National Weather Service Weather Forecast Office

Introduction

Since July 1992, we have identified thirty-two Quasi-Linear Convective Systems (QLCSs) across the Mid-Mississippi Valley Region and surrounding areas.  From the thirty-two events, we have completed the study of twenty-eight QLCSs.  All twenty-eight MCSs were 'forward propagating MCSs'  and were responsible for the production of damaging winds. Non-supercell tornadoes occurred in fifteen of the twenty-eight systems.  During the Bow Echo and MCV Experiment (BAMEX) project (spring - summer 2003) Dr. Nolan Atkins (Lyndon State College VT), Gary Schmocker, and myself documented and studied  the storm evolution of the (June 10, 2003) which intensified over central Missouri and weakened over south-central Illinois.  Since many of the tornadic mesovortices formed over parts east central Missouri and southwest Illinois, the location of the KLSX WSR88D resulted in an excellent viewing angle of observing and tracking the tornadic and non-tornadic mesovortices KLSX. The non-supercell tornadoes occurred during the period of QLCS organization and early evolution of the mesoscale Rear Inflow Jet (RIJ).  In other words we observed the formation of strong mesovortices during the 'linear to early bowing' of the convective line segment.  Earlier thoughts keyed upon mesovortex evolution only during the period of bowing of the convective line.  However, this is not necessarily true.  In reviewing several earlier cases we studied, our data set showed these similar characteristics within the reflectivity and velocity patterns where system organization, mesoscale RIJ formation and mesovortex formation occurred in concert.  The June 10th event was the only major damaging wind producer during the spring - summer 2003 convective season.  Three QLCS occurred during the late spring through early summer season of 2004.  Two of the three MCSs were responsible for the production of non-supercell tornadoes. Only one of the three events have been investigated. Of the twenty-eight cases studied, twelve events fit the classification of a derecho while the remaining sixteen MCSs failed to meet the criteria.    

Our current work completed focuses on the morphology and evolution of the reflectivity and Doppler velocity structures of the twenty-eight MCSs.  We also investigated the pre- and near-storm environment for these events.   Analysis of surface, upper level charts, and soundings analysis (CAPE / Bulk Shear) were completed.  Profiler data in many of our cases was also used to determine the strength of Bulk Shear.  Magnitudes of Bulk Shear were calculated for the 0 - 3 km and the 0 - 5 km layers.  Dr. Jeff Trapp from Purdue University suggested the examination of the 0 - 5 km layer Bulk shear. The MCSs studied occurred during the period from March through September 1992 - 2004 (with the majority of the events occurring during the warm season months).  This is part of a continuing study of Quasi-Linear Convective Systems across our region.  We specifically continued to investigate the 'Intensifying Stage' (Leary and Houze 1979; Rasmussen and Rutledge 1993; or 'Stage 2' from Weisman's 1993 numerical simulations of bow echo - squall line evolution (pre-bow echo to early stages of a bow echo evolution)). Only a limited amount of work has been completed in this area.  Four conceptual models (Storm Types) of this stage were derived from twenty-five of the twenty-eight cases.  Three (maybe four) MCS events did not conform to any of the four storm types and were placed  in a fifth category classified as 'other.'   Examples supporting each of these conceptual storm types will be shown in the following sections.  These illustrations also show the 'variability' of mesovortex evolution observed in our events.  Some of our cases revealed  isolated cell - convective line mergers, where storm and mesovortex development-intensification occurred.  Based on our observations, we will also present preliminary information on the characteristics of 'Line-end vortices or "Bookend vortices' and their role with damaging winds.   

Single Doppler velocity data (10 cm) from KLSX (St. Louis), KEAX (Pleasant Hill MO), KILX (Lincoln IL), and KDMX (Des Monies, IA) were used in the study to analyze Doppler velocity patterns and convective-scale vortex evolution.  Archive Level II data were used in 95% of the cases studied.  Over eighty 80 convective- (gamma) scale vortices from the twenty-eight cases have been surveyed.  Time-height Rotational Velocity (Vr) traces were completed for each circulation to show their evolutionary characteristics and determine time of tornado occurrence (if a tornado occurred with the vortex).   We specifically examined the 1st and 2nd core circulations, frequently observed in the vicinity or south of the intersection of a low-level outflow boundary (or quasi-stationary frontal boundary) and the larger convective line or in the vicinity of the cyclonic shear side of the developing bow.  Additionally, 3rd and successive cores which form along the cyclonic shear side or near (or just north of) the apex of a bow are currently under study.  However, it must be pointed out that not all cases conformed to this kind of evolution. Mean magnitudes of Vr of the 1st, 2nd and successive core circulations were compared to an updated study of mesocyclone statistics reported by Burgess et al. 1982 and Burgess et al. 1995.  Dr. Mike Biggerstaff from the University of Oklahoma reported to us of a bowing MCS case across south-central Texas exhibiting a similar evolution to one of our storm types (Group 1 event).  A strong circulation formed in the  vicinity of an intersecting low-level boundary - larger convective line.  Recent observations reported by Angela Lese (WFO SGF) from the morning July 4, 2004 QLCS over southwest Missouri also showed tornadic mesovortex evolution near the intersection of a surface boundary and the larger convective line.  During the BAMEX field experiment, one of the recent cases sampled by the NOAA P3 aircraft and the High Altitiude Jet occurred over parts of Indiana and Ohio on July 4, 2003. The case also appeared to fit Group 1 events with several gamma-scale vortices occurring near and southwest of the intersection of an old outflow boundary from a earlier MCS and the northern part of a QLCS.

Goals of the Study:

1) Diagnose the pre-convective environment (instability, vertical wind shear, meso and storm-scale settings) and determine their effects on storm type and evolution. 

2) Study the role of surface boundaries (baroclinic zones) intersecting or in the vicinity of convective lines or segments.  What role do these boundaries play on vortex evolution? 

3) Determine the types of reflectivity patterns - initial circulation development preceding bowing of the convective line.

4) Investigate isolated cell - convective line mergers.  What role do these mergers have on storm - circulation evolution. 

5) Investigate the evolution of convective- (gamma) scale vortices (tornadic / non-tornadic) associated with convective lines. 

6) Compare evolutionary characteristics of convective-scale vortices associated with convective lines and traditional mesocyclones associated with supercells (Burgess et al. 1982; Burgess et al. 1995).

A comprehensive (ground truth) damage assessment was completed for 85% of our cases (1992 - 2004).  In most cases, the SOO, a forecaster from the local or nearby WFO and or a graduate student from Saint Louis University or the University of Missouri participated in the survey.  Damage assessments were extremely helpful towards comparing wind (tornadic) damage tracks / time of damage to the location of circulation or bowing segment observed on WSR-88D imagery.  

The following graphics below represent the pathways of the MCSs we have studied thus far for the periods of 1992 - 1997 and 1998 - 2002.  Note: in many of our cases, the MCS originated across the area from west-central through north-central Missouri / south-central Iowa.  (Graphics below will be updated periodically).

Twenty-five of the twenty-eight QLCSs fell into four storm morphologies (storm types) during the  'Intensifying Stage' of MCS evolution.   The fifth category was classified as 'Other' where a three bow echo events evolved with the absence of any identifiable strong - persistent circulation near the northern end or cyclonic shear side of the bowing convective line.  Discussions on each of these storm types / conceptual models are shown in categories listed below.

Table 2: Mesovortex Characteristics associated with storm morphologies across the Mid-Mississippi Valley Region

Sections of each Storm Type will be updated periodically.  Check BAMEX Scientific Links page for ppt presentations relating to QLCS events.