Unique Vertical Characteristics of Hydrometeors Observed in the Tropical Cyclone Outer Rainband Region

This study investigates the vertical cloud microphysics in the stratiform sectors of Hurricane Tammy's outer rainband region and their relationship to local environments. We used in situ Optical Array Probe (OAP) measurements and tail‐Doppler radar data from four NOAA P‐3 spiral modules on 19–22 October 2023, as Tammy intensified from a tropical storm to a Category 1 hurricane. The four spirals sampled different environments, including embedded‐convective stratiform, mature stratiform, weak‐echo with mid‐level dry air entrainment, and near‐eyewall stratiform conditions. Vertical profiles of particle size distributions (PSDs), area ratios, and bulk parameters reveal distinct microphysical processes at different levels and under various environmental conditions. Above the melting layer, aggregation dominated in both intensification and mature phases. Riming was identified between −3 and −1°C layer during intensification, indicated by the increasing area ratios. During the mature phase, PSDs and bulk properties displayed vertical oscillations, suggesting contributions from secondary ice production. The near‐eyewall stratiform case contained more large particles above the melting layer. Across all four spirals, the melting layers exhibited a sharp decrease in hydrometeor number concentration and a transition to spherical hydrometeors, but their thicknesses varied substantially. Gamma‐fit parameter relationships differed among the four spirals, indicating environmental modulation of the total PSDs, whereas habit‐specific parameter relationships were controlled primarily by hydrometeor habit rather than by instantaneous temperature or horizontal wind speed. These observations highlight the need for high‐resolution in situ measurements to improve microphysics parameterizations in TC models, particularly for accurately representing habit‐dependent PSDs.