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On the Trail of Thunder: Studying Storms First-Hand

Late spring is when many NCAR scientists find their thoughts turning to convection. For those who study thunderstorms in the Mesoscale and Microscale Meteorology Division (MMM), this is prime time for field work. Here is the latest word on two such programs this spring and summer involving staff from MMM, the Atmospheric Technology Division (ATD), and UCAR's Office of Field Project Support (OFPS). See the February issue of Staff Notes Monthly for details on another project, the Small Cumulus Microphysics Study, taking place in Florida.


Image of 22 May tornado

Pictured above is a composite image of reflectivity (dbZ) in the Garden City storm of 16 May as observed by ELDORA at a height of one-half kilometer. The Electra's path is indicated by the plane icons at left; an icon is shown for every eighth scan taken by ELDORA between 4:16 and 4:21 p.m. local time. The storm's core (darkest gray) is just north of the hook echo that encompassed the tornado. (Image courtesy Wen-Chau Lee and Susan Stringer, ATD.)

Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX)

It's been called "the largest tornado chase in history." Yet VORTEX is much more systematic than the usual storm chase, and its goals are different. The project aims not just to study tornadoes but also to test a set of hypotheses that involve documenting many aspects of long-lived supercell thunderstorms, whether or not they are tornado producers. A giant array of vehicles has been pursuing individual storms and gathering some of the densest data sets ever collected on supercells.

VORTEX is a multi-institutional program, now ending its second of two field seasons. It's based at the National Severe Storms Laboratory in Norman, Oklahoma. Major players include NSSL, NOAA, the University of Oklahoma and its Center for Analysis and Prediction of Storms (CAPS), and NCAR. On hand for much of the experiment have been a host of scientists, programmers, technicians, and pilots from MMM, ATD, and OFPS.

Scheduled to run from 1 April through 15 June, VORTEX has encountered plenty of severe weather in its study region, which runs from southern Kansas to western and northern Texas. Getting to the tornadoes has not been quite so easy, but by early June, several twisters had been intercepted by the full VORTEX armada. This set of vehicles includes 12 cars equipped with roof-mounted weather stations that take observations every five seconds, whether the vehicle is at rest or in motion. Also collecting data are several portable Doppler radars; ATD's cross-chain Loran atmospheric sounding systems (CLASS), both fixed and mobile; and two aircraft, the NOAA P-3 and the NCAR Electra, each equipped with dual Doppler radar.

The aircraft, with their ability to respond quickly to changing weather over a large area, have found some of the best opportunities for gathering data. A major tornado or tornado family ripped across 80 kilometers in south-central Oklahoma on 7 May. While VORTEX ground teams were tracking another storm more than 100 kilometers away, the Electra zipped to the tornadic cell and its Electra Doppler radar (ELDORA) collected data on a well-developed mesocyclone, a circulation that precedes most major tornadoes. ELDORA had even better luck in western Kansas on 16 May, catching its first actual tornado (see "Noteworthy") and the twister's 20-minute development process. The data quality from ELDORA's first field test at full capacity has been "excellent," says Peter.

Photo of ELDORA aircraft in flight

The largest single instrument ever developed at NCAR, the Electra Doppler radar (ELDORA) aboard this aircraft demonstrated its full capabilities for the first time as it tracked tornadoes over the southern Plains for VORTEX. (Photo by Bob Bumpas.)

Bruce Morley, project manager for the Research Aviation Facility, accompanied many of the Electra flights in May. "We're flying next to supercells," says Bruce, "but we're normally on the inflow side. We usually have very little precipitation and good visibility." Henry Boynton, Mike Heiting, and Jerry Tejcek have been at the Electra's controls, while ELDORA engineer Craig Walther and master technician Rick Parsons have operated the on-board radar system. ATD's Wen-Chau Lee and Peter Hildebrand, flying as project scientists, have worked with Sherrie Smith and Susan Stringer on data analysis, both aboard the Electra and in Norman.

Another new development, Doppler on Wheels, has found success in its debut season. This collaboration between NSSL, NCAR, and OU is a fully portable, pulsed Doppler radar that sits on a flatbed truck. Its high-resolution capacity means that a storm or tornado can be sampled in volumes as small as 75 meters on each side. The hardware design was led by OU assistant professor Josh Wurman, who recently concluded two years in ATD. Signal processing is accomplished through PC integrated radar acquisition (PIRAQ), a creation of ATD's Mitch Randall that packs traditionally bulky radar-processing tools into a personal computer. On its first day in the field-16 May-Doppler on Wheels collected data from within ten kilometers of a long-lived tornado near Jetmore, Kansas, that succeeded the twister tracked by ELDORA earlier in the day.

Photo of Jim Moore

Jim Moore. (Photo by Bob Bumpas.)

Back in the Norman operations center, OFPS has kept data collection humming throughout the experiment. Dick Dirks, Jim Moore, Scot Loehrer, and José Meitin have been in Norman orchestrating CLASS launches and special soundings from six National Weather Service sites, relaying data to project nowcasters (short-term forecasters) and the field coordination team. One key to the CLASS work, says Jim, has been finding and training residents of the small Oklahoma towns where fixed sites were installed for the spring. "It saves money and gets the local people involved. One guy in Woodward has five sons who get out and help him. They enjoy it-it's a good exchange all around."

