Search Results
You are looking at 1 - 5 of 5 items for
- Author or Editor: James J. Toth x
- Refine by Access: All Content x
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Conditions leading up to an outbreak of severe hailstorms in northeast Colorado are examined using satellite and surface data. A persistent mesoscale ridge of surface-high pressure, caused by outflow from a mesoscale- convective system, is seen to coincide with the occurrence of large hail which was confined to a narrow band extending nearly parallel to the Front Range of the Rocky Mountains. The development of the mesoscale ridge during its most intense stages is documented using a procedure that yields an approximate streamfunction for the surface geostrophic wind. Unlike alternatives over sloping terrain, this method is quick and can be adjusted to minimize the error over a limited portion of the analysis area.
Abstract
Conditions leading up to an outbreak of severe hailstorms in northeast Colorado are examined using satellite and surface data. A persistent mesoscale ridge of surface-high pressure, caused by outflow from a mesoscale- convective system, is seen to coincide with the occurrence of large hail which was confined to a narrow band extending nearly parallel to the Front Range of the Rocky Mountains. The development of the mesoscale ridge during its most intense stages is documented using a procedure that yields an approximate streamfunction for the surface geostrophic wind. Unlike alternatives over sloping terrain, this method is quick and can be adjusted to minimize the error over a limited portion of the analysis area.
Abstract
Surface wind data from the program for Regional Observing and Forecasting Services (PROFS) have been analyzed to investigate the diurnal wind flow pattern over the broad drainage are of the South Platte River in northeast Colorado. A consistent diurnal pattern appears in monthly averages as well as on most undisturbed individual days, and is similar to the classic descriptions of mountain-valley wind flows. It is observed that rather than occurring simultaneously at all elevations, downslope-to-upslope and upslope-to-downslope surface flow transitions along the Front Range of northeast Colorado begin near the foothills of the Rocky Mountains and propagate eastward across the plains.
During the summer months, local confluence is found at midday along major east-west ridges in the region (e.g., Cheyenne Ridge and Palmer Lake Divide). Consequently, in addition to the north–south Continental Divide, these east–west ridges are preferred regions for initial afternoon thunderstorm development The late afternoon transition to downslope flow often appears to be associated with the propagation of thunderstorms from the mountains and ridges eastward to the plains.
Abstract
Surface wind data from the program for Regional Observing and Forecasting Services (PROFS) have been analyzed to investigate the diurnal wind flow pattern over the broad drainage are of the South Platte River in northeast Colorado. A consistent diurnal pattern appears in monthly averages as well as on most undisturbed individual days, and is similar to the classic descriptions of mountain-valley wind flows. It is observed that rather than occurring simultaneously at all elevations, downslope-to-upslope and upslope-to-downslope surface flow transitions along the Front Range of northeast Colorado begin near the foothills of the Rocky Mountains and propagate eastward across the plains.
During the summer months, local confluence is found at midday along major east-west ridges in the region (e.g., Cheyenne Ridge and Palmer Lake Divide). Consequently, in addition to the north–south Continental Divide, these east–west ridges are preferred regions for initial afternoon thunderstorm development The late afternoon transition to downslope flow often appears to be associated with the propagation of thunderstorms from the mountains and ridges eastward to the plains.
Abstract
This study examines surface features associated with a mature-to-dissipating midlatitude mesoscale convective system that occurred on 23–24 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central. The primary data sources include a 400 × 500 km surface mesonetwork on a 50 km grid, rawinsonde observations from 12 supplementary sites in Kansas and Oklahoma and radar measurements from conventional as well as dual-Doppler networks.
The mesoscale convective system under investigation developed in an environment with weak vertical shear and had a lifetime of 9–12 h. It consisted in its mature stage of a southward-moving arc-shaped line of deep convective cells with a trailing stratiform precipitation region to the north. Thirty-three percent of the surface rain in the portion of the mesonetwork experiencing storm passage was from the stratiform region. An intense mesoscale downdraft developed beneath the stratiform cloud with a strong mesohigh at the surface. A wake low was positioned just to the rear of the trailing stratiform region. Local “heat bursts” were observed within the wake low. These phenomena am tentatively attributed to downbursts (which develop in a nearly dry-adiabatic environment created by the mesoscale downdraft) that penetrate a shallow, stable layer near the ground.
