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- Author or Editor: William R. Cotton x
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Abstract
A one-dimensional time-dependent cumulus model is developed and discussed. Data predicted by the model along with a bulk entrainment model are compared with a case study observation and Warner's mean profile of Q/QA . While a great deal of the discrepancy between observed and predicted data can he attributed to the transient nature of convection, the consistent pattern of overprediction of such cloud properties as Q/QA and vertical velocity is indeed disturbing. It is concluded that neither the entrainment model nor the scalar nonlinear eddy viscosity model can adequately treat the general problem of turbulent transport in convective clouds. There is, however, sufficient evidence suggesting the models can he of practical value if their use is limited to dynamically active clouds and, in the case of the entrainment model, to a restricted portion of the cloud cycle life. Furthermore, there is little doubt that the entrainment coefficient is not a universal constant while the universality of the mixing length coefficients in the eddy viscosity models is still in question.
Abstract
A one-dimensional time-dependent cumulus model is developed and discussed. Data predicted by the model along with a bulk entrainment model are compared with a case study observation and Warner's mean profile of Q/QA . While a great deal of the discrepancy between observed and predicted data can he attributed to the transient nature of convection, the consistent pattern of overprediction of such cloud properties as Q/QA and vertical velocity is indeed disturbing. It is concluded that neither the entrainment model nor the scalar nonlinear eddy viscosity model can adequately treat the general problem of turbulent transport in convective clouds. There is, however, sufficient evidence suggesting the models can he of practical value if their use is limited to dynamically active clouds and, in the case of the entrainment model, to a restricted portion of the cloud cycle life. Furthermore, there is little doubt that the entrainment coefficient is not a universal constant while the universality of the mixing length coefficients in the eddy viscosity models is still in question.
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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
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
Currently, there is no adequate cumulus parameterization suitable for use in mesoscale models having horizontal resolutions between 5 and 50 kilometers. Based on the similarity of the temporal and spatial evolution of the vertical variances between a CCOPE supercell and a generic tropical squall line as explicitly simulated by the Regional Atmospheric Modeling System developed at Colorado State University, a convective parameterization scheme is developed that represents microscale turbulence with a modified second-order closure scheme and cumulus draft-scale eddies with a convective adjustment scheme. The microscale turbulence scheme is based upon the Mellor and Yamada 2.5-level closure modified to predict solely on
The cumulus draft-scale tendencies of heat, moisture, and hydrometeors are specified by a mesoscale compensation term and a convective adjustment term. The convective adjustment term is the difference between a cloud model-derived properly and its environmental value, and is modulated by a time scale determined through a moist static energy balance. The mesoscale compensation term is a product of the vertical gradient of the appropriate scalar and a convective velocity equal to (
One unique feature of this approach is that the parameterization is not simply a local grid column scheme;
Abstract
Currently, there is no adequate cumulus parameterization suitable for use in mesoscale models having horizontal resolutions between 5 and 50 kilometers. Based on the similarity of the temporal and spatial evolution of the vertical variances between a CCOPE supercell and a generic tropical squall line as explicitly simulated by the Regional Atmospheric Modeling System developed at Colorado State University, a convective parameterization scheme is developed that represents microscale turbulence with a modified second-order closure scheme and cumulus draft-scale eddies with a convective adjustment scheme. The microscale turbulence scheme is based upon the Mellor and Yamada 2.5-level closure modified to predict solely on
The cumulus draft-scale tendencies of heat, moisture, and hydrometeors are specified by a mesoscale compensation term and a convective adjustment term. The convective adjustment term is the difference between a cloud model-derived properly and its environmental value, and is modulated by a time scale determined through a moist static energy balance. The mesoscale compensation term is a product of the vertical gradient of the appropriate scalar and a convective velocity equal to (
One unique feature of this approach is that the parameterization is not simply a local grid column scheme;
Abstract
A two-way interactive, nested-grid simulation of a rotating supercell thunderstorm was performed. After 90 min the genesis of a descending incipient tornado vortex initially located aloft was simulated. The associated pressure-deficit tube subsequently built downward into the subcloud layer, where it continually fed upon a low-level source of vertical vorticity possibly introduced by the low-level downdraft. The pressure-deficit tube then drew in the low-level vorticity-rich air, allowing it to descend to the surface. A strong vortex thus formed in the subcloud field.
Abstract
A two-way interactive, nested-grid simulation of a rotating supercell thunderstorm was performed. After 90 min the genesis of a descending incipient tornado vortex initially located aloft was simulated. The associated pressure-deficit tube subsequently built downward into the subcloud layer, where it continually fed upon a low-level source of vertical vorticity possibly introduced by the low-level downdraft. The pressure-deficit tube then drew in the low-level vorticity-rich air, allowing it to descend to the surface. A strong vortex thus formed in the subcloud field.
