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Abstract
Large-eddy simulations (LESs) were performed to study the dynamical, microphysical, and radiative processes in the 26 November 1991 FIRE II cirrus event. The LES model inherits the framework of the RAMS version 3b, developed at Colorado State University. It includes a new two-stream radiation model developed by Harrington and a new subgrid-scale model developed by Kosovic.
The LES model successfully simulated a single thin cloud layer for LES-1 and a deep cloud structure for LES-2. The simulations demonstrated that latent heat release can play a significant role in the evolution of thick cirrus clouds. For the thin cirrus in LES-1, the latent heat release was insufficient for the cirrus clouds to become positively buoyant. However, in some special cases such as LES-2, positively buoyant cells can be embedded within the cirrus layers. The updrafts from these cells induced its own pressure perturbations that affected the cloud evolution.
Vertical profiles of the total radiative and latent heating rates indicated that for well-developed, deep, and active cirrus clouds, radiative cooling and latent heating could be comparable in magnitude in the cloudy layer. This implies that latent heating cannot be neglected in the construction of a cirrus cloud model.
The probability density function (PDF) of the vertical velocity (w) was analyzed to assist in the parameterization of cloud-scale velocities in large-scale models. For the more radiatively driven, thin cirrus case, the PDFs are approximately Gaussian. However, in the interior of the deep, convectively unstable case, the PDFs of w are multimodal and very broad, indicating that parameterizing cloud-scale motions for such clouds can be very challenging.
Abstract
Large-eddy simulations (LESs) were performed to study the dynamical, microphysical, and radiative processes in the 26 November 1991 FIRE II cirrus event. The LES model inherits the framework of the RAMS version 3b, developed at Colorado State University. It includes a new two-stream radiation model developed by Harrington and a new subgrid-scale model developed by Kosovic.
The LES model successfully simulated a single thin cloud layer for LES-1 and a deep cloud structure for LES-2. The simulations demonstrated that latent heat release can play a significant role in the evolution of thick cirrus clouds. For the thin cirrus in LES-1, the latent heat release was insufficient for the cirrus clouds to become positively buoyant. However, in some special cases such as LES-2, positively buoyant cells can be embedded within the cirrus layers. The updrafts from these cells induced its own pressure perturbations that affected the cloud evolution.
Vertical profiles of the total radiative and latent heating rates indicated that for well-developed, deep, and active cirrus clouds, radiative cooling and latent heating could be comparable in magnitude in the cloudy layer. This implies that latent heating cannot be neglected in the construction of a cirrus cloud model.
The probability density function (PDF) of the vertical velocity (w) was analyzed to assist in the parameterization of cloud-scale velocities in large-scale models. For the more radiatively driven, thin cirrus case, the PDFs are approximately Gaussian. However, in the interior of the deep, convectively unstable case, the PDFs of w are multimodal and very broad, indicating that parameterizing cloud-scale motions for such clouds can be very challenging.
Abstract
At Colorado State University the Regional Atmospheric Modeling System (RAMS) has been used to study the radiative effect on the diffusional growth of ice particles in cirrus clouds. Using soundings extracted from a mesoscale simulation of the 26 November 1991 cirrus event, the radiative effect was studied using a two-dimensional cloud-resolving model (CRM) version of RAMS, coupled to an explicit bin-resolving microphysics.
The CRM simulations of the 26 November 1991 cirrus event demonstrate that the radiative impact on the diffusional growth (or sublimation) of ice crystals is significant. Even in a radiatively cooled atmospheric environment, ice particles may experience radiative warming because the net radiation received by an ice particle depends upon the emission from the particle, and the local upwelling and downwelling radiative fluxes.
Model results show that radiative feedbacks on the diffusional growth of ice particles can be very complex. Radiative warming of an ice particle will restrict the particle’s diffusional growth. In the case of radiative warming, ice particles larger than a certain size will experience so much radiative warming that surface ice saturation vapor pressures become large enough to cause sublimation of the larger crystals, while smaller crystals are growing by vapor deposition. However, ice mass production can be enhanced in the case of radiative cooling of an ice particle. For the 26 November 1991 cirrus event, radiative feedback results in significant reduction in the total ice mass, especially in the production of large ice crystals, and consequently, both radiative and dynamic properties of the cirrus cloud are significantly affected.
Abstract
At Colorado State University the Regional Atmospheric Modeling System (RAMS) has been used to study the radiative effect on the diffusional growth of ice particles in cirrus clouds. Using soundings extracted from a mesoscale simulation of the 26 November 1991 cirrus event, the radiative effect was studied using a two-dimensional cloud-resolving model (CRM) version of RAMS, coupled to an explicit bin-resolving microphysics.
