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Abstract
Two three-dimensional cloud model simulations are examined and compared in order to define some of the characteristics of the low-level downdraft initiation process within deep precipitating convection. The initial environment of each case displayed similar temperature profiles but different moisture profiles. In one case, relatively dry subcloud layers were capped by relatively moist middle levels, while the opposite moisture stratification existed for the second case. Although both simulations displayed peak low-level downdraft speeds of ∼12 m s−1, downdraft spatial and temporal behavior showed significant differences. These differences can be related to dissimilarities in the environment of each case. In the dry subcloud case (the microburst case), peak downdraft speeds occurred near the 0.8 km level shortly after precipitation arrived at low levels. Low-level downdraft developed very rapidly in this case. In the other moist subcloud case, the low-level downdraft developed less rapidly and exhibited a peak magnitude significantly higher at 1.8 km. In both case the downdraft initiation process occurred within the downshear flank. Downdrafts were forced primarily over the lowest 2 km (below the melting level), where melting and evaporation of precipitation generated negative buoyancy. The results demonstrate that low-level downdraft characteristics are closely controlled by arrival of precipitation at low levels.
Abstract
Two three-dimensional cloud model simulations are examined and compared in order to define some of the characteristics of the low-level downdraft initiation process within deep precipitating convection. The initial environment of each case displayed similar temperature profiles but different moisture profiles. In one case, relatively dry subcloud layers were capped by relatively moist middle levels, while the opposite moisture stratification existed for the second case. Although both simulations displayed peak low-level downdraft speeds of ∼12 m s−1, downdraft spatial and temporal behavior showed significant differences. These differences can be related to dissimilarities in the environment of each case. In the dry subcloud case (the microburst case), peak downdraft speeds occurred near the 0.8 km level shortly after precipitation arrived at low levels. Low-level downdraft developed very rapidly in this case. In the other moist subcloud case, the low-level downdraft developed less rapidly and exhibited a peak magnitude significantly higher at 1.8 km. In both case the downdraft initiation process occurred within the downshear flank. Downdrafts were forced primarily over the lowest 2 km (below the melting level), where melting and evaporation of precipitation generated negative buoyancy. The results demonstrate that low-level downdraft characteristics are closely controlled by arrival of precipitation at low levels.
Abstract
This paper presents results from a comprehensive investigation in which observations from several case studies an integrated with three-dimensional cloud model results to examine the general kinematic structure of downdrafts associated with High Plains precipitating convection. One particular downdraft type, the low-level precipitation-associated downdraft, is the focus of this paper.
General airflow and trajectory patterns within low-level downdrafts are convergent from 0.8 km upwards to downdraft top, typically less than 5 km AGL. Observed mass flux profiles often increase rapidly with height as a result of strong buoyancy forcing below the melting level. Inflow to the low-level downdraft although vertically continuous, can be separated into two branches. The up-down branch originating within the planetary boundary layer initially rises up to 4 km and then descends within the main precipitation-associated downdraft. The midlevel branch, usually more pronounced during early downdraft stages, originates from above the PBL and transports low-valued θ e air to low levels.
The depth of the low-level downdraft can be approximated by the environmental sounding transition level, defined as the vertical height interval separating conditionally unstable to neutral air below from roughly stable air above. The low-level downdraft is most frequently located along the upshear storm flank relative to the updraft, although a downshear downdraft may become common under higher environmental wind shear.
Abstract
This paper presents results from a comprehensive investigation in which observations from several case studies an integrated with three-dimensional cloud model results to examine the general kinematic structure of downdrafts associated with High Plains precipitating convection. One particular downdraft type, the low-level precipitation-associated downdraft, is the focus of this paper.
General airflow and trajectory patterns within low-level downdrafts are convergent from 0.8 km upwards to downdraft top, typically less than 5 km AGL. Observed mass flux profiles often increase rapidly with height as a result of strong buoyancy forcing below the melting level. Inflow to the low-level downdraft although vertically continuous, can be separated into two branches. The up-down branch originating within the planetary boundary layer initially rises up to 4 km and then descends within the main precipitation-associated downdraft. The midlevel branch, usually more pronounced during early downdraft stages, originates from above the PBL and transports low-valued θ e air to low levels.
The depth of the low-level downdraft can be approximated by the environmental sounding transition level, defined as the vertical height interval separating conditionally unstable to neutral air below from roughly stable air above. The low-level downdraft is most frequently located along the upshear storm flank relative to the updraft, although a downshear downdraft may become common under higher environmental wind shear.
