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Mesoscale Convective Systems

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Mesoscale Convective Systems OCTOBER 2007 S T E I G E R E T A L . 3303 Total Lightning Signatures of Thunderstorm Intensity over North Texas. Part II: Mesoscale Convective Systems SCOTT M. STEIGER Department of Earth Sciences, State University of New York at Oswego, Oswego, New York RICHARD E. ORVILLE AND LAWRENCE D. CAREY Department of Atmospheric Sciences, Texas A&M University, College Station, Texas (Manuscript received 4 April 2006, in final form 25 January 2007) ABSTRACT Total lightning data from the Lightning Detection and Ranging (LDAR II) research network in addition to cloud-to-ground flash data from the National Lightning Detection Network (NLDN) and data from the Dallas–Fort Worth, Texas, Weather Surveillance Radar-1988 Doppler (WSR-88D) station (KFWS) were examined from individual cells within mesoscale convective systems that crossed the Dallas–Fort Worth region on 13 October 2001, 27 May 2002, and 16 June 2002. LDAR II source density contours were comma shaped, in association with severe wind events within mesoscale convective systems (MCSs) on 13 October 2001 and 27 May 2002. This signature is similar to the radar reflectivity bow echo. The source density comma shape was apparent 15 min prior to a severe wind report and lasted more than 20 min during the 13 October storm. Consistent relationships between severe straight-line winds, radar, and lightning storm cell characteristics (e.g., lightning heights) were not found for cells within MCSs as was the case for severe weather in supercells in Part I of this study. Cell interactions within MCSs are believed to weaken these relationships as reflectivity and lightning from nearby storms contaminate the cells of interest. Another hypothesis for these weak relations is that system, not individual cell, processes are responsible for severe straight-line winds at the surface. Analysis of the total lightning structure of the 13 October 2001 MCS showed downward-sloping source density contours behind the main convective line into the stratiform region. This further supports a charge advection mechanism in developing the stratiform charge structure. Bimodal vertical source density distributions were observed within MCS convection close to the center of the LDAR II network, while the lower mode was not detected at increasing range. 1. Introduction placed trailing stratiform (usually displaced to the north in the Northern Hemisphere), and 3) chaotic, unclassi- A mesoscale convective system (MCS) is “a group of fiable mass of convective and stratiform rain. Certain storms that interacts with and modifies the environ- identifiable lightning patterns have been associated ment and subsequent storm evolution in such a way with these MCS types as different dynamics operate in that it produces a long-lived storm system having di- the stratiform and convective regions. Severe weather mensions much larger than individual storms” (large hail, damaging winds, and tornadoes) occasion- (MacGorman and Rust 1998, p. 258). Houze et al. ally accompanies these systems, mainly in the leading (1990) give a thorough description of the three classic convective line. Lightning has been shown to be a use- MCS reflectivity structures: 1) leading line of deep con- ful indicator of imminent severe weather in supercellu- vection with symmetrically placed trailing stratiform, 2) lar convection (Steiger et al. 2007, hereafter Part I), and leading line of deep convection with asymmetrically it is a major purpose of this study to show lightning’s utility in diagnosing and predicting intense thunder- storm cells within MCSs. Corresponding author address: Dr. Scott M. Steiger, Dept. of Earth Sciences, State University of New York at Oswego, Os- Schuur et al. (1991) and Stolzenburg et al. (1994, wego, NY 13126. 1998) document the electrical charge structure of E-mail: [email protected] MCSs, within both the convective and stratiform re- DOI: 10.1175/MWR3483.1 © 2007 American Meteorological Society Unauthenticated | Downloaded 09/27/21 09:50 AM UTC MWR3483 3304 MONTHLY WEATHER REVIEW VOLUME 135 gions [see Fig. 9 in Stolzenburg et al. (1998) for a con- flashes in the stratiform region are initiated and how ceptual model based on their observations]. They pro- they propagate (e.g., Lang et al. 2004). The relationship pose that two processes are responsible for the charge between reflectivity and lightning will be examined and structure in the stratiform region: growth of mixed- compared to these previous results. phase precipitation particles and subsequent local In MCSs, peak ground flash rates tend to occur later charging in the stratiform region (in situ), and advec- than reports of severe weather (Goodman and tion of charged particles from the convective portion of MacGorman 1986). Cloud-to-ground (CG) flash rates the system into the stratiform region (Rutledge and lag echo volume aloft, which is related to updraft