The OFPS work isn't over with the field phase. The office maintains a variety of operational supporting data (satellite imagery, computer models, radar data, and more) and works with NSSL to perform quality-control checks on what will become the experiment's final archive. This is accomplished using the the Cooperative Distributed Interactive Atmospheric Catalog System (CODIAC), an on-line archive now available for browsing and retrieving data through the World Wide Web. In 1991, three NOAA staff at OFPS on detail for the U.S. Weather Research Program-Wayne Brazille, Ron Murdock, and Dave McGuirk-created CODIAC. Its first big test was making available mountains of data from the Stormscale Operational and Research Meteorology program's Fronts Experiment Systems Test (STORM-FEST). Since then, CODIAC has cataloged hundreds of gigabytes of data from several major field programs.

Image of VORTEX study area

The VORTEX study area and various sources of data from the National Weather Service and other sources are shown above. UCAR's Office of Field Project Support has coordinated the operation of fixed CLASS soundings and the archiving of other VORTEX data. (Illustration courtesy VORTEX.)

All data from the 1994 VORTEX field phase, along with a complete operations summary for that year, is available on the Web via CODIAC. A preliminary summary for 1995, built as the project evolves, has given users up-to-date information on the experiment's
progress. The summaries can be found on the VORTEX home page at http://antietam.nssl.uoknor.edu/mosaic_files/vortex.html

Sailplane Studies of Early Electrification (S2E2)

While the southern Plains' storm season winds down, the eastern Colorado season is in full swing. Dan Breed (MMM) is heading a modest project with a significant goal: finding ways to connect radar measurements to the process that lends charge to an embryonic thunderstorm.

Photo of Dan Breed

Dan Breed. (Photo by Bob Bumpas.)

To do this, he'll be sending a Schweizer SGS 2-32 sailplane out for its first Colorado-based research project in 17 years. The sailplane has been used by NCAR for cloud microphysical and storm electrification studies in various locales since the late 1960s. It's ideal for riding the strong thermals in a developing storm before heavy precipitation and downdrafts make the conditions overly turbulent. This fall a new IMAX film, "Stormchasers," will include airborne footage of the sailplane in its opening segment.

The first links between sailplane data and fine-resolution radar data were made through a 1984 field project in New Mexico involving Dan and MMM senior scientist Jim Dye. They found that electric field strengths can triple within a developing storm in as little time as a minute or two. Follow-up studies took place in 1991 as part of the Florida-based Convection and Precipitation/Electrification Experiment (CaPE).

The goal of this summer's work is to expand the Florida and New Mexico findings to Colorado's cooler, drier climate and to clarify the kinds of particle growth taking place during early electrification. When ice particles collide within a young storm, the results lead to-or at least enhance-the storm's basic charge separation pattern (positive charge aloft, negative charge at mid- and low levels). The details of specific modes of exchange, however, have yet to be worked out. Led by results from cloud chamber experiments, Dan suspects that wet growth (ice in graupel form colliding with water droplets in a high-liquid water content environment) versus dry growth (collection and freezing of smaller droplets at low liquid water contents) is a key distinction that controls how electrification proceeds.

Piloted by Boulder pilot and instructor Bruce Miller, the sailplane will venture forth from Jeffco on promising days, towed by a Cessna 180. The target area is in and around Greeley, where the University of Chicago/Illinois State Water Survey (CHILL) dual-polarization radar, now operated by Colorado State University, will scan the skies for S2E2 and several other field programs. CHILL's detection of signals at two polarizations allows scientists to estimate the sizes of precipitation particles and to determine whether rain or hail predominates in a given part of a cloud. From a starting altitude of around four kilometers, the sailplane will ascend in tight circles less than a kilometer wide up to heights that may exceed ten kilometers, collecting overall electric field data and sampling precipitation particles to measure their size, shape, and individual charges.

Photo of sailplane

The NCAR-operated sailplane heads out of a towering cumulus cloud. (Photo by Bob Bumpas.)

Dan hopes to emerge from the data collection and analysis with a set of thresholds that can be used to gauge the onset of electrification-and thus lightning potential-from radar data. "There are a number of emerging uses of lightning data, ranging from nowcasting to global warming signatures. If we can help establish the physical basis for the development of lightning, it should aid forecasts and other interpretations of those data." --BH


Noteworthy

"I suppose it was the T-shirts that did it," said ATD's Peter Hildebrand the day after ELDORA scored its best data yet in VORTEX on 16 May. As elaborated in an e-mail update, Peter and several colleagues had gone to a brewpub in Norman for dinner the night before. "There we found a local brew called Tornado Alley and T-shirts advertising this well-named brew," which they snapped up: "The plan was to wear the T-shirts until we got a tornado." The crew found success the next day with the help of the Electra's nose-cone radar, newly adapted as a weather radar through Mitch Randall's PIRAQ technology. "The nose radar immediately indicated areas of circulation in the storm . . . that evolved into hook echoes over the next half hour or so." After seeing a number of small-scale circulations, the Electra passengers witnessed "a nice, very distinct, rope-like tornado. . . . These data appear to be exactly what was needed for VORTEX." Peter concluded, "We'll wear the Tornado Alley shirts again." (Back to ELDORA paragraph)


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Edited by Bob Henson, bhenson@ucar.edu
Last revised: Wed Mar 29 16:34:08 MST 2000