Abstract
This paper describes an analysis of a long-lived, microburst-producing storm that evolved within a relatively dry environment having a relatively low CAPE value of 450 J kg−1. The storm displayed a variety of kinematic and echo formations over its 2.5-h lifetime, including 1) a near equality in the strength (∼10 m s−1) of updrafts and downdrafts, 2) strong downdrafts over an extended time period of greater than 60 min, 3) a prevalence of up-down-type downdraft trajectories associated with the strong downdrafts, 4) a prominent echo overhang during the early mature stage, 5) a spearhead-like echo protrusion during the mature storm phase that was indirectly associated with strong downdrafts, and 6) a narrow bow echo and associated weak inflow jet at midlevels during the latter storm stage.
An elongated ascending branch of the up-down downdraft circulation was associated with the echo protrusion. The prominence of the up-down trajectory is corroborated by surface data and 3D numerical simulations, both of which reveal comparable values of equivalent potential temperature in the low-level inflow and downdraft outflow air. Time series plots of saturation point reveal an evaporation line structure typical of evaporation of precipitation into the subcloud boundary layer. Thus, in this case there is little evidence to indicate that significant amounts of downdraft air originated above the atmospheric boundary layer during the sustained mature to dissipating stages.
Abstract
This paper describes an analysis of a long-lived, microburst-producing storm that evolved within a relatively dry environment having a relatively low CAPE value of 450 J kg−1. The storm displayed a variety of kinematic and echo formations over its 2.5-h lifetime, including 1) a near equality in the strength (∼10 m s−1) of updrafts and downdrafts, 2) strong downdrafts over an extended time period of greater than 60 min, 3) a prevalence of up-down-type downdraft trajectories associated with the strong downdrafts, 4) a prominent echo overhang during the early mature stage, 5) a spearhead-like echo protrusion during the mature storm phase that was indirectly associated with strong downdrafts, and 6) a narrow bow echo and associated weak inflow jet at midlevels during the latter storm stage.
An elongated ascending branch of the up-down downdraft circulation was associated with the echo protrusion. The prominence of the up-down trajectory is corroborated by surface data and 3D numerical simulations, both of which reveal comparable values of equivalent potential temperature in the low-level inflow and downdraft outflow air. Time series plots of saturation point reveal an evaporation line structure typical of evaporation of precipitation into the subcloud boundary layer. Thus, in this case there is little evidence to indicate that significant amounts of downdraft air originated above the atmospheric boundary layer during the sustained mature to dissipating stages.
Abstract
The dynamical and thermodynamical properties of precipitation-associated downdrafts are examined using a Lagrangian trajectory analysis approach applied to parcels passing through the low-level downdraft of precipitating convection. Both observations and three-dimensional cloud model results for one particular case presented in Part I are included. For this case, negative buoyancy within the low-level downdraft is very rapidly produced over the lowest 2 km by melting and evaporation of precipitation. Cooling profiles from the melting and evaporation cooling components show a significant overlap in the case considered. Both diagnosed and modeled low perturbation pressure located near the 2 km level appear to be generated by the rapid onset of negative buoyancy.
The diagnosed and modeled behavior along two low-level downdraft branches defined in Part I are examined for this particular case. It is found that the low-level downdraft is forced in varying amounts by condensate loading, negative buoyancy produced by precipitation evaporation and melting, and pressure forces. The relative role of cooling by evaporation and melting varies according to trajectory type. Along midlevel trajectories, evaporation is most significant, whereas along up–down trajectories melting is more important. For the particular case examined, the maximum amplitude in downward acceleration was found in the 1–2 km layer, where effects of loading, melting, evaporation/sublimation and negative vertical pressure gradient contributed to downward acceleration.
Abstract
The dynamical and thermodynamical properties of precipitation-associated downdrafts are examined using a Lagrangian trajectory analysis approach applied to parcels passing through the low-level downdraft of precipitating convection. Both observations and three-dimensional cloud model results for one particular case presented in Part I are included. For this case, negative buoyancy within the low-level downdraft is very rapidly produced over the lowest 2 km by melting and evaporation of precipitation. Cooling profiles from the melting and evaporation cooling components show a significant overlap in the case considered. Both diagnosed and modeled low perturbation pressure located near the 2 km level appear to be generated by the rapid onset of negative buoyancy.
The diagnosed and modeled behavior along two low-level downdraft branches defined in Part I are examined for this particular case. It is found that the low-level downdraft is forced in varying amounts by condensate loading, negative buoyancy produced by precipitation evaporation and melting, and pressure forces. The relative role of cooling by evaporation and melting varies according to trajectory type. Along midlevel trajectories, evaporation is most significant, whereas along up–down trajectories melting is more important. For the particular case examined, the maximum amplitude in downward acceleration was found in the 1–2 km layer, where effects of loading, melting, evaporation/sublimation and negative vertical pressure gradient contributed to downward acceleration.
