ATDD Aiding Efforts to Understand Tornado Formation in the Southeastern U.S.
Temple Lee and Michael Buban of ARL’s Atmospheric Turbulence and Diffusion Division (ATDD) are key participants in an ongoing field campaign called Meso 18-19, along with colleagues from NOAA’s National Severe Storms Laboratory, Earth System Research Laboratory Global Systems Division, National Weather Service (NWS), and multiple universities. Meso 18-19, named for the calendar years during which it takes place, involves the collection of meteorological observations from weather towers, upper air soundings taken via radiosondes (instruments that measure and transmit atmospheric parameters) tied to weather balloons, and observations of temperature and moisture from ATDD’s small unmanned aircraft systems (sUAS).
Meso 18-19 is a follow-on to the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast, or VORTEX-SE, project conducted in Northern Alabama from 2016-2017. For VORTEX-SE, ATDD staff conducted over 100 weather balloon launches at pre-determined experiment sites and installed two 30-foot weather towers complete with instruments to measure temperature, wind, moisture, rainfall, energy exchange between the land and atmosphere, pressure, and radiation. One of the towers is being utilized again for Meso 18-19 as the team attempts to answer the same scientific questions, this time for a much larger region stretching from eastern Texas to northeast Tennessee.
Scientists have hypothesized that the physical processes important to tornado formation in the southern and central plains differ from those in the Southeastern U.S. Therefore, the goal of Meso 18-19 is to gather essential information that will enable a better understanding of how changes in the upper atmosphere contribute to the development of severe thunderstorms and tornadoes in this region. Experiment dates deliberately coincide with the “cool season” in the Southeast, during the months of November through April; a period recognized for enhanced tornadic potential.
Ten intense observation periods are scheduled during Meso 18-19, each lasting roughly 48-hours, when radiosonde launches will be conducted four times per day, or every six hours, at locations approximately 100 miles apart. All participants will maintain the same schedule using similar instrumentation. In comparison, the NWS conducts weather balloon launches only twice per day, or every 12 hours, with 400 miles or more in between locations. Repeated radiosonde launches from the same locations will be key to understanding how the atmosphere changes throughout the day, as well as how those changes affect the development of severe thunderstorms. Developing an accurate representation of the atmosphere requires a long-duration series of observations, something that is not possible during shorter campaigns.
Lee and Buban will also conduct sUAS soundings at a site approximately 20 miles north of Knoxville, Tennessee, where they routinely conduct scientific test flights with the lab’s drones. The site was selected, in part, to help fill a gap in soundings between other sites. With an instrument package mounted directly to its platform, the sUAS will collect the same meteorological information as the balloons, at the same vertical resolution, but with a height restriction of 3,000 feet. Using an sUAS will enable scientists to perform multiple profiles with the same instrumentation and will provide more detailed information on the evolution of the lower atmosphere prior to forecasted severe weather events.
The schedule for the Meso 18-19 campaign was determined by a planning committee that reviewed the science objectives in concert with the optimal timeline, weather, and grant funding. The resulting data from this larger, richer suite of observations will be assimilated into multiple experimental NOAA models in real time. Scientists are hopeful that these efforts will result in improvements to the models’ forecasts of temperatures, dew points, and atmospheric instabilities that will help forecasters to better pinpoint locations where severe storms are likely to form.
About balloon launches
Balloon launches, the “gold standard” for accurate measurements, have been used in the meteorological community for nearly 100 years. They involve tying a radiosonde 30 feet below a weather balloon, where it dangles on a string at the mercy of the wind during its ascent into the atmosphere – often to 50,000 feet or higher. Each radiosonde measures approximately 10-15 centimeters wide, carries instruments and sensors, and is battery powered. Wind speed and direction, pressure, temperature, and relative humidity are among the measurements taken, and transmitted by, the radiosondes to a ground computer.