The CRM simulations of the 26 November 1991 cirrus event demonstrate that the radiative impact on the diffusional growth (or sublimation) of ice crystals is significant. Even in a radiatively cooled atmospheric environment, ice particles may experience radiative warming because the net radiation received by an ice particle depends upon the emission from the particle, and the local upwelling and downwelling radiative fluxes.
Model results show that radiative feedbacks on the diffusional growth of ice particles can be very complex. Radiative warming of an ice particle will restrict the particle’s diffusional growth. In the case of radiative warming, ice particles larger than a certain size will experience so much radiative warming that surface ice saturation vapor pressures become large enough to cause sublimation of the larger crystals, while smaller crystals are growing by vapor deposition. However, ice mass production can be enhanced in the case of radiative cooling of an ice particle. For the 26 November 1991 cirrus event, radiative feedback results in significant reduction in the total ice mass, especially in the production of large ice crystals, and consequently, both radiative and dynamic properties of the cirrus cloud are significantly affected.
Abstract
The production of anomalous supersaturations at cloud edges other than cloud base has presented a vexing challenge for modelers attempting to represent the evolution of a droplet spectrum across an Eulerian grid. Although the problem manifests itself most dramatically for models that explicitly predict on the supersaturation field, it is also present in models with bulk condensation schemes in which condensation happens implicitly. Although the problem has been discussed in the context of truncation errors associated with finite difference approximations to advection, this note demonstrates more generally that the cloud-edge supersaturation problem is a fundamental problem associated with the ubiquitous assumption that the forcings on the droplet spectra are well represented by the mean thermodynamic fields. In certain respects, this assumption is equivalent to failing to represent fractional cloudiness within a grid. Although well-known consequences of this problem are the underprediction of temperature and the erroneous representation of the mean buoyancy flux within a grid box, we also demonstrate that the spurious production of droplets can arise in response to the spurious production of supersaturations in models with detailed microphysical representations.
Abstract
The production of anomalous supersaturations at cloud edges other than cloud base has presented a vexing challenge for modelers attempting to represent the evolution of a droplet spectrum across an Eulerian grid. Although the problem manifests itself most dramatically for models that explicitly predict on the supersaturation field, it is also present in models with bulk condensation schemes in which condensation happens implicitly. Although the problem has been discussed in the context of truncation errors associated with finite difference approximations to advection, this note demonstrates more generally that the cloud-edge supersaturation problem is a fundamental problem associated with the ubiquitous assumption that the forcings on the droplet spectra are well represented by the mean thermodynamic fields. In certain respects, this assumption is equivalent to failing to represent fractional cloudiness within a grid. Although well-known consequences of this problem are the underprediction of temperature and the erroneous representation of the mean buoyancy flux within a grid box, we also demonstrate that the spurious production of droplets can arise in response to the spurious production of supersaturations in models with detailed microphysical representations.
Abstract
In some mesoscale convective complexes (MCCs) the convective regions assume a configuration that resembles a frontal wave, while in other systems the configuration is not classifiable. We examined the evolution of the internal structure and flow of two MCCs that were part of an episode of systems that propagated along a stationary front, and in which the convective activity evolved from a chaotic to a frontal-wave-like pattern. The development of mesoscale vorticity and its possible relationship to the spatial patterns are explored.
Each system developed two dissimilar convective bands that together formed an open-wave pattern. The greatest number of convective clusters and the greatest midlevel convergence and high-level divergence were found in a core region (the “apex” of the wave) where the bands intersected. The north-south convective line was less enduring than the core convection or the cast-west band; it evolved from outflow boundaries of the early storms preceding the mesosystem.
A conceptual model of a frontal-wave-like MCC is developed from Doppler radar and rawinsonde observations. Three airstreams are included in the model: a relatively warm and saturated, ascending flow from the apex into the rear half of the cloud shield; a dry midlevel inflow into the southern flank of the system, in which both ascending and descending motions are observed; and cool inflow into the northern flank of the stratiform cloud below 6 km.
It is found that the frontal-wave configuration is apparently not related to the development of vorticity on the scale of the mesoscale cloud system. Rather, the north-south convective line may evolve ahead of the outflow of the early storms as it propagates into the most unstable air.
It is hypothesized that the unidirectional profile of vertical shear led to an asymmetric distribution of stratiform cloud north of the convective core region. The asymmetry may have suppressed a stronger, more prolonged convergent wind, which inhibited a stronger response of the rotational wind.
