NOAA's National Weather Service Weather Forecast Office

A synthesis of observational studies in the 1980s (mostly from the Pre-Storm Project) has led to the development of a conceptual model of the structure of the mature squall line (Houze et al. 1989).  Key storm reflectivity features include multicellular evolution with new convective cells forming along the storm's leading edge, mature convective cells, and older -dissipating cells immediately to the rear of the mature cells.  A zone of weaker reflectivity referred to as a 'transition zone' separates the leading convective line from the trailing enhanced stratiform rain region.  The trailing 'stratiform rain region' is often marked by a radar bright band (Smull and Houze 1987b hereafter SH87).  There are two primary mesoscale airflow streams: 1) a subsiding layer of rear-to-front flow (rear inflow) and above a layer of upward sloping front-to-rear flow.  The rear inflow comprised of lower theta-e air enters the system from the rear and erodes the stratiform rain region resulting in a weak reflectivity notch (rear inflow notch).  Conversely, the ascending front-to-rear flow advects ice particles detrained from the convective cells rearward and seeds the stratiform rain region.  One of the unresolved issues today is how much, if any of the mesoscale rear inflow penetrates the back edge of the leading convective line.  SH87 have shown with Doppler observations from the 'Pre-storm project', the mesoscale rear inflow extends across the convective system as an uninterrupted channel stretching from mid to upper levels at the back edge of the stratiform rain region towards the lower part of the leading convective line.  They believe that the rear inflow air combines with outflow from convective cell downdrafts to enhance the movement of the leading gust front.   After studying a number of MCSs across the Mid-Mississippi Valley region, this author supports the initial claims of SH87 and believes that the rear inflow enhances the damaging wind potential along the leading edge of the convective line.  However, future Doppler observations planned in the Bow Echo and MCV Experiment (BAMEX) during the spring and early summer of 2003 across the Mid-Mississippi Valley region may either support or reject these initial observations.

Other preliminary studies geared towards improving severe weather warnings of damaging winds with convective lines also occurred during the middle 1980s.  Przybylinski and DeCaire (1985) investigated the reflectivity patterns of 22 warm season Derecho events across parts of the Midwest and Great Lakes region.  They were able to classify the echo patterns into four types or groups.  The four types include: Type 1: Individual convective cells becoming a solid line of storms extending over 150 km in length. One or two bowing segments would evolve within the large convective line. Wind damage would be associated with bowing segments. Type 2: A large bow echo extending between 100 to 150 km in length with a band of scattered to broken cells extending downwind (east of the larger convective line).  Wind damage would be associated with the bow.  This pattern is similar to John's and Hirt's (1983) progressive pattern.  One unique feature with this group was the presence of a band of cells downwind from the northern end of the line and random isolated cells downwind from the bowing line.  In over half of the cases, tornadoes were observed when mergers occurred between the isolated cells (within and outside of the northern band) and larger convective line.  Type 3:  A large bow echo extending between 100 to 150 km in length with one or several isolated storms present along the southwest (upshear) flank of the larger bow.  The isolated storms may become supercells.  Swaths of damaging winds were associated with the bow while large hail, damaging winds and tornadoes were associated with the isolated storms along the southwest flank of the line.  This pattern is similar to the 'Asymmetric pattern' described by Houze et al 1989. Type 4: A High-Precipitation Supercell evolved into a bow echo pattern.  Of the four groups only two cases were documented with this pattern.  Isolated cells formed along the storm's RFD and extend 40 to 80 km southwest of the HP storm.   Swaths of damaging winds occurred from the RFD southwest (upshear) along the developing bow echo.

Common reflectivity characteristics with all four groups included: 1) strong low-level reflectivity gradients along the leading edge of the bow (signifying strong updrafts along the storm's leading edge); 2) Rear Inflow notch or notches immediately along the trailing flank of the leading convective line.  Identification of the notch(s) was a good pre-cursor to where the strongest damaging winds occurred.

Fig. 2. Four conceptual models of the mature stage of bow echoes associated with
widespread damaging wind events.

