Development of UAV-Deployed Air-Launched Drifters (ALD) for Aboveground Thermodynamic Measurements in Supercells

Aboveground thermodynamic and wind measurements in supercells are challenging for both ground-based and airborne sensing systems. Outflow winds behind the rear-flank downdraft (RFD) gust front make the RFD particularly difficult to access with balloon-borne sensors launched from the ground, since strong winds in the horizontal outflow force balloons away from typical regions of interest. A goal of the NSF National Robotics Initiative project “Severe-storm Targeted Observation and Robotic Monitoring” (STORM) is to develop new in-situ atmospheric sensing applications through targeted observations. STORM is a three-year collaboration of researchers from the University of Colorado Boulder (CU), University of Nebraska-Lincoln (UNL), and Texas Tech University (TTU). This paper describes the development and deployment of the Mistral small UAS (sUAS) with an integrated semi-Lagrangian drifter deployment system. The Mistral is the most recent in the line of sUAS, developed at CU, for making meteorological measurements in supercells. These sUAS include the Tempest, first deployed in May 2010 during the “2nd Verification of the Origins of Rotation in Tornadoes Experiment” (VORTEX2), and the TTwistor, a twin-motor derivative of the Tempest first deployed in October 2016 during phase-1 of the NOAA “Environmental Profiling and Initiation of Convection” (EPIC) project. TTwistor was later deployed in May 2017 for EPIC phase 2, and again in June 2017 for the NSSL “Rivers of Vorticity in Supercells” (RiVorS) project.
The Air-Launched Drifter (ALD) system is designed to deploy ALD balloons at an altitude above the supercell outflow, onto trajectories that entrain the balloons into the RFD or other regions of interest, a few kilometers above the ground. The Mistral carries a helium tank to inflate and release up to four drifters during a single flight. Each drifter consists of small sensor board populated with a GPS receiver, pressure, temperature, and humidity sensors, and a communications radio. When deployed, the sensor board hangs from a tether attached to a small mylar balloon that is inflated and released during flight. Telemetry and scientific data are collected from multiple fixed and mobile ground station receivers.
Simulation results are shown for balloons released into the wind field of a high-resolution supercell simulation to target potential balloon release points expected to create trajectories favorable for penetration of the RFD. Results from Mistral airborne-release tests are shown, along with results from a ground-release test during a supercell intercept in the Oklahoma Panhandle on 12 June 2018, where position and meteorological data were delivered from the supercell for a duration of 2 hours and from a distance of greater than 120 km as the mobile ground stations moved away from the storm.