The Severe Thunderstorm Electrification and Precipitation Study (STEPS)

Timothy J. Lang1*, L. Jay Miller2, Morris Weisman2, Steven A. Rutledge1, Llyle J. Barker, III3, V. N. Bringi1, V. Chandrasekar1, Andrew Detwiler4, Nolan Doesken5, John Helsdon4, Charles Knight2, Paul Krehbiel6, Walter A. Lyons, CCM6, Don MacGorman8, Erik Rasmussen8, William Rison6, W. David Rust8, Ron Thomas6

1Colorado State University, Fort Collins, CO

2National Center for Atmospheric Research, Boulder, CO

3National Weather Service, Lincoln, IL

4South Dakota School of Mines and Technology, Rapid City, SD

5Colorado Climate Center, Fort Collins, CO

6New Mexico Institute of Mining and Technology, Socorro, NM

7FMA Research, Inc., Fort Collins, CO

8National Severe Storms Laboratory, Norman, OK

Submitted to Bulletin of the American Meteorological Society

1 June 2003

*Corresponding Author Address:

Timothy J. Lang

Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

Abstract

During May-July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) was conducted in the High Plains, near the Colorado-Kansas border, in order to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included: 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as low-precipitation (LP), classic or medium-precipitation, and high-precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) to verify and improve microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler and polarimetric radar network, as well as a time-of-arrival VHF lightning mapping system, the T-28 armored research aircraft, electric field meters carried on balloons, mobile mesonet vehicles, instruments to detect and classify transient luminous events over thunderstorms (TLEs; e.g., sprites and blue jets), and mobile atmospheric sounding equipment. The project featured significant collaboration with the local National Weather Service office in Goodland, KS, as well as local governments, schools, and the public. The project was a major success, gathering unprecedented data on a wealth of diverse cases, including LP storms, supercells, and mesoscale convective systems, among others. Many of the storms produced mostly positive CG lightning during their lifetimes, and also exhibited unusual electrical structures such as a possibly inverted dipole. The field data from STEPS is expected to bring new advances to our understanding of supercells, positive CG lightning, TLEs, and precipitation formation in convective storms.

1. Introduction

Severe thunderstorms, due to their propensity to injure, kill, and cause extensive property damage, are a primary concern to not only weather forecasters but also the public. However, these storms remain a puzzling scientific and forecasting problem, as they exhibit not only a wide range of electrical activity, but also diversity in precipitation type and amount. Indeed, the incomplete representation of precipitation in convective storms remains a significant impediment to improving the quantitative forecast of warm season precipitation nationwide (e.g., Fritsch et al. 1998, Droegemeier et al. 2000).

One of the more intriguing severe storm types in this regard is the supercell thunderstorm (Browning 1964). In its most pristine state, a supercell is composed of a single, long-lived, rotating updraft that frequently produces large hail, high winds, prolific lightning, and occasionally tornadoes. While the basic dynamics of supercells seem well understood (e.g., Klemp 1987), these storms exhibit a wide variety of precipitation characteristics that are not well understood. For instance, supercells have been classified as either low-precipitation (LP; Donaldson et al. 1965, Davies-Jones et al. 1976, Burgess and Davies-Jones 1979, Bluestein and Parks 1983), classic or medium-precipitation (MP), and heavy-precipitation (HP; Doswell and Burgess 1993, Rasmussen and Straka 1998) based on differences in overall precipitation characteristics. Perhaps the least understood among these storms are LP supercells, which characteristically produce a huge anvil, some large hail, but appear to produce little rain. The visible cloud is a skeleton compared with other supercell storms, and rarely has a visible rain shaft (Bluestein and Parks 1983, Bluestein and Woodall 1990). For a schematic representation of classic and LP supercells, see Fig. 1.