During the final dissipation of the stratiform precipitation (in a matter of 2 h), the surface mesohigh transformed into a mesolow. Observations suggest that at least part of this transformation process can be explained as a collapsing cold pool or spreading density current. This mechanism may also have contributed to the observed development or intensification of a midlevel mesovortex as the storm dissipated. Following the decay of the mesoscale convective system during the nighttime hours, new deep convection broke out in the region of the remnant midlevel circulation the next morning.
Abstract
This study examines surface features associated with a mature-to-dissipating midlatitude mesoscale convective system that occurred on 23–24 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central. The primary data sources include a 400 × 500 km surface mesonetwork on a 50 km grid, rawinsonde observations from 12 supplementary sites in Kansas and Oklahoma and radar measurements from conventional as well as dual-Doppler networks.
The mesoscale convective system under investigation developed in an environment with weak vertical shear and had a lifetime of 9–12 h. It consisted in its mature stage of a southward-moving arc-shaped line of deep convective cells with a trailing stratiform precipitation region to the north. Thirty-three percent of the surface rain in the portion of the mesonetwork experiencing storm passage was from the stratiform region. An intense mesoscale downdraft developed beneath the stratiform cloud with a strong mesohigh at the surface. A wake low was positioned just to the rear of the trailing stratiform region. Local “heat bursts” were observed within the wake low. These phenomena am tentatively attributed to downbursts (which develop in a nearly dry-adiabatic environment created by the mesoscale downdraft) that penetrate a shallow, stable layer near the ground.
During the final dissipation of the stratiform precipitation (in a matter of 2 h), the surface mesohigh transformed into a mesolow. Observations suggest that at least part of this transformation process can be explained as a collapsing cold pool or spreading density current. This mechanism may also have contributed to the observed development or intensification of a midlevel mesovortex as the storm dissipated. Following the decay of the mesoscale convective system during the nighttime hours, new deep convection broke out in the region of the remnant midlevel circulation the next morning.
Abstract
Data from the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to document the effect of variable snow cover on atmospheric boundary layer properties cloudiness and weather conditions over north Colorado on 15 April 1983. On this day an oval-shaped ∼104 km2 area of snow-free ground surrounded by snow-covered ground existed along the Colorado Front Range. While sky conditions on the morning of this day were everywhere clear, cloudiness developed by midday over the snow-free region as a result of the more rapid boundary-layer heating and mixed-layer growth there. During midafternoon snow showers occurred over the snow-free ground whereas sides remained mostly clear over the snow-covered area.
Our analysis suggests that snow boundaries in the region may have acted through the development of a weak solenoidal field to enhance low-level inflow into the snow-free area, thereby assisting with cloud development in the region. Analogous to the sea bronze, this phenomenon might be termed a “snow breeze”. Even without such an enhancement to the circulation, the variable snow cover through its impact on the surface energy budget had a profound effect on the regional weather conditions on that day. This situation represents just one example of a class of complex interactions and feedback processes involving variable surface properties and the large-scale flow.
Abstract
Data from the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to document the effect of variable snow cover on atmospheric boundary layer properties cloudiness and weather conditions over north Colorado on 15 April 1983. On this day an oval-shaped ∼104 km2 area of snow-free ground surrounded by snow-covered ground existed along the Colorado Front Range. While sky conditions on the morning of this day were everywhere clear, cloudiness developed by midday over the snow-free region as a result of the more rapid boundary-layer heating and mixed-layer growth there. During midafternoon snow showers occurred over the snow-free ground whereas sides remained mostly clear over the snow-covered area.
Our analysis suggests that snow boundaries in the region may have acted through the development of a weak solenoidal field to enhance low-level inflow into the snow-free area, thereby assisting with cloud development in the region. Analogous to the sea bronze, this phenomenon might be termed a “snow breeze”. Even without such an enhancement to the circulation, the variable snow cover through its impact on the surface energy budget had a profound effect on the regional weather conditions on that day. This situation represents just one example of a class of complex interactions and feedback processes involving variable surface properties and the large-scale flow.
