and within the storm. Vertical transport of northerly low-level momentum within the updraft imparted a significant blocking on mid-level flow having southerly momentum. Such a blocking affected the movement and characteristics of adjacent, less organized storms. Additional storm-environment interactions produced an organized recirculation of precipitation particles from the mid-level updraft to the low-level updraft.
It is concluded that the steadiness of the storm depended on two factors: 1) the introduction of low-level flow which was directed opposite to mid-level flow, 2) formation of persistent downdrafts of sufficient magnitude to sustain an active gust front.
Abstract
An analysis of an intense, quasi-steady thunderstorm which developed over mountainous terrain is presented. This storm, extensively analyzed using multiple Doppler radar and surface mesonet data, formed within an environment having strong low-level wind shear. The evolution and characteristics of the mesoscale systems prior to storm formation are presented in Part I (Cotton et al., 1982). Such an environment was responsible for several unique storm features, including a quasi-steady primary updraft circulation and movement 50° to the left of the cloud layer (2–8 km AGL) environmental winds.
Several interactions were observed or inferred near and within the storm. Vertical transport of northerly low-level momentum within the updraft imparted a significant blocking on mid-level flow having southerly momentum. Such a blocking affected the movement and characteristics of adjacent, less organized storms. Additional storm-environment interactions produced an organized recirculation of precipitation particles from the mid-level updraft to the low-level updraft.
It is concluded that the steadiness of the storm depended on two factors: 1) the introduction of low-level flow which was directed opposite to mid-level flow, 2) formation of persistent downdrafts of sufficient magnitude to sustain an active gust front.
Abstract
Convective cell identification methods, besides their operational utility, are useful to identify cells, to understand cell interactions within multicell thunderstorms, and to distinguish between convective and stratiform regions within mesoscale convective systems. The method developed in this note was utilized for research on cell interactions within the 9 August 1991 Convection and Precipitation/Electrification (CaPE) multicell thunderstorm. A critical component of such research is an objective method to accurately depict all significant convective cells within an evolving multicell thunderstorm. While conventional methods based upon radar reflectivity can be successfully used in identifying cells, especially when the cells are in their growth stage, the methods are not as useful during the later stages of cell growth. This is because updraft and precipitation cores are not collocated at these advanced stages, and thus the reflectivity (precipitation) core may not be a good indicator of convectively active regions. The method presented in this note uses four objective criteria to define and identify convective cells within multicell thunderstorms. These criteria are chosen from a prestorm proximity sounding using the air parcel theory. The four objective criteria and their threshold values for the CaPE storm included in parentheses are 1) a threshold updraft W d (∼8 m s−1), 2) a threshold cloud-layer depth D d (∼4.9 km), 3) a threshold updraft area A d (∼1 km2), and 4) cell origin within the planetary boundary layer indicated by the W pbl (∼3 m s−1) contour. Since the method is based upon upward motion and not reflectivity factor, multiple Doppler radar data are required to utilize this method.
Abstract
Convective cell identification methods, besides their operational utility, are useful to identify cells, to understand cell interactions within multicell thunderstorms, and to distinguish between convective and stratiform regions within mesoscale convective systems. The method developed in this note was utilized for research on cell interactions within the 9 August 1991 Convection and Precipitation/Electrification (CaPE) multicell thunderstorm. A critical component of such research is an objective method to accurately depict all significant convective cells within an evolving multicell thunderstorm. While conventional methods based upon radar reflectivity can be successfully used in identifying cells, especially when the cells are in their growth stage, the methods are not as useful during the later stages of cell growth. This is because updraft and precipitation cores are not collocated at these advanced stages, and thus the reflectivity (precipitation) core may not be a good indicator of convectively active regions. The method presented in this note uses four objective criteria to define and identify convective cells within multicell thunderstorms. These criteria are chosen from a prestorm proximity sounding using the air parcel theory. The four objective criteria and their threshold values for the CaPE storm included in parentheses are 1) a threshold updraft W d (∼8 m s−1), 2) a threshold cloud-layer depth D d (∼4.9 km), 3) a threshold updraft area A d (∼1 km2), and 4) cell origin within the planetary boundary layer indicated by the W pbl (∼3 m s−1) contour. Since the method is based upon upward motion and not reflectivity factor, multiple Doppler radar data are required to utilize this method.
