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Abstract
The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.
Abstract
The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.
Abstract
This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.
Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.
A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.
Abstract
This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.
Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.
A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.
Abstract
An unusual low-precipitation cumulonimbus that developed in northeastern Colorado is photographically documented in some detail. The storm produced at least 12 funnels, mostly at midlevels on the north side of the main updraft. The base of the cloud consisted of a lenticular “bell” that rotated cyclonically, while a couplet of counterrotating storm-scale eddies prevailed aloft. The funnels originated in a region of enhanced shear between easterly low-level flow on the north side of the bell and westerly flow aloft on the north side of a midlevel anticyclonic eddy.
Abstract
An unusual low-precipitation cumulonimbus that developed in northeastern Colorado is photographically documented in some detail. The storm produced at least 12 funnels, mostly at midlevels on the north side of the main updraft. The base of the cloud consisted of a lenticular “bell” that rotated cyclonically, while a couplet of counterrotating storm-scale eddies prevailed aloft. The funnels originated in a region of enhanced shear between easterly low-level flow on the north side of the bell and westerly flow aloft on the north side of a midlevel anticyclonic eddy.
Abstract
Total area divergence is related to area rainfall using data collected during the Florida Area Cumulus Experiment (FACE) 1975 field experiment over a network that covered 1440 km2. A convergence event is defined as a monotonic decrease in total area divergence of more than 25×10−6 s−1 for more than ten minutes. This change in total area divergence is related to the total amount of area rainfall considered to be associated with the convergence event. For 121 convergence events during July and August 1975, a correlation coefficient of −0.59 is found between change in convergence and rainfall amount. When the ensemble is subdivided, it is found that for slow moving convective systems, or when low-level winds are weak, there is twice the amount of rainfall per convergence event. When middle-level (850–500 mb) relative humidity is in the range 50–65%, the correlation coefficient between convergence and rainfall is −0.81. Data are also partitioned according to stability and buoyancy. Convective outflow and its reflection in total area divergence are examined, and relationships are developed for determining the amount of precipitation for each divergence event. For the 75 rain events during FACE 1975, a correlation coefficient of 0.75 is found between the change in divergence and the rainfall amount.
Abstract
Total area divergence is related to area rainfall using data collected during the Florida Area Cumulus Experiment (FACE) 1975 field experiment over a network that covered 1440 km2. A convergence event is defined as a monotonic decrease in total area divergence of more than 25×10−6 s−1 for more than ten minutes. This change in total area divergence is related to the total amount of area rainfall considered to be associated with the convergence event. For 121 convergence events during July and August 1975, a correlation coefficient of −0.59 is found between change in convergence and rainfall amount. When the ensemble is subdivided, it is found that for slow moving convective systems, or when low-level winds are weak, there is twice the amount of rainfall per convergence event. When middle-level (850–500 mb) relative humidity is in the range 50–65%, the correlation coefficient between convergence and rainfall is −0.81. Data are also partitioned according to stability and buoyancy. Convective outflow and its reflection in total area divergence are examined, and relationships are developed for determining the amount of precipitation for each divergence event. For the 75 rain events during FACE 1975, a correlation coefficient of 0.75 is found between the change in divergence and the rainfall amount.
Abstract
Although they are a fairly consistent feature, the sea-breeze and lake-breeze convergence lines and the associated convection over south Florida during the summer may vary considerably from one day to the next. Daily radar maps indicate a few basic recurring patterns. Analyses of radiosonde data show significant differences corresponding to the different patterns in the local thermodynamic parameters, most notably the mixing ratio. Changes in the synoptic-scale wind field correspond closely to changes in the observed radar patterns and the local thermodynamic conditions. Explanation of the formation and development of the different patterns of convection is given in terms of the complex interaction between the regional-, synoptic-, peninsular- and local-scale circulations.