Supercell updrafts generally are too strong to allow much precipitation growth in a single upward pass. Therefore, some form of recirculation of embryonic precipitation is required to produce larger-sized particles that fall out as raindrops, graupel, or hail (Browning 1977, Nelson 1983, Miller et al. 1988, 1990). One possible explanation as to why some supercells produce large hail with very little rain, while others might produce large amounts of rain and hail of all sizes, is that environmental shear (e.g., Marwitz 1972a,b) and storm-relative flow in the upper levels (Rasmussen and Straka 1998) modulate the recycling process. Consistent with the notion that airflow affects hail production, Nelson (1987) proposed that severe hailfall events are critically dependent on kinematic structure rather than microphysical factors. Therefore, clarified understanding of the workings of supercells should illuminate the mechanisms that influence storm precipitation efficiency in general, as well as the feedbacks between precipitation production and storm dynamics.

Another unusual aspect of severe convective storms, including supercells, is their tendency to produce copious positive cloud-to-ground (+CG) lightning (e.g., Branick and Doswell 1992), in contrast with normal warm-season thunderstorms that transfer mostly negative charge to ground via negative CG lightning (Orville 1994, Orville and Silver 1997). A major question is the location of the source charge regions for +CG flashes in these storms, and how those charge regions develop. Most convection is generally thought to have an approximately tripolar charge structure, with a small amount of lower positive charge below major mid-level negative (generally considered to be the origin location of most negative CG lightning) and upper-level positive charge (Williams 1989). However, more complex electrical structures exist, particularly in thunderstorm downdrafts (Stolzenburg et al. 1998).

A detailed review of +CG hypotheses is provided in Williams (2001), and a schematic representation of these hypotheses can be found in Fig. 2. Some researchers have posited an enhanced low-level positive charge layer as being responsible for most +CGs (tripole hypothesis; Fig. 2d). A similar possibility is an inverted dipole, with mid-level positive charge underlying upper-level negative (Fig. 2c; MacGorman and Nielsen 1991, Williams et al. 1991). Other work points toward upper-level positive charge that is unshielded either due to falling precipitation (Fig. 2b; Carey and Rutledge 1998) or strong wind shear (tilted dipole; Fig. 2a; MacGorman and Nielsen 1991, Branick and Doswell 1992, Curran and Rust 1992). All of these hypotheses suggest interesting yet poorly understood relationships between precipitation formation, airflow dynamics, and lightning production in +CG thunderstorms.

The ability to understand these relationships, however, requires sophisticated tools to observe and analyze thunderstorm characteristics. In particular, for precipitation, research with polarimetric meteorological radars has led to an emerging capability for identifying hydrometeor types remotely (Vivekanandan et al. 1999, Liu and Chandrasekar 2000, Straka et al. 2000). Such work began with efforts to discriminate between hail and rain, but as these radars have become more sophisticated, the number of measurable variables and thus the number of potential discriminants has increased. Some algorithms distinguish between such diverse hydrometeor types as large and small hail, graupel, snow, and mixed-phase precipitation. Hydrometeor identification can be useful in various applications to weather forecasting, aviation weather warnings, as well as in fundamental studies of storm structure and evolution. However, like all remote sensing techniques, polarimetric hydrometeor classification needs in situ verification to establish and improve the scope of its validity.

During May-July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS; Weisman and Miller 2000; was conducted in the High Plains, near the Colorado-Kansas border, in order to investigate all of the above issues. STEPS was intended to achieve a better understanding of the interactions between kinematics, precipitation production, and electrification in severe thunderstorms. Specific scientific objectives included: 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as LP, MP, and HP; 2) understanding lightning formation and behavior in storms, and how it differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly +CG lightning; and 3) to verify and improve microphysical interpretations from polarimetric radar.

In addition to these major research objectives, STEPS provided an opportunity to examine some related issues. The emphasis on +CG lightning enabled research into what is different about the small subset of +CGs from certain storms which trigger mesospheric transient luminous events (TLEs) such as sprites (Lyons et al. 2000, 2003a,b; Williams 1998). In addition, the emphasis on polarimetric radar observations allowed research into how precipitation forms in growing cumulus clouds.