MacGorman 1988). strength in ordinary cells according to Carey and Rut- The convective portion of an MCS analyzed by Carey ledge (1996), and total flash activity in MCSs by 30 min et al. (2005) exhibits a bimodal height distribution of (McCormick 2003). A bipole pattern, the spatial sepa- lightning activity with source peaks at 4.5 km (3°C) and ration between regions of concentrated positive and 9.5 km MSL (Ϫ35°C) while the stratiform region has negative ground flashes, has been noticed in some MCS three peaks at 4.5, 6, and 9 km MSL. These peaks are cases (Orville et al. 1988). Negative ground flashes in thought to be regions of positive charge as the total MCSs are usually associated with the convective region lightning instrument used by Carey et al. [the Lightning while the positive ground flashes concentrate in the Detection and Ranging (LDAR II) research network; stratiform region (Rutledge and MacGorman 1988). same as the one used here] detects more radiation from The average peak ϩCG current in the stratiform region lightning propagating through positive than negative (43 kA) is greater than double the mean peak positive space charge (Rison et al. 1999). The more complex current in the convective region (20 kA) of an MCS charge structure inside the stratiform region agrees studied by McCormick (2003). [Petersen and Rutledge with the results from Stolzenburg et al. (1994, 1998). (1992) and MacGorman and Morgenstern (1998) show Radar reflectivity and lightning source density contours similar results, but there were some exceptions.] The slant downward behind the convective line into the charge regions are more expansive in the stratiform stratiform region (Carey et al. 2005). The excellent region and may provide more charge to a CG flash to agreement between snow trajectories and the slanted produce a larger current (Petersen and Rutledge 1992). path of VHF sources into the stratiform region strongly Sometimes, the convective cells are dominated by posi- supports the charge advection mechanism thought to be tive CG flashes. Typically, the storm that is dominated partly responsible for charging the stratiform regions of by ϩCGs occurs early in the lifetime of the MCS, and MCSs. Carey et al. (2005) caution, however, that in situ is on the southern end of the convective region, where charging cannot be disregarded, as charge advection most severe weather is reported (MacGorman and Rust does not explain the entire charge structure of the 1998, p. 278). stratiform region in the MCS they analyzed. In contrast to the CG flash rates discussed above, As a new cell in an MCS grows and its reflectivity total lightning flashes have shown a better correlation increases, the flash density maximum moves toward the with storm updraft strength (Williams et al. 1999). rear of the cell, eventually merging with lightning in Rapid increases in total lightning rates, or “lightning older cells. McCormick (2003) show a greater number jumps,” can aid in the prediction of severe weather (i.e., of LDAR II flash origins along the back edge of the large hail, severe straight-line winds, and tornadoes) as convective region of a squall line, in the leading edge of they gave 1–15 min of lead time for the storms analyzed the stratiform region, and in the enhanced reflectivity in Williams et al. (1999). “bridge” in the transition zone that connects the con- Mazur and Rust (1983) show that in one storm the vective and stratiform regions. There are relatively few number of cloud flashes was 40 times the ground flash LDAR II flashes associated with the leading edge of count. In contrast to this observation and expectations the convective line and in the stratiform region. LDAR from the MacGorman et al. (1989) elevated charge re- II flashes begin in regions of moderate reflectivity (Ͻ45 gion hypothesis, McCormick (2003) shows intracloud to dBZ) above convective cores at altitudes mostly Ͼ5km cloud-to-ground (IC:CG) flash ratios were at relative MSL, and LDAR II sources are at higher altitudes minimum values (near 12.7) when cells in the 13 Octo- above enhanced reflectivity aloft (indicative of a strong ber 2001 MCS were most intense. This suggests the updraft). LDAR II source density maxima in the con- storm conditions controlling IC and CG flash rates are vective line of the MCS discussed in Carey et al. (2005) more complicated than in the elevated charge hypoth- are well correlated with reflectivity enhancements. esis. Indeed, Mansell et al. (2002) show CG flashes re- These data can also help resolve the issue of where quire the development of a lower charge region (near Unauthenticated | Downloaded 09/27/21 09:50 AM UTC OCTOBER 2007 S T E I G E R E T A L . 3305 cloud base) of opposite sign to the midlevel charge re- lightning contamination, flashes with positive peak gion to initiate. Another MCS analyzed by McCormick currents less than 10 kA were removed from the NLDN (2003) had a mean IC:CG ratio of 7 and its stratiform dataset (Cummins et al.
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