Abstract
Although they are a fairly consistent feature, the sea-breeze and lake-breeze convergence lines and the associated convection over south Florida during the summer may vary considerably from one day to the next. Daily radar maps indicate a few basic recurring patterns. Analyses of radiosonde data show significant differences corresponding to the different patterns in the local thermodynamic parameters, most notably the mixing ratio. Changes in the synoptic-scale wind field correspond closely to changes in the observed radar patterns and the local thermodynamic conditions. Explanation of the formation and development of the different patterns of convection is given in terms of the complex interaction between the regional-, synoptic-, peninsular- and local-scale circulations.
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
Radar and synoptic data obtained during the Florida Area Cumulus Experiment have been used in an exploratory study to investigate the effects of synoptic and regional circulations on the development of convective activity in south Florida. The radar data were used to stratify the days into four groups according to their degree of shower coverage. Mean soundings and typical synoptic maps were constructed for each group. These products were then analyzed to identify and quantify the principal factors that are associated with the production of convective showers. In general, the overall differences in activity among the four groups are a result of the influence of large-scale regional and synoptic flow patterns on the underlying local mesoscale and sea-breeze circulations. This influence is reflected in the properties of the air mass in which the convection is developing, as well as in the additional large-scale dynamic effects that are provided.
Abstract
Radar and synoptic data obtained during the Florida Area Cumulus Experiment have been used in an exploratory study to investigate the effects of synoptic and regional circulations on the development of convective activity in south Florida. The radar data were used to stratify the days into four groups according to their degree of shower coverage. Mean soundings and typical synoptic maps were constructed for each group. These products were then analyzed to identify and quantify the principal factors that are associated with the production of convective showers. In general, the overall differences in activity among the four groups are a result of the influence of large-scale regional and synoptic flow patterns on the underlying local mesoscale and sea-breeze circulations. This influence is reflected in the properties of the air mass in which the convection is developing, as well as in the additional large-scale dynamic effects that are provided.
Abstract
Radar data from the Florida Area Cumulus Experiment were used to study the ensemble characteristics of echo populations and also the structure of echo systems and their phenomenological growth and development process. The diurnal development of the convective field was explored as well. The overall distribution of echoes turned out to be of the truncated lognormal type, which is indicative of growth process that favor the larger, more vigorous clouds. Three principal scales of convective activity were apparent: single cell echoes conglomerates of several individual cells, and large areas of convection that normally form by the joining of existing cloud conglomerates by growth of new cells in the intervening space. The distributions of the radar characteristics of the individual cells were found to be very skewed, which indicates that the large majority of the cells are small and weak, and that only sporadically do a few large and strong cells appear. A considerable number. of the cells originated below the -10°C level indicating that warm rain formation is common in south Florida. Growth curves for the cells indicate fast formation and dissipation stages. A study of the diurnal development of the field of echoes revealed a tendency with time toward more complex echo structures starting from isolated showers to large merged systems. It was also seen that the large multicelled systems are preferred area for the formation of new cells, even at the expense of the population of smaller echoes.
Abstract
Radar data from the Florida Area Cumulus Experiment were used to study the ensemble characteristics of echo populations and also the structure of echo systems and their phenomenological growth and development process. The diurnal development of the convective field was explored as well. The overall distribution of echoes turned out to be of the truncated lognormal type, which is indicative of growth process that favor the larger, more vigorous clouds. Three principal scales of convective activity were apparent: single cell echoes conglomerates of several individual cells, and large areas of convection that normally form by the joining of existing cloud conglomerates by growth of new cells in the intervening space. The distributions of the radar characteristics of the individual cells were found to be very skewed, which indicates that the large majority of the cells are small and weak, and that only sporadically do a few large and strong cells appear. A considerable number. of the cells originated below the -10°C level indicating that warm rain formation is common in south Florida. Growth curves for the cells indicate fast formation and dissipation stages. A study of the diurnal development of the field of echoes revealed a tendency with time toward more complex echo structures starting from isolated showers to large merged systems. It was also seen that the large multicelled systems are preferred area for the formation of new cells, even at the expense of the population of smaller echoes.