2. STEPS Design and Execution

The STEPS project centered on a unique suite of complementary observing platforms in eastern Colorado and western Kansas. This portion of the High Plains region of the U. S. has been observed to climatologically favor supercell storms, particularly of the LP variety (Bluestein and Parks 1983). This is primarily due to the warm-season presence in this region of the dry line, the boundary between moist air from the Gulf of Mexico and drier continental air, which has been strongly associated with the occurrence of LP storms (Bluestein and Parks 1983). This region also is favorable for thunderstorms that produce predominantly +CG lightning (Zajac and Rutledge 2001, Carey and Rutledge 2003, Carey et al. 2003), as well as severe hailstorms (Changnon 1977). Thus, the STEPS domain was ideally located for studying the storms of interest.

The field measurements and analysis for STEPS were specifically designed to explore the mechanisms of precipitation formation and lightning production in supercell storms. The instrumentation (Table 1) included two S-band polarimetric radars, the Colorado State University CSU-CHILL and the NCAR S-Pol - along with the NWS WSR-88D Doppler radar at Goodland, KS. Collectively, these radars were used to determine the internal airflow and precipitation structure of storms. The deployable Lightning Mapping Array (LMA) from New Mexico Institute of Mining and Technology was used to map the three-dimensional total lightning distribution, while the National Lightning Detection Network (NLDN) provided CG flash data. The South Dakota School of Mines and Technology (SDSMT) armored T-28 aircraft was used to provide in situ microphysical, electric field, and particle charge data. Mobile sounding systems from NOAA/NSSL were used to obtain balloon-borne measurements of electric fields inside storms (EFM balloons). NCAR mobile sounding systems (M-GLASS) and NOAA/NSSL Mobile Mesonet vehicles were used to characterize the storm environment. Finally, the Yucca Ridge Field Station (YRFS), located a few hundred km NW of the STEPS domain, provided observations of TLEs during STEPS.

The basic geographical layout of the project is shown in Fig. 3. For more information on each of these observing platforms see Table 1. The combination of all of these observations provided a thorough depiction of the co-evolving kinematic, microphysical, and electrical structures of STEPS thunderstorms, along with an understanding of each storm's mesoscale environment. Due to the detailed observing network, the STEPS data provides the best opportunity to answer key questions about precipitation formation and electrification within severe storms. Additionally, the presence of two polarimetric radars and in situ observations provided a unique opportunity to evaluate and improve radar-based hydrometeor identification and quantification algorithms.

The Operations Center for STEPS was situated at the CSU-CHILL radar facility, which was temporarily re-located from its home base at Greeley, CO, to Burlington, CO. Mobile facilities and STEPS personnel generally were based out of Burlington, CO, and Goodland, KS. STEPS received excellent support from the local NWS forecast office in Goodland, KS (see sidebar), and daily forecast and observational platform status briefings occurred each morning at this NWS facility.

Based on each briefing, operations plans were formulated for the afternoon and evening. The research radars (CSU-CHILL and S-Pol) typically were running surveillance scans by noon. When convection was forecasted, M-GLASS soundings were released at various locations and vehicle platforms (Mobile Mesonet, EFM balloons) were deployed in strategic locations where activity was expected. Once convective targets were identified the vehicles and T-28 aircraft were vectored to the storm via two-way radio communications with the operations center. In addition, the research radars would begin synchronized sector-based PPI and RHI scans of the target storm.

The main focus of observations were storms that occurred within or passed through the dual-Doppler lobes formed by each radar pair within the STEPS network (see Fig. 3). Of these, the highest priority was given to supercell storms, especially those with LP characteristics, as well as thunderstorms observed to be producing predominantly +CGs. The two research radars, CSU-CHILL and S-Pol, provided both polarimetric and velocity information while the KGLD WSR-88D operational radar provided additional velocity measurements for wind syntheses, both dual- and triple-Doppler depending on storm location.

Despite a drought during much of the operations period, STEPS investigators were able to obtain unprecedented data on a wealth of diverse cases, including LP storms, supercells, and mesoscale convective systems (MCSs), among others (Table 2). Many of the storms produced predominantly positive CG lightning during all or a portion of their lifetimes, and also exhibited unusual electrical structures, such as a possibly inverted dipole.

