Mesoscale Environment and Internal Structure of Severe Cold Season QLCS’s over the Southeast U.S

Severe cold-season QLCSs are relatively common over the Southeast – northern Alabama in particular. Over the 14-year period 2004-2017, 25 tornado days (56 tornadoes) have occurred within 120 km of the ARMOR radar (northern Alabama) between late November and early March. About ⅔of these tornado events have been associated with QLCSs, and 80% of the total have developed during the more stable nocturnal period. Low-level stratocumulus clouds, with an average cloud base near 700 m, are prevalent ~3 hr (or more) prior to QLCS passage.

 This paper examines the internal structure and evolution of a severe cold season QLCS on 25 February 2011 using WSR-88D radars (KGWX and KHTX), the ARMOR radar, and instruments that comprise the Mobile Integrated Profiling System (MIPS), including an X-band profiling radar (XPR), 915 MHz wind profiler, microwave profiling radiometer, and lidar ceilometer. In this case, the QLCS was two-dimensional and highly slabular (value near 1.0) around the KGWX radar (NE MS), and then evolved to a more undulating structure (with continued high values of slabularity) as it moved over the ARMOR-MIPS and KHTX domain. Passage over the MIPS XPR revealed a striking low-level updraft/downdraft couplet of +18/-6 m s^-1at low levels (<3 km AGL). This vertical motion pattern, when combined with surface measurements (4 hPa permanent increase in pressure and temporary wind shift), indicate that this QLCS was propagating as a bore in a sufficiently stable low-level environment, as confirmed by temperature profiles provided by the 15-channel microwave radiometer, and Bragg scatter provided by the 915 MHz wind profiler. System-wide kinematic properties will be analyzed with the KGWX, ARMOR, and KHTX radars utilizing three different approaches: (a) 2-D analysis of airflow in the plane perpendicular to the QLCS, (b) synthetic dual Doppler analyses, and (c) dual Doppler analyses using the ARMOR and KHTX radars (65 km baseline). Analysis of the propagating characteristics of this event will be compared with several other cold season QLCS events observed with the MIPS, in which the mechanism of propagation was aided by density currents, bores, and solitary waves at the leading edge of the QLCS.