Abstract
In this paper, storm-relative helicity (SRH) and low-level vertical shear of the horizontal wind fields were investigated on the mesoscale and stormscale in regions where tornadoes occurred for four case studies using data collected during the Verification of the Origin of Rotation in Tornadoes Experiment. A primary finding was that SRH was highly variable in both time and space in all of the cases, suggesting that this parameter might be difficult to use to predict which storms might become tornadic given the available National Weather Service upper-air wind data. Second, it was also found that the shear between the lowest mean 500-m wind and the 6-km wind was fairly uniform over vast regions in all of the four cases studied; thus, this parameter provided little guidance other than that there was possibly enough shear to support supercells. It was contended that forecasters will need to monitor low-level features, such as boundaries or wind accelerations, which might augment streamwise vorticity ingested into storms. Finally, it was suggested that one reason why one storm might produce a tornado while a nearby one does not might be due to the large variations in SRH on very small spatial and temporal scales. In other words, only those storms that move into regions, small or large, with sufficient SRH might produce tornadoes.
Abstract
In this paper, storm-relative helicity (SRH) and low-level vertical shear of the horizontal wind fields were investigated on the mesoscale and stormscale in regions where tornadoes occurred for four case studies using data collected during the Verification of the Origin of Rotation in Tornadoes Experiment. A primary finding was that SRH was highly variable in both time and space in all of the cases, suggesting that this parameter might be difficult to use to predict which storms might become tornadic given the available National Weather Service upper-air wind data. Second, it was also found that the shear between the lowest mean 500-m wind and the 6-km wind was fairly uniform over vast regions in all of the four cases studied; thus, this parameter provided little guidance other than that there was possibly enough shear to support supercells. It was contended that forecasters will need to monitor low-level features, such as boundaries or wind accelerations, which might augment streamwise vorticity ingested into storms. Finally, it was suggested that one reason why one storm might produce a tornado while a nearby one does not might be due to the large variations in SRH on very small spatial and temporal scales. In other words, only those storms that move into regions, small or large, with sufficient SRH might produce tornadoes.
Abstract
On 2 June 1995, the large-scale environment of eastern New Mexico and western Texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm-relative helicity, as well as sufficient convective available potential energy (CAPE). Indeed, many supercells occurred, but the only storms to produce tornadoes were those supercells that crossed, or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection. Surface conditions, vertical vorticity, and horizontal vorticity near this boundary are documented using conventional and special observations from the VORTEX field program. It is shown that the boundary was locally rich in horizontal vorticity, had somewhat enhanced vertical vorticity, and enhanced CAPE. Theoretical arguments indicate that the observed horizontal vorticity (around 1 × 10−2 s−1), largely parallel to the boundary, can be readily produced with the type of buoyancy contrast observed. It is hypothesized that such local enhancement of horizontal vorticity often is required for the occurrence of significant (e.g., F2 or stronger) tornadoes, even in large-scale environments that appear conducive to tornado occurrence without the aid of local influences.
Abstract
On 2 June 1995, the large-scale environment of eastern New Mexico and western Texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm-relative helicity, as well as sufficient convective available potential energy (CAPE). Indeed, many supercells occurred, but the only storms to produce tornadoes were those supercells that crossed, or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection. Surface conditions, vertical vorticity, and horizontal vorticity near this boundary are documented using conventional and special observations from the VORTEX field program. It is shown that the boundary was locally rich in horizontal vorticity, had somewhat enhanced vertical vorticity, and enhanced CAPE. Theoretical arguments indicate that the observed horizontal vorticity (around 1 × 10−2 s−1), largely parallel to the boundary, can be readily produced with the type of buoyancy contrast observed. It is hypothesized that such local enhancement of horizontal vorticity often is required for the occurrence of significant (e.g., F2 or stronger) tornadoes, even in large-scale environments that appear conducive to tornado occurrence without the aid of local influences.
