An Update on the Use of the Mid-Altitude
 Radial Convergence (MARC) Signature in
 Forecasting Damaging Winds

Gary K. Schmocker - WFO St. Louis
Ron W. Przybylinski - WFO St. Louis
Yeong-Jer Lin - St. Louis University

I. Introduction - Radar Based Precursors of Damaging Winds
  1. Reflectivity Characteristics (Fujita, Przybylinski & Gery)
        - bowing of line echo
        - WECs or RINs
        - strong reflectivity gradient
        - displaced echo top
   2. VIL
   3. Base Velocity (limited range)
   4. Microburst Studies (Eilts et al. -DDPDA)
        - descending reflectivity core
        - initial core development at a higher height
        - mid-altitude radial convergence (>22m/s)
   5. MARC in Squall Lines - Przybylinski et al.
   6. DCZ in Supercells - Lemon, Burgess & Parker

II. Definition and Dynamics of MARC (Mid-Altitude Radial  Convergence)
   1. We are surveying a component of the convective squall line's sloping
       updraft/downdraft currents along the MCS's forward flank (intensifying
       stage).
       - region of strong outbound velocities signifies a component of the storm's
         updraft current and FTR flow (with respect to approaching storm west of
         radar)
       - region of strong inbound velocities depicts the storm's convective scale
         downdrafts and the origin of the mesoscale RIJ

                                      

  

 

 

 2. Close coupling of the inbound (outbound) velocity maxima reflects where
       the radial convergence is strongest and signifies the strength of the
       mid-level, hydrostatic "Lemone Low" within that part of the convective line.

 

  3. Enhanced velocity differentials or areas of strong convergence are usually
       located in or just downwind of the high reflectivity cores along the leading
       edge of the line.
   4. Persistent areas of radial convergence (enhanced velocity differentials)
       within the larger zone of convergence along the forward flank of the
        convective line appears to be linked to the greatest degree of wind damage.
   5. We often identify 1 to 3 MARC velocity signatures (persistent areas of
       enhanced radial convergence) embedded within the larger zone of
       convergence.  These convergent areas are less than 15 km in length and
       less than 7 km in width.  A strong velocity gradient between the inbound
       and outbound maxima (nearly gate to gate) yields the strongest actual
       convergence.        
   6. Once radial velocity differentials reach 25 m/s or greater (actual
       convergence values of 2.5 x 10-2 to 5.6 x 10-3), the potential for
       severe straight line winds increase.
   7. Convective-scale vortices (tornadic as well as non-tornadic) often form in
       the zone or interface between the two drafts (mainly on the updraft side)
       where cyclonic or negative horizontal vorticity is strong.

III. Case Sample & MARC Characteristics
   1. 13 warm season (May-September) MCS cases studied so far
       - 7 afternoon/evening
       - 4 late night/early morning
       - 2 mid-late morning

   2. Differences between afternoon/evening and nocturnal (late night/early
       morning cases
       - afternoon/evening cases have greater CAPE, but lower 0-3 km shear
       - in nocturnal cases MARC is weaker, shallower, and found at a lower
          height
       - horizontal extent of overall convergent region along the forward flank of
          the convective line is also less in the nocturnal cases
       - the MARC signature has shown greater lead time in the afternoon/eveningg cases

IV. Three Example Cases
  1. July 2, 1992

                            A - B / C - D are identified tracks of MARC (location of strongest magnitudes).
                            (W) represent locations of wind damage. (T) denotes occurrences
                            of tornadoes.

     

 

 

 

 

 

 

 

 

 

          
 

2303 UTC July 1992   Reflectivity / Storm-relative velocity images at 0.5 degree slice.  Note on the SRM velocity image we observe an area of strong outbounds (red 35 kts) adjacent to a region of strong inbounds (blue 35 to 45 kts).  Velocity difference is over 70 kts at this elevation slice.

 

2321 UTC   2 July 1992  Reflectivity image (0.5 degree slice) / SRM velocity image (1.5 degree slice). At this time the SRM velocity image shows a strong mesovortex near the cyclonic shear side (northern side) of the small bow echo and MARC signature south of the apex of the bow. The magnitude of MARC was 70 kts. 

 

Time-height cross-section of MARC for track A - B.  Magnitudes of MARC are m/s.
(W) represents time of wind damage occurrence. 

 

2. May 25, 1996 case

Identified tracks of MARC (strongest magnitudes) (W) represent locations of wind damage.

  

 

 

 

 

 

 

 

 

 

 

2224 UTC  25 May 1996   Reflectivity / Storm-relative velocity at 1.5 degree slice

 

2236 UTC  25 May 1996   Reflectivity / Storm-relative velocity images at 1.5 degree slice

                Time-height cross-section of MARC for southern track (284° - 302°). (W) represents time of wind damage.  

 

3. June 14, 1998

Identified locations of MARC (strongest magnitudes)
(W) represent locations of wind damage.

 

1041 UTC  14 June 1998   Reflectivity / Storm-relative velocity images at 1.5 degree slice.

 

1102 UTC  14 June 1998   Reflectivity / Storm-relative velocity images at 2.4 degree slice.

                          Time-height cross-section of MARC (track #2) 14 June 1998. Magnitudes of MARC are in m/s. 
                          (W) represent time of wind damage. 

 

V. Summary and Key Findings
1. The MARC velocity signature (magnitudes of 25 m/s or greater) provided lead times on the average of 20 minutes prior to the first report of damaging winds in our cases.
 - often identified before the development of a well defined bow echo, or strong vortices (mesocyclone, line-end or bookend vortex)
2. MARC identified at a height between 4-5 Km along the forward flank of the convective line (in or just downwind from the high reflectivity cores within  the line).
3. Preliminary results indicate that the MARC signature is not as identifiable with nocturnal convection compared to convection occurring during the afternoon/evening hours (weaker magnitudes and shorter lead times with nocturnal cases examined so far).
4. Importance of the viewing angle
- MARC will be underestimated when the convective line is not orthogonal
  (perpendicular) to the radial.