Abstract
The Regional Atmospheric Modeling System (RAMS), developed at Colorado State University, was used to predict boundary-layer clouds and diagnose fractional cloudiness. The primary case study for this project occurred on 7 July 1987 off the coast of southern California. On this day, a transition in the type of boundary-layer cloud was observed from a clear area, to an area of small scattered cumulus, to an area of broken stratocumulus, and finally, to an area of solid stratocumulus. This case study occurred during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment field study. RAMS was configured as a nested-grid mesoscale model with a fine grid having 5-km horizontal grid spacing covering the transition area.
Various fractional cloudiness schemes found in the literature were implemented into RAMS and tested against each other to determine which best represented observed conditions. The complexities of the parameterizations used to diagnose the fractional cloudiness varied from simple functions of relative humidity to a function of the model's subgrid variability. It was found that some of the simpler schemes identified the cloud transition better, while others performed poorly.
Abstract
The Regional Atmospheric Modeling System (RAMS), developed at Colorado State University, was used to predict boundary-layer clouds and diagnose fractional cloudiness. The primary case study for this project occurred on 7 July 1987 off the coast of southern California. On this day, a transition in the type of boundary-layer cloud was observed from a clear area, to an area of small scattered cumulus, to an area of broken stratocumulus, and finally, to an area of solid stratocumulus. This case study occurred during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment field study. RAMS was configured as a nested-grid mesoscale model with a fine grid having 5-km horizontal grid spacing covering the transition area.
Various fractional cloudiness schemes found in the literature were implemented into RAMS and tested against each other to determine which best represented observed conditions. The complexities of the parameterizations used to diagnose the fractional cloudiness varied from simple functions of relative humidity to a function of the model's subgrid variability. It was found that some of the simpler schemes identified the cloud transition better, while others performed poorly.
Abstract
The characteristics of a severe squall line are examined using data acquired from the 1981 Cooperative Convective Precipitation Experiment (CCOPE). The case is unusual in that the squall line was decoupled from an immediate, surface-based inflow source due to a mesoβ-scale (20–200 km) outflow pool produced by a separate mesoscale convective system. Both systems participated in the development of a mesoscale convective complex which subsequently produced sustained, severe surface winds throughout its entire life cycle. Despite the absolutely stable, presquall atmospheric boundary layer, the squall line produced radar reflectivity values of 70 dBZ and storm-induced outflow of 30 m s−1. Aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high-valued θc air overriding the cold dome produced by the developing MCC.
The strongest surface winds were located upshear from the convective line and consisted of a northerly (alongline) component. Because the middle-level environmental flow was from the southwest, a simple vertical transport of middle-level momentum cannot account for the observed surface flow. The strong surface winds were primarily a result of the local surface pressure gradient associated with a mesohigh–mesolow couplet that accompanied the squall line.
The squall line also maintained a strong, quasi-steady, supercell-like cell that could be tracked by radar for several hours. The kinematic structure, derived from a multiple Doppler radar analysis, shows that significant alongline flow was also generated by the rotational characteristics of the supercell. This feature distinguishes this system from tropical squall lines and many midlatitude squall lines which are composed of transient ordinary cells.
Abstract
The characteristics of a severe squall line are examined using data acquired from the 1981 Cooperative Convective Precipitation Experiment (CCOPE). The case is unusual in that the squall line was decoupled from an immediate, surface-based inflow source due to a mesoβ-scale (20–200 km) outflow pool produced by a separate mesoscale convective system. Both systems participated in the development of a mesoscale convective complex which subsequently produced sustained, severe surface winds throughout its entire life cycle. Despite the absolutely stable, presquall atmospheric boundary layer, the squall line produced radar reflectivity values of 70 dBZ and storm-induced outflow of 30 m s−1. Aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high-valued θc air overriding the cold dome produced by the developing MCC.
The strongest surface winds were located upshear from the convective line and consisted of a northerly (alongline) component. Because the middle-level environmental flow was from the southwest, a simple vertical transport of middle-level momentum cannot account for the observed surface flow. The strong surface winds were primarily a result of the local surface pressure gradient associated with a mesohigh–mesolow couplet that accompanied the squall line.
The squall line also maintained a strong, quasi-steady, supercell-like cell that could be tracked by radar for several hours. The kinematic structure, derived from a multiple Doppler radar analysis, shows that significant alongline flow was also generated by the rotational characteristics of the supercell. This feature distinguishes this system from tropical squall lines and many midlatitude squall lines which are composed of transient ordinary cells.
Abstract
Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation.
We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines.
A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.
Abstract
Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation.
We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines.
A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.
Abstract
A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere.
Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool.
Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed.
Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.
Abstract
A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere.
Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool.
Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed.
Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.
