High Temporal Resolution Profiles of the Near Storm Environment During the 13 July 2015 Nickerson, Kansas, Tornado

As part of the 2015 Plains Elevated Convection At Night (PECAN) field experiment, a series of five instrumented sites were temporarily deployed in the central Great Plains of the United States. While the purpose of the experiment was to investigate nocturnal convection, these instrumentation sites operated 24 hours a day and captured the environments associated with numerous other meteorological events. Each site contained a high-temporal resolution wind profiler and an Atmospheric Emitted Radiance Interferometer (AERI), a ground-based infrared hyperspectral radiometer that can be used to generate profiles of boundary layer temperature and moisture. Together, these instruments provided near-continuous thermodynamic and kinematic profiles of the boundary layer with a temporal resolution of five minutes.

Perhaps one of the most interesting events captured by one of the PECAN observation sites was the 13 July tornado that formed near Nickerson, Kansas, in the south central part of the state, and approximately 50 km west of one of the fixed PECAN observing sites. With a path less than 10 km long and a rating of EF 3 on the Enhanced Fujita scale, this tornado was notable for a number of reasons. First, the surface temperatures were extremely warm and dry, with temperatures in excess of 35 °C and dew point depressions more than 10 °C in magnitude. As a result, cloud bases were high-based, with lifting condensation levels more than 1.5 km above the surface. Second, somewhat unusually for this region, supercell motion was towards the southwest. Finally, environmental storm-relative helicity observations were extraordinarily low; in fact the environment was much more similar to one that supports non-supercellular tornadoes than the supercell-based one that did form. As a result of the extremely dry air and weak kinematic support, forecasters did not consider the region as a potential location for tornado development and neither a tornado nor a severe thunderstorm watch was issued.

Due to the atypical storm motion, the fixed-site observations remained within the storm inflow throughout its life cycle despite being situated east of the storm. Therefore, they can provide insight into the initiation and development of this event. The thermodynamic profiles indicate that a strong nocturnal inversion mixed out after sunrise enabling deep mixing, but residual weak inhibition could not be overcome by surface-based convection due to the extreme dryness of the air at low levels. The arrival of a weak outflow boundary provided enough lift for parcels to reach the level of free convection (LFC), and subsequent parcel accelerations were enough to tilt and stretch ambient vertical wind shear that was generated at the intersection of the outflow boundary and a pre-existing pressure trough. Time series of stability parameters reveal both the promise and limitations of continuously operating environmental profilers.