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Abstract
Based on numerical experiments in droplet collection with a stochastic model similar to Berry's, a new quantitative definition of autoconversion is discussed. The new formulation of autoconversion is compared with Kessler's and with Berry's. The new formulation has the decisive advantage over Berry's model of being directly compatible with Kessler's accretion model.
Abstract
Based on numerical experiments in droplet collection with a stochastic model similar to Berry's, a new quantitative definition of autoconversion is discussed. The new formulation of autoconversion is compared with Kessler's and with Berry's. The new formulation has the decisive advantage over Berry's model of being directly compatible with Kessler's accretion model.
Abstract
A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model.
The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concentrations of ice crystals. In such clouds, extensive riming and vapor deposition growth of crystals in concentrations of several thousand per liter are required before they make significant contributions to the dynamic structure of the cloud.
Finally, it was found that the warm-cloud precipitation process can either invigorate or retard the dynamic behavior of a supercooled cloud, depending upon the height and magnitude of the precipitation process.
Abstract
A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model.
The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concentrations of ice crystals. In such clouds, extensive riming and vapor deposition growth of crystals in concentrations of several thousand per liter are required before they make significant contributions to the dynamic structure of the cloud.
Finally, it was found that the warm-cloud precipitation process can either invigorate or retard the dynamic behavior of a supercooled cloud, depending upon the height and magnitude of the precipitation process.
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.
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
In this paper the authors address one type of severe weather: strong straight-line winds. The case of a mesoscale convective system that developed in eastern Colorado on 12–13 May 1985 was studied. The system formed in the afternoon, was active until early morning, and caused strong winds during the night.
A multiscale nonhydrostatic full physics simulation was performed to formulate a conceptual model of the main airflow branches of the system, and to gain understanding of the physical processes involved in the strong wind generation in this storm. Four telescopically nested grids covering from the synoptic-scale down to cloud-scale circulations were used. A Lagrangian model was employed to follow trajectories of parcels that took part in the updraft and downdraft, and balances of forces were computed along the trajectories.
The strong nocturnal winds were caused by downdrafts reaching the surface and by a dynamically forced horizontal pressure gradient force. The most important branch of the downdraft had an “up–down” trajectory. Parcels originated close to the ground, were lifted up by a strong upward-directed pressure gradient force, and became colder than their surroundings as they ascended in a stable environment. Then, as they went through the precipitation shaft, they sank due to negative buoyancy enhanced by condensate loading. The upward pressure gradient force was partially related to midlevel perturbation vorticity in the storm.
Abstract
In this paper the authors address one type of severe weather: strong straight-line winds. The case of a mesoscale convective system that developed in eastern Colorado on 12–13 May 1985 was studied. The system formed in the afternoon, was active until early morning, and caused strong winds during the night.
A multiscale nonhydrostatic full physics simulation was performed to formulate a conceptual model of the main airflow branches of the system, and to gain understanding of the physical processes involved in the strong wind generation in this storm. Four telescopically nested grids covering from the synoptic-scale down to cloud-scale circulations were used. A Lagrangian model was employed to follow trajectories of parcels that took part in the updraft and downdraft, and balances of forces were computed along the trajectories.
The strong nocturnal winds were caused by downdrafts reaching the surface and by a dynamically forced horizontal pressure gradient force. The most important branch of the downdraft had an “up–down” trajectory. Parcels originated close to the ground, were lifted up by a strong upward-directed pressure gradient force, and became colder than their surroundings as they ascended in a stable environment. Then, as they went through the precipitation shaft, they sank due to negative buoyancy enhanced by condensate loading. The upward pressure gradient force was partially related to midlevel perturbation vorticity in the storm.
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
Vertical divergence of the mountain wave's momentum flux has recently been hypothesized to be an important contribution to the global momentum budget. Wavebreaking theories and envelope orography have been employed to explain the divergence of the momentum flux. Here, cloud-top radiational cooling is shown to locally destabilize the environment and disrupt the propagation of the mountain wave in idealized two-dimensional simulations, thus drastically altering the expected momentum flux profile. Also, simulations of two-dimensional mountain waves indicate that nonlinearities can increase the wave response if the lower layer is decoupled from the flow aloft or decrease the wave response by providing multiple reflection levels for the incident mountain wave. The onset of wavebreaking and the level at which the wave breaks can be influenced by the ambient thermodynamic profile.