A number of studies published during the 1980s showed a 'snapshot' of the mature phase of squall line evolution.  However, a very limited amount of work was conducted with the earlier and later stages of MCS evolution.   One of the few studies that was able to address the full MCS evolution was completed by Leary and Houze (1979).  Using reflectivity data from the GARP Atlantic Tropical Experiment (GATE) they were able to show that linear MCSs evolved through four stages:
a) formative stage - line of isolated cells
b) intensifying stage - breaks fill in between cells and reflectivity increases
c) mature stage - mesoscale precipitation features are present
d) dissipating stage - leading edge convection dissipates

Using a special Doppler data set from Australia, Rasmussen and Rutledge (1993) showed that certain characteristics of the squall line, seen in both the reflectivity and kinematic structure, help determine the stage of evolution.  Much of their work focused upon the 'Intensifying' and 'mature' stages of MCS evolution.   They described the later part of the 'intensifying' stage when convective cells reached their greatest vertical extent and largest updraft speeds.  Their cases showed that the changes in the flow structure occurred much more rapidly after the line becomes solid.  They noted that during and towards the end of this stage the developing buoyancy pertubation is much greater in along-line extent than in line-normal extent.  The divergence near the summit of the leading edge induces relative Front-To-Rear flow to the rear of the line, while the Rear-to-Front flow accelerations occur in the lower and middle levels and extends rearward from the trailing edge of the convective line.   The changes in these flow fields combine to drive the horizontal vorticity.  The generation of vorticity is a system-scale phenomena where a 'sloping zone of negative horizontal vorticity' extends from the surface to higher elevations rearward from the leading edge.  This implies that buoyancy gradients occur across the entire squall line and through much of its depth, not just near the leading edge of the low-level cold pool. Sublimation, melting, and evaporation in the expanding rearward cloudy region as well as condensate loading near the leading edge contributes to the negative buoyancy anomaly. Conversely, heating due to condensation and freezing maintains the positive buoyancy anomaly in the ascending region.  Much of our investigations over the past ten years focuses upon storm and circulation characteristics associated with the 'Intensifying Stage' of MCS evolution.

Note: The 'intensifying stage' presented here is also similar to Weisman's Stage II numerical simulations - where the cold pool circulation balances the circulation inherent by the ambient shear - resulting in vertical convective towers (Weisman 1993)).

An updated study of the structure of the mesoscale rear inflow was completed by Klimowski (1994).  He investigated the reflectivity and velocity structures of an MCS from CP4 and CP5 Doppler radars which moved across the southern part of North Dakota. Some of the observations he documented included: 1) The rear inflow was initiated near high reflectivity cores of the squall line and largely elevated. 2) The rear inflow was not homogeneous along the length of the squall line. 3) Rear inflow was stronger where the trailing stratiform precipitation formed. and 4) There was a slight positive correlation between the development of the rear inflow and the development of the ascending branch of the front-to-rear flow.

Role of Low-level Boundaries

Early work completed by Maddox et al. 1980, and Weaver and Nelson 1982 focused their studies on the interaction of convective storms with shallow baroclinic zones (e.g. typically warm frontal boundaries, statinary fronts and old slow-moving thunderstorm cold-air outflow boundaries).  Maddox showed a number of examples where storms spawned weak short-lived as well as strong tornadoes as they crossed or moved along strong thermal boundaries. Weaver and Nelson showed a case in Oklahoma where a supercell containing multiple updraft centers (HP supercell as we call it today?) along the storm's southeastern flank intersected and
tracked along an old low-level boundary from a supercell located 70 km to the east.   A weak tornado occurred just north of the intersection of the old low-level outflow boundary from the downwind supercell and RFD of the upwind supercell (in the region where multiple updraft centers were observed and recorded on NSSL Doppler and conventional radars).  Investigations completed during the early to middle 1990s (Przybylinski et al. 1993; Weaver et al 1994; and Weaver and Purdom 1995) further confirmed these earlier observations. 

Low-level Thermal Boundaries serve in two ways:
1) Local forcing mechanism for convective initiation
2) As a source of local vorticity augmentation for convective-scale (or mesoscale) vortices.

From the 'VORTEX project', Rasmussen et al. 1994 and Markowski et al. 1998 also studied the role of low-level boundaries and their relationship to mesocyclogenesis.  Their definition of boundaries include all identifiable boundaries except for the forward flank boundary and rear flank gust fronts of the conceptual supercell model of Lemon and Doswell (1979).   Their investigations showed that nearly 70% of the tornadoes they sampled were assciated with boundaries and tornadoes associated with boudaries usually occurred in a zone from up to 10 km on the warm side to 30 km on the cold side of these boundaries.   A number of investigators including Rasmussen and Markowski showed that horizontal vorticity can be produced along baroclinic boundaries (e.g old outflow boundaries, warm fronts, etc) with parcels residence times of as little as 10 minutes.  Horizontal vorticity generated at boundaries is an important vortcity source for low-level mesocyclogenesis via tilting and stretching. In some cases when large accelerations in the storm's inflow is present, baroclinically generated horizontal vorticity can be amplified by horizontal stretching even prior to reaching the supercell's updraft.   The diagram below is a conceptual model from Markowski et al. 1998 showing how an updraft-boundary interaction may lead to low-level mesocyclogenesis.

