Airborne Radio Occultation as an Observing System for Tropical Cyclones

Airborne radio occultation (ARO) data are collected as part of an ongoing collaboration with the National Oceanic and Atmospheric Administration (NOAA) to provide thermodynamic profiles in Atlantic and East Pacific hurricanes. An ARO instrument has flown on the NOAA Gulfstream-IV (G-IV) for the 2020, 2022 and 2023 seasons in experimental mode. The technology complements existing dropsonde deployments through the ability to collect profiles away from the flightpath without increasing expendable costs. A typical ARO profile collected from the G-IV aircraft can be described as a curve comprised of hundreds of observations starting at a flight level of approximately 14 km and extending 300 km out to the side of the flightpath and down to an altitude of 4 km. For an aircraft tasked with a synoptic survey such as the G-IV, which may circumnavigate the storm center at a radius of 300 km, this provides the ability to retrieve observations from the inner core and near core environment, an area that is relatively sparsely surveyed by existing dropsondes. In extremely intense tropical cyclones where flying the aircraft through the storm is not feasible, ARO sampled from afar can mitigate missing dropsonde observations in these critical areas. Comparisons between the spatial patterns of dropsondes in hurricanes and the sampling patterns from ARO in storm-centric coordinates are presented by examining their differences in terms of quadrant, radial distance, and storm intensity. With the best coverage between 4-14 km altitude, data are expected to be most useful for research related to impacts of dry air intrusions, evaluating forecast improvements and for their potential for assimilation into hurricane models. For some flights into Hurricanes Franklin, Idalia and Lee, observations were also collected from an advanced ARO signal recorder currently under development that is capable of extending sensing to the ocean’s surface. The 2023 deployment also included the NOAA WP-3 and 10 US Air Force WC-130s for the first time. These aircraft fly at a low flight level into the storm core, contrasting with high altitude synoptic surveys from the G-IV. These recent developments enable the possibility of expanding the utility of ARO from mid-troposphere observations into the lower troposphere and marine atmospheric boundary layer.