The purpose of this project is to develop and utilize unmanned aircraft systems (UAS) to obtain critical meteorological observations aloft in the rear flank region of supercell thunderstorms. This is a pilot project with the initial emphasis being on the system development and obtaining experience in utilizing such systems in severe storm environments. If the pilot development project is successful, the Principal Investigators intend to participate in the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX 2). The investigators will utilize the expertise in unmanned aircraft operations of the University of Colorado to develop a system that is sufficiently safe to obtain a Certificate of Authorization for operation from the Federal Aviation Administration. Observations will be used to evaluate specific hypotheses related to the baroclinic generation of vorticity at the rear of the supercell updraft, and subsequent reorientation of that vorticity into the observed counter-rotating low-level vortices. The cyclonic member of this pair appears often to be the antecedent tornadic vortex.
Intellectual Merit Supercell tornadogenesis is the result of a complex series of processes. Evidence suggests that the vorticity in a tornado originates as horizontal vorticity between the supercell updraft and a trailing rear flank downdraft (RFD). The RFD, in turn, appears to be partially the result of small-scale precipitation structures unique to supercells: the hook echo and/or a narrow descending reflectivity core. In some supercells, the vorticity generated between the major vertical drafts is drawn upward in the updraft, leading to arched vortex lines and associated counter-rotating vortices in the rear flank gust front convergence zone. Under certain conditions, that appear to be governed by the degree of negative buoyancy in the RFD, tornadogenesis can occur in the vicinity of the cyclonic member of the counter-rotating pair.
Broader Impacts An important goal of this work is that it will contribute to the development of a rapidly-deployable mesoscale and stormscale UAS sensing system. The system will be suitably designed to greatly reduce regulatory hurdles to its future deployment. Important knowledge will be gained on technical and regulatory issues that will allow future deployments of UAS for weather research.
Collected observations will improve understanding of RFD buoyancy and tornado cyclone genesis. This information may lead to improvements in the tornado warning process via the diagnosis of rear-flank precipitation morphology and hydrometeor structure using Doppler radars with dual-polarization diversity capability. Further, it is anticipated that RFD buoyancy can be estimated through knowledge of the low-level thermodynamic stratification. Eventually, it is likely that operational meteorologists can make much better discriminations between potentially tornadic and non-tornadic supercells.
The Tempest Unmanned Aircraft System (UAS) is the first UAS to collect temperature, pressure, and humidity data in a supercell thunderstorm. Data collected during a flight beneath a tornado-producing supercell show new details that may lead to a deeper understanding of tornado formation. The average tornado warning time is currently about 13 min. The Tempest UAS demonstrated data-gathering capability that may transform the understanding of why and how tornadoes form in supercells. This will lead to improved models and forecasts to increase warning time and save lives. The rear-flank downdraft is a sinking column of air that forms on the backside of a supercell thunderstorm that appears to be directly linked to tornado formation. The supercell is the most violent type of severe thunderstorm and its rear-flank downdraft is too dangerous to be sampled by manned aircraft, and balloons cannot carry instruments through the sinking air. The Tempest UAS was designed and developed by students and faculty of the Research and Engineering Center for Unmanned Vehicles (RECUV) at the University of Colorado Boulder in collaboration with meteorology students and faculty from the University of Nebraska Lincoln and the University of Oklahoma. The primary mission of the Tempest UAS is to track and penetrate the supercell rear-flank downdraft, and other parts of the supercell, for in situ measurements of temperature, pressure, humidity, and wind velocity. The VORTEX2 (second Verification of the Origin of Rotation in Tornadoes Experiment) field experiment was conducted May-June of 2009 and 2010. The Tempest UAS development effort was funded by NSF as a VORTEX2 "pilot project" to demonstrate the capability of a small UAS designed to make atmospheric measurements in severe storms, while satisfying Federal Aviation Administration requirements for operations and safety.
