Severe thunderstorm initiation along the dryline: A mesoscale case study

Observing systems were deployed to investigate the mesoscale structure of drylines and dryline initiation of thunderstorms as part of the Cooperative Oklahoma Profiler Studies-1991 (COPS-91) field program. These observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and the KTLX WSR-88D Doppler radar. On 16 May 1991 a synoptically active dryline moved rapidly across the Southern Plains, and by late afternoon tornadic thunderstorms developed along it in east-central Oklahoma and in southern Kansas.

Analysis focuses on convective cloud and storm initiation in the mid- to late afternoon along the dryline. Aircraft crossings of the dryline and comparison of satellite and radar fields reveal that convective clouds were forming at or a few kilometers from the dryline rather than a significant distance east of the dryline. The dryline moved toward the east and northeast in northern Oklahoma and in the late afternoon retreated toward the west-northwest in southern Oklahoma. Therefore a pivot point was present where the dryline moved little, and it was just to the south of this point where storms developed. Three significant precipitating cells developed within a 30-minute period along a 20 km section of the dryline. It is shown that development in this area appears to have been aided by an area of enhanced warming in the dry air that resulted in backed winds east of the dryline in the moist air. Farther north along the dryline in Oklahoma there was a gap where few clouds formed. This coincided with an swath of cooler satellite-sensed surface temperatures that resulted from thunderstorms moving across northern Oklahoma the previous night. It was along the northern edge of this cool swath that the second storm complex developed at the intersection of the dryline and a line of convective clouds that extended into the dry air.

Dryline behavior following the initiation of convection is also illustrated. A stepped traverse pattern by the aircraft provided data for east-west vertical cross-sections just west of the developing storms. A double structure, also present in the radar reflectivity field (fine lines), was clearly shown in vertical cross-sections of water vapor and across-dryline winds. Two sharp gradients in water vapor spaced at about 15 km coincided with convergent signatures in the wind field. In the area south of the pivot point the double fine-line structure and convective clouds retreated slowly to the west-northwest in late afternoon. In addition, between 2300 and 0000 UTC a westward redevelopment of the dryline occurred, wherein the dryline at the eastern location lost identity and a pre-existing boundary strengthened about 100 km to the west. The redevelopment process is illustrated through analysis of a series of clear air reflectivity and velocity fields from the KTLX radar.