MARC Signature

Introduction

The MARC velocity signature is a Doppler radar-velocity based precursor towards forecasting the initial onset of damaging straight-line winds in a linear Quasi_Linear Convective System (QLCS) or bowing convective system. One of the challenges in the severe storm warning process is forecasting the initial onset of damaging winds. Since the late 1970's, the bow echo reflectivity signature has been used extensively as a severe weather signature for forecasting damaging winds. However, storm surveys and observations have shown that damaging winds often precede the initial bowing of the convective line segment. Thus we looked at other reflectivity and velocity precursors in forecasting the initial onset of damaging winds.

Our initial thoughts in tackling this problem keyed upon studies associated with the isolated pulse microbursts storms.  Eilts et al. (1996) from the National Severe Storms Laboratory examined Doppler data from 85 microburst / downburst cases across Oklahoma. They discovered that one of the three most important precursors in forecasting damaging winds included strong and deep radial convergence at the storm's mid-altitudes (11 - 14 km).  We fully understood that the dynamics between a pulse 'isolated' storm and a linear convective line segment comprised of a collection of storms which are radically different. However, we wanted to observe how effective this radial velocity signature might be with a linear convective line.  Thus, we tested  the first time along with Vertical Integrated Liquid (VIL) on the June 8, 1994 linear MCS which produced damaging winds over parts of the St. Louis metro area and adjacent counties.  

After this initial case, we surveyed the MARC raidal velocity signature on 14 other linear MCSs which evolved across the Mid-Mississippi Valley Region during the period from 1992 through 2004.  We initially surveyed the reflectivity and storm-relative velocity fields of each MCS which did or did not produce damaging wind swaths. Archived level 2 data data was used in the study starting with the 2 July 1992 derecho which formed over parts of northeast Missouri and moved through east-central sections of the state into south-central Illinois. In each of these events, the linear convective line eventually evolved into bowing segment or larger bow echo. Swaths of damaging winds occurred with 12 of the 16 events. The overall length of the linear MCSs ranged from 30 to as long as 120 km.  We discovered that a 25 m s-1 velocity difference along three to five (maybe six) radials between the area of oubound to inbound velocities was an initial signal or starting point for damaging winds to occur. When calculating MARC you add the magnitude of the maximum outbound value and the maximuim inbound value.  In some of the cases, pockets or small areas of MARC have magnitudes > 40 m s-1 differences were embedded within the larger ribbon of the MARC velocity signature which may extend 80 to 140 km in length.  Preliminary results suggest that enhanced wind damage is possible under a local area of enhanced radial convergence (SRM velocity differential of 35 to 40 m s-1) within the larger ribbon of MARC.

The first conceptual models of a convective line's "Intensifying Stage" and "Late Mature Stage" was documented by Rasmussen and Rutledge 1993 (AMS JAS). Yes, this is Erik Rasmussen's early work. Figure 1 shows an example of the "intensifying stage" where the schematic diagram shows a component of two opposing currents along the convective line's forward or down shear flank. When viewing a QLCS or nearly linear convective line segment approaching your WSR-88D site from the west or northwest, the ribbon of strong out-bound velocities are frequently observed along the leading convective line at mid altitudes (4 km - 8 km). This area of strong outbound velocities represent the convective line's steep sloping updraft or the ascending branch of the mesoscale flow current. The region of strong inbound velocities within the mature to weakening convective cores within the convective line represent steep sloping downdraft region and the orgins of the mesoscale rear inflow jet (RIJ).
 

MARC Characteristics: 

The information below is based on 15 QLCSs and bow echo events during the warm season (May - September).  There were 8 afternoon - early evening cases, 5 late night-early moning events and 2 mid morning cases. 

Environmental Issues:

Difference between the late afternoon - early evening events and early to mid morning cases include:

1) afternoon and evening cases are associated with greater ML and MU CAPE compared to the late night cases.

2) 0 -3 km shear magnitudes are a little weaker during the late afternoon and early evening cases to the late night and early morning cases.
 
3) MARC magnitudes associated with the noctural late night and early moring events were weaker, shallower depth and was found at a lower heigh compared to their late afternoon or early evening counterparts.
 
4) The overall horizontal extent of the ribbon of radial convergence along the forward flank of convective lines was less with noctural events compared to the afternoon and early evening events.

5) The MARC velocity signature has shown to have greater lead times with the afternoon - early evening cases compared to the noctural cases.

MARC velocity characteristics

1) Horizontal extent of the MARC velocity signature varied from 30 to as large as 120 km in length.  Local enhanced velocity maximums were embedded within the sheet of inbound (outbound) velocities.  We continue to investigate the relationship between these enhanced velocity maximums and the degree of surface damage. These local velocity maximums are often co-located with a high-rellectivity core immediately to the west (up-shear flank).

2) Average depth of MARC was 6.2 km. Strongest magnitudes of MARC were located between 4 and 7 km.

3) Width of the zone of radial convergence varied from 2 to 6 km. Be alert for strong velocity gradients between the inbound and outbound velocities. Aliased velocities may be mistaken for the magnitudes of Inbound and Outbound Velocities.

4) Once radial velocity differences within 6 km along one or several radials reach 25 m s-1 or greater, the potential for severe straight-line winds increases..Actual magnitudes of convergence is 2.54 x 10-2 ro 5.6 x10-3.  Average lead time from the initial identification of this magnitude (25 m s-1) to the first reports of severe wind damage was 20 minutes. In a few cases, the lead time was 25 - 30 minutes.

5) Recent observations revealed that the MARC velocity signature has been observed more frequently with nearly a solid linear convective line segment compared to discrete convective cells along the southern flank of an asymmetric MCS.  

6) The viewing angle is extremely important when viewing the MARC velocity signature. The convective line must be nearly perpendicular to the radial which you are calculating MARC -  Personal discussions (Jim Wilson - NCAR).  

7) Enhanced velocity differences or smaller areas of strong convergence embedded within the larger area of MARC are usually located just downwind from high reflectivity cores located along the leading edge of the convective line. These areas (sometimes very persist) appear to be linked to the greatest degree of wind damage.

Examples of the MARC velocity signature:
 

The first example of how MARC is identified was published in the AMS 15th Conf. on Weather Analysis and Forecasting (Vienna Virginia). The 02 July 1992 Derecho case which formed east of Kansas City and moved across east-central Missouri and into south-central sections of Illinois was one of the first cases used in the MARC study.
 

Table 1:  Pre-convective environments,  MARC characteristics and lead times from the initial identification of MARC to first reports of damaging winds are summarized.