Tornadoes are very important because they are a life-threatening public hazard. It is assumed that our ability to provide more timely warnings of them requires a better physical understanding of what causes and sustains them. To this end, this research grant has improved our understanding of why tornadoes form, what modulates their intensity, and what determines the direction and speed of their motion. 

Our approach was to use a state-of-the art, truck-mounted, rapid-scan, Doppler radar that has polarimetric capability. This instrument, when deployed near (so that they are visible), but at a safe distance (so as not to be blown over or hit by debris) from tornadoes, can provide details of the wind, precipitation, and debris fields in and around tornadoes as they form, evolve, and dissipate. At the same time the radar collected data, photographs and videos were taken, and afterward damage surveys were conducted. All of the aforementioned were combined to improve our understanding of tornado behavior.

During the course of this study the radar, named RaXPol (rapid-scan, X-band, polarimetric), probed numerous supercell storms, the type that most commonly produce major tornadoes, in portions of the Plains of the U. S., over a four-year period, mainly during the late spring. The rapid-scan ability allows us to observe tornado evolution with much better temporal resolution than conventional radars allow. The polarimetric capability allows us to determine with high likelihood what type of targets are being scanned by the radar. Being in the right location at the right time is most challenging and we made our own forecasts based on both observations in the operational network of instruments maintained by the National Weather Service, and on computer models run by NOAA. The most significant findings supported the importance of the tilt of the tornado vortex, the asymmetry of the airflow around the vortex, the rotation of tornadoes around a parent, wider vortex, and updrafts and downdrafts in the parent storm near the parent vortex. 

A significant serendipitous finding was that the formation and structure of a rare anticyclonic (rotating in the opposite direction of the Earth’s rotation about its axis, i.e., in the Northern Hemisphere, in a clockwise manner) tornado was documented. It was found this tornado was much shallower than most, typical cyclonic tornadoes and formed when sinking air hit the ground and rotated around a companion cyclonic tornado, which was dissipating, and hit the leading edge of earlier-produced sinking air.

Another significant development was that for the first time, raw (as opposed to processed, data), at slow scan (a slower scanning rate is needed to collect enough samples to come up with acceptably accurate target motions), were collected in a tornado debris cloud for an extended period of time, allowing for polarimetric spectra to be computed. This analysis is continuing with support from a subsequent NSF grant. Polarimetric spectra allow us to not only identify tornado debris clouds, but also to determine when there is a mixture of debris, whose motions do not follow the wind, and precipitation type (in an intense vortex the debris is centrifuged radially outward), whose motions, especially those of raindrops or dust, more closely follow the wind. We need to disregard debris motions when trying to determine what the true wind field is.

This research grant supported, to varying degrees, four graduate students, who gained valuable knowledge when participating in field programs and processing and analyzing real data they collected, while thinking about what physical effects are important. The students will join the current workforce and in the future contribute to public service (in the National Weather Service and private companies) and to research efforts (at universities and national laboratories). Publications and media coverage provided both colleagues and the public with an insight into how scientific research is conducted and more widely disseminated the knowledge that was acquired.

Last Modified: 03/20/2025
Modified by: Howard B Bluestein