SECTION 1: PROJECT OBJECTIVES AND ACCOMPLISHMENTS
Project objectives: 1) To document the primary atmospheric processes that accompany winter lightning across the southeast U.S. and suggest relationships of lightning flash density, polarity and multiplicity with location, intensity, and duration of winter precipitation. Processes that distinguish cases with abundant lightning vs. those with little lightning were of particular interest. 2) To formulate a classification routine for lightning events based on the spatial and temporal pattern of lightning and primary atmospheric processes. 3) To integrate cloud-to-ground lightning flash data with other data sets to forecast and nowcast type, amount, and spatial extent of winter precipitation. This will be accomplished via utilization of research software, but may easily be applied to AWIPS.
Participant Responsibilities: Seven case studies were identified, and these were distributed equitably to participants at the three institutions (UGA, WFO DDC, and the National Severe Storms Laboratory [NSSL]). Although the case studies were carried out independently at each locale, there was frequent coordination/discussion via telephone, email and meetings.
R&D Activities and Accomplishments: The centerpiece of each case was a survey of spatial and temporal lightning patterns. This was accomplished via the overlay of cloud-to-ground (CG) flash locations and polarities on WSR-88D radar reflectivity data. The primary software tool in this endeavor was KEDIT, developed by Kurt Hondl of NSSL. A secondary tool was the WSR-88D Algorithm Testing and Display System (WATADS). Karen Cooper of NSSL ported a lightning-capable version of WATADS to an HP workstation for use by Hunter. Finally, synoptic analyses of each case were done using various data sources. The WSR-88D U.S. composite 2km, 15min instantaneous reflectivity data, which proved very valuable later in the project. This was because scripts were eventually developed to overlay lightning data on these composites, which encompassed a much larger area than that covered by a single radar. This is important because the precipitation shields and associated lightning patterns were usually more extensive than a single radar's umbrella. The composite reflectivity and lightning locations were displayed via several software packages, including WATADS and KEDIT.
Per objective 2) above, we proposed a binary classification of winter lightning storms based on the seven cases. First are those associated with a rapidly translating, intensifying cyclone. These cyclones initially moved slowly east or southeast in the south-central states then accelerated to the northeast over the eastern third of the country. The lightning produced by these lows were almost entirely confined to the warm sectors, far removed from any frozen precipitation. The CG flashes in these cases came almost exclusively from static instability and upright convection and flash densities were large. The second classification involves a quasi-stationary arctic cold front oriented west-east or southwest-northeast. Strong southwest flow aloft, caused by the southern branch of a split polar jet stream, produced strong isentropic upglide and warm moist advection over the shallow frontal surface. The southwest flow pattern was alluded to for other winter lightning studies by Holle and Cortinas (1999). Although most lightning was found south of the arctic boundary, there was substantial elevated convection and some lightning above the front that produced freezing precipitation as it fell through the cold air mass. Although these cases are of greater interest to the exploration of the lightning/convection relationship to heavy frozen precipitation, the first type may generate a larger number of CG flashes in winter over the southeast U.S.
The second type, that with the quasistationary arctic front, exhibited lightning flash locations in or near the subfreezing air and heavy frozen precipitation. Sounding and model initialization data indicated static instability above the cold air that could easily support elevated convection there. Nevertheless, there were also local maxima of elevated (most evident in the 850 - 700 hPa layer) frontogenesis that was nearly collocated with bands of heavy frozen precipitation, similar to Trapp et. al (1999). Further contributing to upward vertical motion were ageostrophic circulations attending extremely powerful upper jet streaks. In two of the three arctic front cases, vertical motion may have been augmented by coupling of circulations near the southern and northern branches of a split polar jet stream (Hakim and Uccellini, 1992). The resulting upper divergence likely joined frontogenesis and the isentropic upglide to produce full tropospheric vertical motion and widespread heavy frozen precipitation in the arctic air mass.
We hypothesized from the outset that convection (erect or slantwise) near or above the arctic frontal surface generated ice crystals near the convective cloud tops. These were subsequently advected (generally northeast) toward the subfreezing airmass that contained a stratiform precipitation area. The ice crystals fell into the stratiform cloud and "seeded" that region, thereby enhancing the (frozen) precipitation there. This is similar to the "seeder-feeder" process treated by Houze (1993), chapter 6.2.