To improve tornado warning capabilities, they suggested that it is fundamentally necessary to distinguish between operationally 'important'  boundaries from those which are less significant for mesocyclone generation.  Additionally, they indicated that studies on storm- boundary interactions in tornado production are needed over other regions of the United States outside the plains.    

Of the 25 MCS cases we have studied, 18 events appeared to be directly influenced by a low-level thermal boundary.  One of our goals in the MCS - bow echo study at LSX is to investigate the vortex evolution not only along the leading edge of bowing systems but also at low-level boundary - convective line intersections.  Check MCS Groups 1 and 2 for more detailed information on meso-vortex evolution near these intersections.

References:

Houze, R.A., S.A. Rutledge, M.I. Biggerstaff, and B.F. Smull, 1989: Interpretation of Doppler weather radar displays of mid-latitude mesoscale convective systems. Bull. Amer. Meteor. Soc., 70, 608-619.

Johns, R.H. and W.D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 7, 588-612.

Leary, C.A. and R.A. Houze, Jr. 1979: The structure and evolution of convection in a tropical cloud cluster. J. Atmos. Sci., 36, 437-457.

Klimowski, B.A., 1994: Initiation and development of rear inflow within the 28-29 June 1989 North Dakota mesoconvective system. Mon. Wea. Rev., 122, 765-779.

Lemon, L.R. and C.A. Doswell III, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107, 1184-1197.

Maddox, R.A., L.R. Hoxit, and C.F. Chappell, 1980: A study of tornadic thunderstorm interactions with thermal boundaries.  Mon. Wea. Rev. 108, 332-336.

Markowski P.M., E.N. Rasmussen and J.M. Straka, 1998: The occurrence of tornadoes in supercells interacting with boundaries during VORTEX-95.  Wea Forecasting, 13, 852-859.

Przybylinski, R.W. and D.M. DeCaire, 1985: Radar signatures associated with the Derecho, A type of mesoscale convective system.  Preprints, 14th Conf. on Severe Local Storms. Indianapolis, Amer. Meteor. Soc. 228-231.  

Przybylinski, R.W., T.J. Shea, D.L. Ferry, E.H. Goetsch, R.R.Czys, and N.E. Wescott, 1993: Doppler radar observations of High-Precipitation supercells over the Mid-Mississippi Valley region. Preprints, 17th Conf. on Severe Local Storms. St. Louis, Amer. Meteor. Soc. 158-163. 

Rasmussen, E.N. and S.A. Rutledge, 1993: Squall line evolution. Part 1: Kinematic and reflectivity structure. J. Atmos. Sci., 50, 2584-2606.

Rasmussen, E.N., J.M. Straka, R.P. Davies-Jones, C.A. Doswell III, F.H. Carr, M.D. Eilts, and D.R. MacGorman, 1994: The Verifications of the Origins of Rotation in Tornadoes Experiment: VORTEX. Bull. Amer. Meteor. Soc., 75, 997-1006.

Smull, B.F. and R.A. Houze, Jr. 1987: Rear Inflow in squall lines with trailing stratiform precipitation. Mon. Wea. Rev., 115, 2869-2889.

Thorpe, A.J., M.J. Miller, and M.W. Moncrieff, 1982: Two-dimensional convection in non-constant shear: A model of mid-latitude squall lines. Quart. J. Roy. Meteor. Soc.,
108, 739-762.

Weaver J.F. and S.P. Nelson: 1982: Multiscale aspects of thunderstorm gust fronts and their effects on subsequent storm development.  Mon. Wea. Rev. 110, 707-718.

Weaver, J.F. and E.J. Szoke, 1994: Some mesoscale aspects of the 6 June 1990 Limon Colorado tornado case.  Wea. Forecasting, 9, 45-61.

Weaver, J.F. and J.F.W. Purdom, 1995: An interesting mesoscale storm-environment interaction observed just prior to changes in severe storm behavior.  Wea and Forecasting, 10, 449-453.