One of our main goals was to understand why some storms with rotation well above the ground (supercells) produce tornadoes and some do not.  We performed idealized studies using computer simulations based on an observed case from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign to examine how supercell thunderstorms were affected by changes in the environmental wind profile.  In particular, we used conditions near a supercell that made a tornado and those near a supercell that did not, and we found that environmental changes in wind speed and direction with height (i.e., vertical wind shear) over the lowest 1 km were a distinguishing feature.  This low-level shear was important for the formation and intensification of strong vortices near the surface.  Additionally, when this 0-1km shear is increased in time as a storm matures, stronger vortices are produced than in simulations for which this shear does not increase in time.  These differences are tied to changes in the storm, including stronger rotation at the 1 km AGL level and greater ability to contract (e.g., “spin-up”) the near-surface rotation.  A storm’s response to increasing shear is sensitive to the timing of the shear increase.  These findings are helpful for understanding the changing tornadic potential of storms during the early evening transition (i.e., the period near sunset) when shear often increases.

We next investigated the reasons that a nearly tornadic case from the VORTEX2 field campaign failed to produce a tornado, using wind observations from mobile radar along with in situ thermodynamic measurements from instrumented cars.  Several factors resulted in the inability to produce a tornado: outflow underneath the storm was quite cold, air did not follow a favorable trajectory to result in rotation reaching the area where it could be amplified to tornado strength, and there was an unfavorable orientation of the shear within the near-storm environment, leading to weaker upward motion above the near-surface circulation.  Understanding the reasons for tornado failure in marginal environments is crucial for improving tornado warnings.

As a supplement to this grant, we were able to develop and deploy a new method for obtaining temperature, humidity, and pressure measurements above ground within supercell storms using sensors that approximately follow the air current within the storm.  These were the first 3-D measurements of this type ever obtained in a supercell storm, and more measurements like these will be crucial for understanding tornado formation.

This project also included a study of the effects of terrain (hills, mountains) on tornado potential within supercell thunderstorms.  Computer simulations are necessary because in the case of observed storms passing over terrain features, it is not possible to know how the storms would have behaved in the absence of terrain.  However, in computer simulations, the terrain can be removed while keeping everything else the same.  Our study found that tornado potential can be enhanced within eastward-moving supercell storms that pass over the southern flank of a hill/mountain.  If tornadoes occur owing to terrain-enhancement effects, they tend to occur on the southeast slope of the hill, or occasionally east of the hill/mountain.  However, the outcome is very sensitive to small changes in the track of the storm relative to the terrain feature.    

Lastly, the project included a study of how localized cold anomalies can influence tornadoes.  Such "surges" of cold air frequently occur in thunderstorms in conjunction with downdrafts or rain shafts.  In high-resolution computer simulations of surges introduced near tornadoes (or developing, incipient tornadoes), it was found that weak cold surges, associated with cold pockets less than 2 degrees Celsius colder than the ambient temperature, could temporarily intensify tornadoes.  However, strong cold surges, associated with cold pockets 2 or more degrees Celsius colder than the ambient temperature, abruptly weakened tornadoes, though occasionally the abrupt weakening was preceded by brief intensification similar to the case of a weak cold surge.  The outcome seems to depend on a delicate balance: a well-positioned cold surge can force existing angular momentum nearer to the axis of rotation (like forcing a figure skater to draw their arms inward), but cold air's eventual ingestion into the overlying storm updraft can weaken the updraft and facilitate the storm's demise.  The findings have implications for understanding short-term tornado behavior and perhaps even the prediction of tornado formation.

Last Modified: 12/29/2020
Modified by: Yvette P Richardson