"Lessons Learned": At the outset we thought that Convective Symmetric Instability (CSI) might play a major role in generating slantwise convection and lightning near frontal surfaces. A few months into this study we encountered the work of Schultz and Schumacher (1999), that exposed the wide misunderstanding and misapplication of CSI. CSI as it has been commonly evaluated is actually Potential Symmetric Instability (PSI). The difference between the two has to do with parcel vs. layer lift and using saturated equivalent potential temperature and simply equivalent potential temperature, respectively. When the lifted volume of air becomes saturated, PSI and CSI are equivalent and the old method of calculating CSI will serve for both. In the interesting arctic front cases, this is likely the case. Both are subtypes of the general classification MSI. Although no conclusive evidence of Moist Symmetric Instability (MSI) was found in these regions, MSI and frontogenetic circulations have been intimately linked in the literature. Therefore the presence of MSI, which would give rise to slantwise convection, cannot be ruled out in such cases.
As stated heretofore, however, there was strong evidence for static instability above the frontal surfaces in the arctic front cases. In such a case, the growth rate of upright convection is much greater than that of slantwise convection and so the former would come to be the dominant convective mode. Static instability was indicated by the presence of large 70-50 hPA temperature differences of 13-18 EC, which are large values for winter (Janish et al., 1996). Axes of such large lapse rates above the cold air were also nearly coincident with aforementioned axes of 85-70 hPA frontogenesis, suggesting that slantwise and erect convection may have been at least initially coexistent.
Another clear pattern emerged from scrutiny of the synoptic data. This had to do with the presence of a "split flow" pattern, either before or during the winter precipitation event. This split flow was present in all but one of the seven cases, resulting in a northern and southern branch of the polar jet stream. In all but one case, the split flow eventually merged to form one extremely powerful jet stream. We believe that this often occurs with major winter storms, with the southern jet providing warm moist air and the northern jet arises because of the presence of unusually cold air to its north. When and where the jets merge the warm moist air is lifted over the cold air mass and precipitates into it, the precipitation subsequently freezing by the time it reaches the surface. The timing of the jet merger relative to heavy precipitation onset did not seem to be distinct between the arctic front and migratory cyclone cases. A strong southern branch of the polar jet has been associated with the El Nino Southern Oscillation (ENSO). Buechler et al. (1999) found a statistically significant increase in lightning frequency over the Gulf of Mexico when compared to a 10-year climatological mean. This increase was not evident over the southeast U.S., however. None of the seven cases presented here occurred during a major ENSO winter.
Finally, we wish to comment on the climatology of winter thunderstorms. Although seven cases does not an extensive climatology make, we find Fig. 1 of Hunter et al. (1998) suggestive. This figure outlines the geographic extent of the heavy frozen precipitation impacts from the seven storms. The impacted areas tend to be further south in the central U.S. than in the eastern part of the country. This gives a composite orientation of southwest-to-northeast. This may be a reflection of the tendency of winter migratory cyclones to propagate in this direction. It is also probably a manifestation, particularly in the arctic front cases, for polar or arctic outbreaks to penetrate further south in the Plains compared to the eastern third of the country, because of the damming effect of the Rocky Mountain chain. An extensive climatology is offered by Holle and Cortinas (1998), who studied thunderstorm observations when surface temperatures were at and below freezing. They showed maxima of such thunderstorms in the central U.S. (from north Texas through Iowa) and near the Great Lakes (see their Fig. 1). Their central U.S. maximum is to the northwest of our cases, but this is partly because our initial focus was the southeast U.S. and our cases were chosen accordingly. Another explanation is that our cases were largely extreme events, where cold air outbreaks sagged deep into the southern U.S., well south of their "normal" climatological reach.
SECTION 2: SUMMARY OF UNIVERSITY/ NWS EXCHANGES
During the past year, four research meetings have been conducted: Chattanooga, TN (6/97), Athens, GA (4/98), Morristown, TN (4/98), and Norman, OK (7/99). In addition to these meetings, we have used several conference calls and various forms of electronic communication, particularly e-mail. Materials used in the investigation were placed on a web site so that all participants could access material. Steve Hunter and have also met with Dave Schultz about Moist Symmetric Instability and Karen Cooper about using WATADS for our analyses.
