Intense Surface and Flight-Level Wind Speeds in Hurricanes: When Conventional Rules Break Down

Surface wind speed observations are important for understanding the overall structure of tropical cyclones and are one of the primary indicators of a storm’s current intensity. Aircraft observations from GPS dropsondes are reliable over the full spectrum of TC surface winds but provide limited spatial coverage of the surface winds as point observations on a case-by-case basis. On the other hand, wind speeds collected by flight-level (~3 km altitude) instrumentation and from the stepped frequency microwave radiometer (SFMR) provide high-resolution, along track estimates of the wind speeds in the form of radial profiles. This observation strategy captures the sharp wind gradient found near the hurricane eyewall and is especially useful for intense hurricanes with maximum wind speeds reaching Category 3 strength or higher. NOAA and Air Force Reserve hurricane hunter aircraft observed several intense TCs during the active 2017 Atlantic hurricane season, including Category 4 and 5 hurricanes Harvey, Irma, and Maria. However, several of the peak wind speeds observed by the SFMR were questioned due to instances of the surface winds exceeding their coincident flight-level wind speeds and quality control flags.

With the attention given to these significantly impactful hurricanes in 2017, the purpose of this present study is three-fold. For the first goal, we seek to show that the SFMR algorithm upgrade in 2015 on all operational SFMR significantly reduces a low bias relative to dropsonde data within the strong hurricane wind regime and improves the estimate of surface winds from SFMR in these conditions. A brief discussion of the algorithm upgrade is provided with statistical examination of the bias. Secondly, we compare the reprocessed maximum wind speeds from several historical hurricanes and compare them to their official ‘best track’ intensity estimates. An evaluation and discussion of the impact of these improved wind speeds on previous intense hurricanes intensity estimates is provided. The final goal of this presentation is to answer the question related to the likelihood of surface wind speeds exceeding their coincident flight-level wind speeds by studying the brightness temperatures associated with the specific instances from 2017 compared to previous events. Using an idealized, high-resolution Weather Research and Forecasting (WRF) simulation that undergoes rapid intensification and develops a very small eye, we suggest that mesovortices near the eyewall provide a means by which this phenomenon occurs.