In-situ Microphysical Observations of the Tropical Cyclone Mixed-Phase Region

Cloud microphysical processes play a crucial role in the organization and intensification of tropical cyclones (TCs). Despite this scientific importance, direct in situ measurements of hydrometeors above the freezing level in TCs are rare due to challenges with aircraft icing in the mixed-phase region. A novel approach to collecting in situ measurements in the mixed phase region was conducted during the 2020 - 2023 Atlantic Hurricane seasons through the use of microphysical spiral ascents and descents with the National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft. The spirals limit the time the aircraft spends in potential icing conditions and allow for in-situ measurements in quasi-columns within the stratiform region of TC rainbands. Microphysical spirals have been conducted previously in continental mesoscale convective systems during the BAMEX (2003) and PECAN (2015) field campaigns, but these new measurements are the first of their kind in TCs. Spiral ascents and descents were conducted in Teddy and Delta (2020), Ida and Sam (2021), Earl and Ian (2022), and Tammy (2023) from altitudes of 3 km up to 7 km altitude. The measurements were conducted as part of a joint effort from the Office of Naval Research Tropical Cyclone Rapid Intensification (TCRI) Departmental Research Initiative and NOAA Advancing the Prediction of Hurricanes Experiment (APHEX). Optical probe measurements were collected using a Droplet Measurement Technologies Cloud Imaging Probe (CIP) and a Precipitation Imaging Probe (PIP). The CIP and PIP have 64 elements of 25- and 100-micron resolution, respectively, which allow for the detection and characterization of hydrometeors ranging from 25 microns to 6.2 mm. The measurements span a temperature range from +10 to -10 C, providing a valuable new dataset in the mixed-phase region of TC rainbands for physical process studies and numerical model verification. An overview of these novel observations will be presented, including particle size distributions and ice habit type. The observations show the presence of supercooled liquid water and graupel in the stratiform precipitation, suggesting transport from stronger convection in other regions of the storm. High concentrations of pristine ice and aggregates were also observed, indicating a complex combination of microphysical processes were occurring. Implications for microphysical parameterization and TC intensity and structure change will be discussed.