SECTION 3: PRESENTATIONS AND PUBLICATIONS
Hunter, S.M., R.L. Holle, S.J. Underwood, and T.L. Mote, Winter lightning in the eastern U.S. and its relation to heavy frozen precipitation. Manuscript in preparation for submission to Weather and Forecasting.
Hunter, S.M., S.J. Underwood, R.L. Holle, and T.L. Mote, 1998: Winter lightning in the southeast U.S. and its relation to heavy frozen precipitation. Preprints, 19th Conf. on Severe Local Storms, Minneapolis MN, Amer. Meteor. Soc., 701-704.
Steve Hunter presented a talk at the second annual High Plains Conference jointly sponsored by the High Plains Chapter of AMS/NWA and Hastings College, August 18, 1998 at Hastings, NE.
Steve Hunter presented a seminar on Moist Symmetric Instability at the Dodge City NWS office in March, based on the items in "Lessons Learned."
SECTION 4: SUMMARY OF BENEFITS AND PROBLEMS ENCOUNTERED
Thomas Mote moved from the University of Georgia to the University of North Dakota during the middle of the project period, then returned to the University of Georgia near the end of the period. This substantially slowed his contribution to the project. (He retains an adjunct appointment at the University of Georgia.) The Northwest Airlines strike left Jeff Underwood (a doctoral student at UGA who is leading the research there) unable to attend the Minneapolis SLS conference in September. This conference was supposed to serve as a meeting for all involved in the project.
The identification of quasistationary arctic fronts with strong upper jet circulations (at times coupled) as the primary precursor to lightning/convection in or near the front is a valuable forecast tool. When lightning is seen in or near the subfreezing airmass in such cases, forecasters should be aware of the potential for convective (erect or slantwise) enhancement of frozen precipitation in that airmass. The forecaster will not be able to use lightning data in such a predictive manner in the case of migratory cyclones with lightning confined to their warm sectors.
The ability to overlay low-level (85-70 hPA) frontogenesis and static stability fields (e.g. 70-50 hPA temperature difference) may give the forecaster advance notice of the potential for slantwise and erect convection, respectively. This may be in advance of the first CG lightning from either type of convection. The subsequent flash density or frequency is probably a far better forecast (or nowcast) tool for gauging the magnitude of ice crystal seeding of the stratiform region in the cold air and resulting frozen precipitation amounts. Now that all WFO's have CG display capabilities with AWIPS, this should present a new frontier of opportunity for forecasting and future research.
REFERENCES
Buechler, S.J., S.J. Goodman, E.W. McCaul, and K. Knupp, 1999: The 1997-98 El Nino event and related lightning variations in the southeastern United States. Proceedings, 11th International Conf. on Atmospheric Electricity, Guntersville, AL, NASA MSFC, 519-522.
Holle, R.L., and J.V. Cortinas, 1998: Thunderstorms observed at surface temperatures near and below freezing across North America. Preprints, 19th Conf. on Severe Local Storms, Minneapolis MN, Amer. Meteor. Soc., 705-708.
Houze, R.A., 1993: Cloud Dynamics, Academic Press, San Diego CA, 570 pp.
Hunter, S.M., S.J. Underwood, R.L. Holle, and T.L. Mote, 1998: Winter lightning in the southeast U.S. and its relation to heavy frozen precipitation. Preprints, 19th Conf. on Severe Local Storms, Minneapolis MN, Amer. Meteor. Soc., 701-704.
Janish, P.R., C.A. Crisp, J.V. Cortinas Jr., and R.L. Holle, 1996: Development of an ingredients based approach to forecasting hazardous winter weather in an operational environment. Preprints, 15th Conf. Weather Analysis and Forecasting, Norfolk VA, Amer. Meteor. Soc., 56-59.
Schultz, D.M., and P.N. Schumacher, 1999: The Use and Misuse of Conditional Symmetric Instability. Accepted for publication in Monthly Weather Review. Also located at on the World Wide Web at: http://www.nssl.noaa.gov/~schultz/csi/csimwr.html.
Trapp, R.J., D.M. Schultz, A.V. Ryzhkov, and R.L. Holle, 1999: Diagnosis of a winter precipitation event using dual-Doppler and dual-polarization radar data. Preprints, Eighth Conf. on Mesoscale Processes, Boulder CO, Amer. Meteor. Soc., 170-174.