Abstract
The evolution of the turbulent structure of an intense, quasi-steady thunderstorm is examined using Doppler radar estimates of turbulent kinetic energy dissipation rates (ε) and radial shears of raw radial velocity (ΔV r/ΔR). A comparison of turbulent patterns with mean storm airflow is made.
Observations taken during the quasi-steady mature stage reveal that turbulent intensity and activity peaked at mid to upper storm levels. The primary storm updraft was nearly turbulence-free at low levels, but exhibited increasingly turbulent activity at higher levels indicating a transition from quasi-laminar flow to bubble-like flow. Comparison of ε and ΔV r/ΔR patterns with environmental parameters such as equivalent potential temperature and momentum suggests that buoyancy and wind shear acted together to generate turbulent eddies, some greater than 500 m in size, at middle storm levels. At mid to upper storm levels, patterns of ε and ΔV r/ΔR exhibited considerable spatial and temporal variability, and maximum estimated dissipation rate estimates approached 0.15 m2 s−3. During one particular time period, 11 local ε maxima were estimated, some with magnitudes exceeding 0.07 m2 s−3.
Abstract
The evolution of the turbulent structure of an intense, quasi-steady thunderstorm is examined using Doppler radar estimates of turbulent kinetic energy dissipation rates (ε) and radial shears of raw radial velocity (ΔV r/ΔR). A comparison of turbulent patterns with mean storm airflow is made.
Observations taken during the quasi-steady mature stage reveal that turbulent intensity and activity peaked at mid to upper storm levels. The primary storm updraft was nearly turbulence-free at low levels, but exhibited increasingly turbulent activity at higher levels indicating a transition from quasi-laminar flow to bubble-like flow. Comparison of ε and ΔV r/ΔR patterns with environmental parameters such as equivalent potential temperature and momentum suggests that buoyancy and wind shear acted together to generate turbulent eddies, some greater than 500 m in size, at middle storm levels. At mid to upper storm levels, patterns of ε and ΔV r/ΔR exhibited considerable spatial and temporal variability, and maximum estimated dissipation rate estimates approached 0.15 m2 s−3. During one particular time period, 11 local ε maxima were estimated, some with magnitudes exceeding 0.07 m2 s−3.
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
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.
Abstract
Using high-resolution three-dimensional numerical experiments, this paper shows that the cell separation distance scales as 0.75 times the planetary boundary layer (PBL) depth for successful cell mergers between constructively interacting cells within multicell thunderstorms. This boundary layer scaling is determined from several simulations of convective cell pairs with a fixed PBL depth and is shown to be valid for other sensitivity simulations with larger PBL depths. This research establishes a robust and quantitative relation between prestorm ambient conditions and cell merger potential useful for research efforts on the multifaceted cell merger process of multicell thunderstorms. The weakly sheared ambient prestorm conditions of the 9 August 1991 Convection and Precipitation/Electrification Experiment (CaPE) multicell thunderstorm are used to initialize the cell pair simulations.
Since ambient wind and wind shear are assumed to be zero, only simple cell mergers, defined in this study as those between cell updraft cores joined but not overlapping in the convective stage, are shown to be possible. The coarse-resolution simulations of Stalker suggest that ambient wind shear may be necessary for forced cell mergers, defined in this study as those in which the initial updraft cores are found apart. The scenarios of overlapping initial updraft cores for cell merger are considered physically invalid in this study.
Abstract
Using high-resolution three-dimensional numerical experiments, this paper shows that the cell separation distance scales as 0.75 times the planetary boundary layer (PBL) depth for successful cell mergers between constructively interacting cells within multicell thunderstorms. This boundary layer scaling is determined from several simulations of convective cell pairs with a fixed PBL depth and is shown to be valid for other sensitivity simulations with larger PBL depths. This research establishes a robust and quantitative relation between prestorm ambient conditions and cell merger potential useful for research efforts on the multifaceted cell merger process of multicell thunderstorms. The weakly sheared ambient prestorm conditions of the 9 August 1991 Convection and Precipitation/Electrification Experiment (CaPE) multicell thunderstorm are used to initialize the cell pair simulations.
Since ambient wind and wind shear are assumed to be zero, only simple cell mergers, defined in this study as those between cell updraft cores joined but not overlapping in the convective stage, are shown to be possible. The coarse-resolution simulations of Stalker suggest that ambient wind shear may be necessary for forced cell mergers, defined in this study as those in which the initial updraft cores are found apart. The scenarios of overlapping initial updraft cores for cell merger are considered physically invalid in this study.