3. Current STEPS Research

Although a variety of storms passed through the network, supercells were the main focus of data collection in STEPS. Therefore, we have selected two cases to represent the range of supercells observed: a classic supercell that occurred on 29 June 2000, which was observed by every available platform; and an LP supercell that occurred on 5 July.

a. Overview of the 29 June Classic Supercell

The weather scenario for the afternoon of 29 June 2000 was characterized by an unstable airmass in western Kansas, with temperatures near 30 °C and dew points near 15 °C. Winds were 10-15 m s-1 from the south at the surface, veering to 15-25 m s-1 from the northwest aloft, producing sufficient shear for supercell-type storms. Surface dew points decreased toward the west into eastern Colorado, but a distinct dry line was not evident. A short line of convective cells developed around 2200 UTC in the northwest corner of Kansas as a weak upper-level disturbance moving southeastward out of Wyoming approached the more unstable airmass. The convection subsequently moved southeastward, remaining in a multicellular phase for nearly 1.5 hours before making a 35° right turn, as it became more supercellular in character.

Around this time storm size and radar reflectivity increased dramatically and a tornado first touched ground (2328 UTC). The tornado was on the ground for about 16 min and was tracked by the Mobile Mesonets throughout its lifetime. (A description and photogrammetric analysis of the tornado courtesy of Erik Rasmussen is available at The mid-life intensification of the storm radically altered its kinematics, microphysics, and lightning production, based on a detailed analysis of the radar and lightning data (S. Tessendorf and K. Wiens, personal communication, 2003). Prior to the intensification, there was little CG lightning of either polarity and little radar evidence of hail aloft. After intensification and the right turn, large hail began to dominate radar returns aloft (as revealed by polarimetric data) and the storm simultaneously began producing large numbers of +CG flashes. There also was an increase in total lightning activity.

During its lifetime this storm underwent several convective surges, with updraft speeds peaking near 50 m s-1 as estimated by multiple-Doppler synthesis (S. Tessendorf, personal communication, 2003). The most important of these surges was the mid-life intensification mentioned earlier. Peaks in hail production aloft, largely around the altitude of -10 °C, were well correlated with the convective surges, as was +CG lightning production (Fig. 4; surges A and C in Fig. 5). The +CG discharges were usually initiated on the edge of hail regions and progressed into the hail regions and/or into the downshear part of the storm (e.g., Fig. 6). Trends in total flash rates for this storm closely followed trends in volumes of updraft, reflectivity, and hail with maximum flash rates near 300 min-1.

Pulsations in updraft strength also closely matched observations of bounded weak echo regions (BWERs; Browning and Donaldson 1963, Browning 1964, 1965) in the reflectivity field, as well as "holes" in VHF sources detected by the LMA (Fig. 7). The ring-like structure of the lightning hole and the BWER indicates that charged precipitation particles were "wrapped around" the updraft by the storm rotation. The tornado occurred on the western side of the lightning hole and updraft region. Similar observations were obtained with the LMA from a storm producing an F0 tornado during the MEaPRS Project in Oklahoma in 1998 (Krehbiel et al. 2000).

The LMA observations indicate that, in its very initial stages, the storm contained a lower positive and mid-level negative charge; i.e., the lower half of a tripole structure (Fig. 8a; interval 'a' of Fig. 5). The inferred positive charge regions were well-correlated with the locations of significant graupel and hail concentrations, suggesting that the charging resulted from a non-inductive charging process imparting charge to the precipitation-sized particles (e.g., Takahashi 1978). As the storm evolved, the LMA indicated the rapid development of alternating positive and negative charge layers above the lower dipole (Fig. 8b; interval 'b' of Fig. 5). The storm then went on to develop a dominant, deep mid-level positive charge region, with negative charge above the positive (the inverse of a normal polarity dipolar structure) and produced numerous inverted polarity IC flashes between the two charge regions (e.g., Fig. 6). The inverted electrical structure developed less than an hour into the storm and persisted for the remainder of the storm's life.