Formation of Mesoscale Lines of Precipitation: Nonsevere Squall Lines in Oklahoma during the Spring

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.