Abstract
In some mesoscale convective complexes (MCCs) the convective regions assume a configuration that resembles a frontal wave, while in other systems the configuration is not classifiable. We examined the evolution of the internal structure and flow of two MCCs that were part of an episode of systems that propagated along a stationary front, and in which the convective activity evolved from a chaotic to a frontal-wave-like pattern. The development of mesoscale vorticity and its possible relationship to the spatial patterns are explored.
Each system developed two dissimilar convective bands that together formed an open-wave pattern. The greatest number of convective clusters and the greatest midlevel convergence and high-level divergence were found in a core region (the “apex” of the wave) where the bands intersected. The north-south convective line was less enduring than the core convection or the cast-west band; it evolved from outflow boundaries of the early storms preceding the mesosystem.
A conceptual model of a frontal-wave-like MCC is developed from Doppler radar and rawinsonde observations. Three airstreams are included in the model: a relatively warm and saturated, ascending flow from the apex into the rear half of the cloud shield; a dry midlevel inflow into the southern flank of the system, in which both ascending and descending motions are observed; and cool inflow into the northern flank of the stratiform cloud below 6 km.
It is found that the frontal-wave configuration is apparently not related to the development of vorticity on the scale of the mesoscale cloud system. Rather, the north-south convective line may evolve ahead of the outflow of the early storms as it propagates into the most unstable air.
It is hypothesized that the unidirectional profile of vertical shear led to an asymmetric distribution of stratiform cloud north of the convective core region. The asymmetry may have suppressed a stronger, more prolonged convergent wind, which inhibited a stronger response of the rotational wind.
Abstract
Dual-Doppler radar, surface mesonet, satellite, and upper-air sounding data from the 1985 Preliminary Regional Experiment for STORM-Central field experiment are used to analyze the early growth stages of a mesoscale convective complex (MCC) that developed in the network on 3 June 1985. This MCC was characterized by a complex distribution of convective clusters and intervening stratiform echo as it grew from its initial stage to the typical meso-α-scale cloud shield structure at its mature stage. The MCC exhibited two very different states of organization as it grew. The early state was characterized by a relatively weak and disorganized surface pressure pattern and a highly variable three-dimensional mesoscale flow structure. The later state was characterized by a well-developed mesohigh-wake-low surface pressure pattern and more organized mososcale flow fields. The evolution between these two regimes occurred about 1 h after the upper-level cloud shield reached MCC proportions and manifested itself as a rapid, almost discrete transition that took place over a period of about 30 win.
The flow structure in this system was highly complex compared to the two-dimensional squall-line conceptual model. Five separate flow branches coexisted and interacted with one another throughout the observed development of the MCC, and the structure of some of them changed considerably as the system evolved. Notably, the rear inflow evolved from a highly variable westerly flow that ascended in its northern half and descended in the south, to a more uniformly descending rear-inflow jet. This transition was dynamically linked to the development of an upper-tropospheric mesohigh, which we hypothesize blocked the upper-trapospheric flow and partially forced the descent of the rear inflow.
Abstract
Dual-Doppler radar, surface mesonet, satellite, and upper-air sounding data from the 1985 Preliminary Regional Experiment for STORM-Central field experiment are used to analyze the early growth stages of a mesoscale convective complex (MCC) that developed in the network on 3 June 1985. This MCC was characterized by a complex distribution of convective clusters and intervening stratiform echo as it grew from its initial stage to the typical meso-α-scale cloud shield structure at its mature stage. The MCC exhibited two very different states of organization as it grew. The early state was characterized by a relatively weak and disorganized surface pressure pattern and a highly variable three-dimensional mesoscale flow structure. The later state was characterized by a well-developed mesohigh-wake-low surface pressure pattern and more organized mososcale flow fields. The evolution between these two regimes occurred about 1 h after the upper-level cloud shield reached MCC proportions and manifested itself as a rapid, almost discrete transition that took place over a period of about 30 win.
The flow structure in this system was highly complex compared to the two-dimensional squall-line conceptual model. Five separate flow branches coexisted and interacted with one another throughout the observed development of the MCC, and the structure of some of them changed considerably as the system evolved. Notably, the rear inflow evolved from a highly variable westerly flow that ascended in its northern half and descended in the south, to a more uniformly descending rear-inflow jet. This transition was dynamically linked to the development of an upper-tropospheric mesohigh, which we hypothesize blocked the upper-trapospheric flow and partially forced the descent of the rear inflow.
Abstract
An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean.
Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs found by other authors are consistent with those studied here. In addition, significant similarities were found between the structure of these MCCs and developing tropical cloud clusters. It is concluded that the MCCs analyzed here are basically tropical in nature.
A number of previously unreported features are found common to the two MCCs studied here. Among thew are a 50 kPa divergence/convergence couplet, hypothesized to be an adjustment of the flow around an “obstacle,” and a ring of convergence at 20 kPa surrounding the large circular, divergent anvil region. Also, the high-speed upper-tropospheric outflow in the vicinity of the MCCs is shown to be shallow, indicating that the effect of these systems on the upper-tropospheric flow, in terms of changes in total kinetic energy, may not be as large as implied in previous work. Finally, computations show that while the two MCCs generated vertical velocities comparable to those associated with cyclogenesis, they transported virtually no heat meridionally, suggesting that MCCs are primarily driven by buoyant forces.
Abstract
An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean.
Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs found by other authors are consistent with those studied here. In addition, significant similarities were found between the structure of these MCCs and developing tropical cloud clusters. It is concluded that the MCCs analyzed here are basically tropical in nature.
A number of previously unreported features are found common to the two MCCs studied here. Among thew are a 50 kPa divergence/convergence couplet, hypothesized to be an adjustment of the flow around an “obstacle,” and a ring of convergence at 20 kPa surrounding the large circular, divergent anvil region. Also, the high-speed upper-tropospheric outflow in the vicinity of the MCCs is shown to be shallow, indicating that the effect of these systems on the upper-tropospheric flow, in terms of changes in total kinetic energy, may not be as large as implied in previous work. Finally, computations show that while the two MCCs generated vertical velocities comparable to those associated with cyclogenesis, they transported virtually no heat meridionally, suggesting that MCCs are primarily driven by buoyant forces.
Abstract
An investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April–August) of 1996–98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previously defined categories and two new categories: mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), and meso-β elongated convective systems (MβECSs). Around two-thirds of the MCSs in the study fell into the larger satellite-defined categories (MCCs and PECSs). These larger systems produced more severe weather, generated much more precipitation, and reached a peak frequency earlier in the convective season than the smaller, meso-β systems. Overall, PECSs were found to be the dominant satellite-defined MCS, as they were the largest, most common, most severe, and most prolific precipitation-producing systems.
In addition, 2-km national composite radar reflectivity data were used to analyze the development of each of the systems. A three-level radar classification scheme describing MCS development is introduced. The classification scheme is based on the following elements: presence of stratiform precipitation, arrangement of convective cells, and interaction of convective clusters. Considerable differences were found among the systems when categorized by these features. Grouping systems by the interaction of their convective clusters revealed that more than 70% of the MCSs evolved from the merger of multiple convective clusters, which resulted in larger systems than those that developed from a single cluster. The most significant difference occurred when classifying systems by their arrangement of convective cells. In particular, if the initial convection were linearly arranged, the mature MCSs were larger, longer-lived, more severe, and more effective at producing precipitation than MCSs that developed from areally arranged convection.
Abstract
An investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April–August) of 1996–98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previously defined categories and two new categories: mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), and meso-β elongated convective systems (MβECSs). Around two-thirds of the MCSs in the study fell into the larger satellite-defined categories (MCCs and PECSs). These larger systems produced more severe weather, generated much more precipitation, and reached a peak frequency earlier in the convective season than the smaller, meso-β systems. Overall, PECSs were found to be the dominant satellite-defined MCS, as they were the largest, most common, most severe, and most prolific precipitation-producing systems.
In addition, 2-km national composite radar reflectivity data were used to analyze the development of each of the systems. A three-level radar classification scheme describing MCS development is introduced. The classification scheme is based on the following elements: presence of stratiform precipitation, arrangement of convective cells, and interaction of convective clusters. Considerable differences were found among the systems when categorized by these features. Grouping systems by the interaction of their convective clusters revealed that more than 70% of the MCSs evolved from the merger of multiple convective clusters, which resulted in larger systems than those that developed from a single cluster. The most significant difference occurred when classifying systems by their arrangement of convective cells. In particular, if the initial convection were linearly arranged, the mature MCSs were larger, longer-lived, more severe, and more effective at producing precipitation than MCSs that developed from areally arranged convection.
Abstract
The hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems (MCSs) is explored by sampling environments that supported the growth of MCSs in the Preliminary Regional Experiment for STORM (Stormscale Operational and Research Meteorology) (PRE-STORM) network with high quality special soundings. Secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the PRE-STORM field program. Additional examples of MCS environments were examined using data from the Mesoscale Analysis and Prediction System. Results show strong divergence and cross-stream accelerations occurred at upper-tropospheric levels where inertial instabilities were present. These accelerations were not uniform over the domain but were focused in the regions of instability. Also, the analyses of these data showed that regions of inertial instability may be more commonplace than is typically assumed.