Abstract
Vertical divergence of the mountain wave's momentum flux has recently been hypothesized to be an important contribution to the global momentum budget. Wavebreaking theories and envelope orography have been employed to explain the divergence of the momentum flux. Here, cloud-top radiational cooling is shown to locally destabilize the environment and disrupt the propagation of the mountain wave in idealized two-dimensional simulations, thus drastically altering the expected momentum flux profile. Also, simulations of two-dimensional mountain waves indicate that nonlinearities can increase the wave response if the lower layer is decoupled from the flow aloft or decrease the wave response by providing multiple reflection levels for the incident mountain wave. The onset of wavebreaking and the level at which the wave breaks can be influenced by the ambient thermodynamic profile.
Abstract
The interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the Rockies is shown to lead to the growth of a mesoscale convective system (MCS). An organic MCS observed during the 1977 combined South Park Area Cumulus Experiment and High Plains Experiment is numerically simulated with a two-dimensional nonhydrostatic cloud model covering spatial scales of 1000 km. In this numerical investigation,mesoγ-, mesoβ- and mesoα-scales of motion are represented simultaneously. As a result, interesting features of cloud-mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks. Based on the results of this simulation, a six-stage conceptual model of orogenic development is given.
Abstract
The interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the Rockies is shown to lead to the growth of a mesoscale convective system (MCS). An organic MCS observed during the 1977 combined South Park Area Cumulus Experiment and High Plains Experiment is numerically simulated with a two-dimensional nonhydrostatic cloud model covering spatial scales of 1000 km. In this numerical investigation,mesoγ-, mesoβ- and mesoα-scales of motion are represented simultaneously. As a result, interesting features of cloud-mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks. Based on the results of this simulation, a six-stage conceptual model of orogenic development is given.
Abstract
A detailed analysis of the dynamics and thermodynamics responsible for the structure, growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made. The process of scale interaction is addressed through Fourier analysis and Reynolds averaging analysis of representative predicted variables, diabatic forcing and momentum acceleration terms. Additional dynamical analysis is accomplished through sensitivity experiments in which Coriolis, diabatic heating and ambient airflow are varied.
The general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems. The heating scales range over a nearly continuous spectrum ranging from 10–250 km. The heating occurs in response to primary advective gravity modes. The larger-scale gravity-wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns. The largest gravity-wave modes are modulated by constraints of the slope flow circulation, namely a phasing of an advective mode with a localized break in the plains inversion.
The simulated growth to mesoα-scale proportions occurs from the horizontal expansion of the disturbance through interaction with the mountain-plains scale slope flow circulation. Similar to upscale two-dimensional turbulence cascade, the mountain plains solenoid deforms thermal patterns, increasing their scale. As the scale reaches mesoα-scale proportions, geostrophic adjustment frequencies are sufficient to allow the thermal fields to persist. Implications to the problem of cumulus parameterization and limitations of the two-dimensional framework of this numerical study are discussed.
Abstract
A detailed analysis of the dynamics and thermodynamics responsible for the structure, growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made. The process of scale interaction is addressed through Fourier analysis and Reynolds averaging analysis of representative predicted variables, diabatic forcing and momentum acceleration terms. Additional dynamical analysis is accomplished through sensitivity experiments in which Coriolis, diabatic heating and ambient airflow are varied.
The general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems. The heating scales range over a nearly continuous spectrum ranging from 10–250 km. The heating occurs in response to primary advective gravity modes. The larger-scale gravity-wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns. The largest gravity-wave modes are modulated by constraints of the slope flow circulation, namely a phasing of an advective mode with a localized break in the plains inversion.
The simulated growth to mesoα-scale proportions occurs from the horizontal expansion of the disturbance through interaction with the mountain-plains scale slope flow circulation. Similar to upscale two-dimensional turbulence cascade, the mountain plains solenoid deforms thermal patterns, increasing their scale. As the scale reaches mesoα-scale proportions, geostrophic adjustment frequencies are sufficient to allow the thermal fields to persist. Implications to the problem of cumulus parameterization and limitations of the two-dimensional framework of this numerical study are discussed.
