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- Author or Editor: Michael H. Jain x
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Abstract
Four distinct kinds of severe, mesoscale convective-line development are identified in Oklahoma during the spring based on the analysis of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma. The primary classes of fine formation are broken line, back building, broken areal and embedded areal. Each is described in detail, along with illustrative examples. Comparisons are made with other observations and with numerical model simulations. The former two classes of line formation have been previously documented, while the latter two have not. Only the broken-areal squall line has been realistically simulated numerically.
The environment for each of the types of line development was determined from data from the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. It was found that broken-line formation tends to occur along cold fronts in a multicell environment, while back building occurs along any boundary in a supercell environment. The former formation is associated with a steering level with respect to cell motion, while the others are not. A steering level with respect to line motion exists around 6 or 7 km MSL in all cases. Cells within back-building squall lines have high relative helicity, like supercells, while cells within broken-line squall lines have low relative helicity. Most lines were oriented approximately 40° to the left of the pressure-weighted vertical shear vector in the troposphere, along the pressure-weighted vertical shear vector in the lowest 1 km and at a large angle to the shear somewhere in the lower portion of the middle troposphere.
Abstract
Four distinct kinds of severe, mesoscale convective-line development are identified in Oklahoma during the spring based on the analysis of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma. The primary classes of fine formation are broken line, back building, broken areal and embedded areal. Each is described in detail, along with illustrative examples. Comparisons are made with other observations and with numerical model simulations. The former two classes of line formation have been previously documented, while the latter two have not. Only the broken-areal squall line has been realistically simulated numerically.
The environment for each of the types of line development was determined from data from the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. It was found that broken-line formation tends to occur along cold fronts in a multicell environment, while back building occurs along any boundary in a supercell environment. The former formation is associated with a steering level with respect to cell motion, while the others are not. A steering level with respect to line motion exists around 6 or 7 km MSL in all cases. Cells within back-building squall lines have high relative helicity, like supercells, while cells within broken-line squall lines have low relative helicity. Most lines were oriented approximately 40° to the left of the pressure-weighted vertical shear vector in the troposphere, along the pressure-weighted vertical shear vector in the lowest 1 km and at a large angle to the shear somewhere in the lower portion of the middle troposphere.
Abstract
The development of mesoscale lines of nonsevere convection in Oklahoma during the spring is discussed. This documentation complements the study by Bluestein and Jain of severe, mesoscale convective-fine development. Three major classes of squall line formation,broken line, back building. and broken areal, are identified from analyses of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma.
The environment for each of the types of squall-line development was determined from data store the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. The convective available potential energy for, each type of nonsevere squall-line development is significantly less than that for severe squall-line development, while the convective inhibition is greater. Cells within nonsevere squall lines are characterized by low relative helicity. The environmental vertical shear associated with all types of nonsevere squall-line development is less than that associated with supercells and severe, back-building squall lines. Most fines were oriented approximately 50° to the left of the pressure-weighted vertical shear vector in the troposphere, along the shear vector in the lowest 0.6 km, and normal to the shear in the lower portion of the middle troposphere.
Abstract
The development of mesoscale lines of nonsevere convection in Oklahoma during the spring is discussed. This documentation complements the study by Bluestein and Jain of severe, mesoscale convective-fine development. Three major classes of squall line formation,broken line, back building. and broken areal, are identified from analyses of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma.
The environment for each of the types of squall-line development was determined from data store the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. The convective available potential energy for, each type of nonsevere squall-line development is significantly less than that for severe squall-line development, while the convective inhibition is greater. Cells within nonsevere squall lines are characterized by low relative helicity. The environmental vertical shear associated with all types of nonsevere squall-line development is less than that associated with supercells and severe, back-building squall lines. Most fines were oriented approximately 50° to the left of the pressure-weighted vertical shear vector in the troposphere, along the shear vector in the lowest 0.6 km, and normal to the shear in the lower portion of the middle troposphere.
Abstract
A brief field project was conducted during July 1988 to assess the potential for Next Generation Weather Radar (NEXRAD), 404-MHz radar wind profilers, and digital sounding systems to monitor the low-level wind field during clear-air conditions. The low-level jet was chosen as the phenomenon of interest because it is neither well sampled nor resolved by the current upper-air network, yet it is a common feature of mesoscale convective system and severe thunderstorm environments. Data were collected under quiescent synoptic conditions during several low-level jet events using a 10-cm NEXRAD-like Doppler radar and a digital sounding system colocated in Norman, Oklahoma. These data suggest that the areal-averaged horizontal winds calculated from the Doppler radar data using the Velocity Azimuth Display (VAD) technique are comparable with the winds observed using a digital sounding system, except under weak wind conditions. However, the vertical spacing of 304 m (1000 ft) between levels of horizontal VAD calculated winds, as currently proposed for NEXRAD, may not be of sufficient resolution to document the detailed wind structure of these events. The height of the maximum wind speed of the low-level jet on all days studied was below the planned lowest observation range gate of the 404-MHz radar wind profiler, indicating that a combination of NEXRAD and profiler data might be needed to sample the important wind field structure of the lower atmosphere. Lastly, the National Weather Service rawinsonde data processing software affects the vertical resolution of the low-level wind field in operational, and therefore archived, upper-air soundings. The procedure used to calculate NWS 1000 ft winds actually damps the wind speed profile and artificially increases the height of the level of maximum wind speed associated with the low-level jet. The appropriateness of these highly smoothed 1000 ft winds for input into sophisticated mesoscale weather prediction models should be considered.
Abstract
A brief field project was conducted during July 1988 to assess the potential for Next Generation Weather Radar (NEXRAD), 404-MHz radar wind profilers, and digital sounding systems to monitor the low-level wind field during clear-air conditions. The low-level jet was chosen as the phenomenon of interest because it is neither well sampled nor resolved by the current upper-air network, yet it is a common feature of mesoscale convective system and severe thunderstorm environments. Data were collected under quiescent synoptic conditions during several low-level jet events using a 10-cm NEXRAD-like Doppler radar and a digital sounding system colocated in Norman, Oklahoma. These data suggest that the areal-averaged horizontal winds calculated from the Doppler radar data using the Velocity Azimuth Display (VAD) technique are comparable with the winds observed using a digital sounding system, except under weak wind conditions. However, the vertical spacing of 304 m (1000 ft) between levels of horizontal VAD calculated winds, as currently proposed for NEXRAD, may not be of sufficient resolution to document the detailed wind structure of these events. The height of the maximum wind speed of the low-level jet on all days studied was below the planned lowest observation range gate of the 404-MHz radar wind profiler, indicating that a combination of NEXRAD and profiler data might be needed to sample the important wind field structure of the lower atmosphere. Lastly, the National Weather Service rawinsonde data processing software affects the vertical resolution of the low-level wind field in operational, and therefore archived, upper-air soundings. The procedure used to calculate NWS 1000 ft winds actually damps the wind speed profile and artificially increases the height of the level of maximum wind speed associated with the low-level jet. The appropriateness of these highly smoothed 1000 ft winds for input into sophisticated mesoscale weather prediction models should be considered.