The Regional Atmospheric Modeling System was used to increase the understanding of the basic processes and secondary circulations that enhance MCS growth in inertially unstable environments. Model results indicate that the strength of the divergent outflow was strongly linked to the degree of inertial stability in the local environment. The results also showed a strong dependence on the magnitude of the Coriolis parameter. Finally, experiments using varying degrees of vertical stability indicated that there was also significant sensitivity to this parameter.
Abstract
The hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems (MCSs) is explored by sampling environments that supported the growth of MCSs in the Preliminary Regional Experiment for STORM (Stormscale Operational and Research Meteorology) (PRE-STORM) network with high quality special soundings. Secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the PRE-STORM field program. Additional examples of MCS environments were examined using data from the Mesoscale Analysis and Prediction System. Results show strong divergence and cross-stream accelerations occurred at upper-tropospheric levels where inertial instabilities were present. These accelerations were not uniform over the domain but were focused in the regions of instability. Also, the analyses of these data showed that regions of inertial instability may be more commonplace than is typically assumed.
The Regional Atmospheric Modeling System was used to increase the understanding of the basic processes and secondary circulations that enhance MCS growth in inertially unstable environments. Model results indicate that the strength of the divergent outflow was strongly linked to the degree of inertial stability in the local environment. The results also showed a strong dependence on the magnitude of the Coriolis parameter. Finally, experiments using varying degrees of vertical stability indicated that there was also significant sensitivity to this parameter.
Abstract
This paper is the first in a two part series in which the interactions between a growing mesoscale convective system (MCS) and its surrounding environment are investigated. The system studied here developed in northeastern Colorado on 19 July 1993 and propagated into Kansas as a long-lived nocturnal MCS. High-resolution dual-Doppler and surface mesonet data collected from this system are discussed in Part I, while the results of a numerical simulation are discussed in Part II.
The observations show that organized mesoscale surface pressure and flow features appeared very early in the lifetime of this system, long before the development of any trailing stratiform precipitation. Most of the stratiform anvil advected ahead of the convective line in the strong upper-tropospheric westerlies. In accordance with this, most of the mid- and upper-tropospheric storm-relative flow behind the line remained westerly, or rear-to-front.
Despite the westerlies, the strongest flow perturbations with respect to the ambient winds developed to the rear of the line. The structure of these perturbations was similar to the upper-tropospheric front-to-rear and midtropospheric rear-to-front flows typically found in more mature leading-line/trailing-stratiform systems. The presence of these perturbations on the upwind side of the convective line indicates that gravity wave propagation was primarily responsible for their development.
Abstract
This paper is the first in a two part series in which the interactions between a growing mesoscale convective system (MCS) and its surrounding environment are investigated. The system studied here developed in northeastern Colorado on 19 July 1993 and propagated into Kansas as a long-lived nocturnal MCS. High-resolution dual-Doppler and surface mesonet data collected from this system are discussed in Part I, while the results of a numerical simulation are discussed in Part II.
The observations show that organized mesoscale surface pressure and flow features appeared very early in the lifetime of this system, long before the development of any trailing stratiform precipitation. Most of the stratiform anvil advected ahead of the convective line in the strong upper-tropospheric westerlies. In accordance with this, most of the mid- and upper-tropospheric storm-relative flow behind the line remained westerly, or rear-to-front.
Despite the westerlies, the strongest flow perturbations with respect to the ambient winds developed to the rear of the line. The structure of these perturbations was similar to the upper-tropospheric front-to-rear and midtropospheric rear-to-front flows typically found in more mature leading-line/trailing-stratiform systems. The presence of these perturbations on the upwind side of the convective line indicates that gravity wave propagation was primarily responsible for their development.
Abstract
The authors describe eighth- and sixth-order polynomial fits to Wexler's and Hyland-Wexler's saturation-vapor-pressure expressions. Fits are provided in both least-squares and relative-error norms. Error analysis is presented. The authors show that their method is faster in comparison with the reference expressions when implemented on a CRAY-YMP.
Abstract
The authors describe eighth- and sixth-order polynomial fits to Wexler's and Hyland-Wexler's saturation-vapor-pressure expressions. Fits are provided in both least-squares and relative-error norms. Error analysis is presented. The authors show that their method is faster in comparison with the reference expressions when implemented on a CRAY-YMP.
