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Abstract Accurate prediction of tropical cyclone (TC) intensity remains a significant challenge partially due to physics deficiencies in forecast models. Improvement of boundary layer physics in the turbulent “gray zone” requires a better understanding of spatiotemporal variations of turbulent properties in low-level high-wind regions. To fill the gap, this study utilizes Anduril’s Altius 600, a small uncrewed aircraft system (sUAS), that collected data in the eye and eyewall regions of category 5 Hurricane Ian (2022) at altitudes below 1.4 km. The highest observed wind speed (WSPD) exceeded 105 m s⁻¹at 650-m altitude. The Altius measured turbulent kinetic energy (TKE) and momentum fluxes that were in good agreement with previous crewed aircraft observations. This study explores the scale-awareness turbulent structure by quantifying turbulence-scale (100 m–2 km) and mesoscale (2–10 km) contributions to the total flux and TKE. The results show that mesoscale eddies dominate the horizontal wind variances compared to turbulent eddies. The horizontal wind variances contribute 70%–90% of the total TKE, while the vertical wind variances contribute 10%–30% of the total TKE. Spectral and wavelet analyses demonstrate eddy scales from a few hundred meters up to 10 km, with unique distributions depending on where observations were taken (e.g., eye vs eyewall). These findings underscore the complex and multiscale nature of TKE and momentum fluxes in intense hurricanes and highlight the critical need for advanced observational tools within the high-wind hurricane boundary layer environment. Significance StatementIt is crucial to improve the understanding of turbulent processes in the low-level high-wind regions of tropical cyclones (TCs) for accurate intensity forecasts. Traditional data collection methods involving crewed aircraft are too risky to access these critical regions. This study demonstrates the use of a small uncrewed aircraft system (sUAS) to collect data at low levels within an intense Hurricane Ian (2022). The wind speed measured by the sUAS exceeded 105 m s⁻¹. Important turbulence parameters are estimated and presented as a function of wind speed, height, and radial locations. We found that mesoscale (2–10 km) eddies contributed to a significant portion of the total momentum transfer relative to turbulence-scale (100 m–2 km) eddies. This work demonstrates the usefulness of sUASs for improving the basic understanding of key physical processes in the high-wind hurricane boundary layer. 

Abstract This study documents the capabilities of the StreamSonde, a lightweight (24 g) instrument manufactured by Skyfora that measures atmospheric temperature, pressure, humidity, and wind velocity. Unique features of the StreamSonde are its wind speed accuracy enabled by a dual-band Global Navigation Satellite System (GNSS) receiver, the ability to vary the terminal fall velocity, a theoretical maximum communication distance between the instrument and the deployment aircraft of 250 km, and the ability to simultaneously operate up to eight instruments (50 in the future). Skyfora’s GNSS receiver receives signals on two bands from U.S. global positioning system (GPS) (L1/L5), European Galileo (E1/E5a), and Chinese BeiDou (B1I/B2a) satellites to calculate the wind speed. The combination of dual GNSS and lower terminal fall velocity results in more accurate wind retrievals than from single-band GPS potentially allowing us calculate turbulence quantities, especially near the surface. StreamSondes were launched as dropsondes from the NOAA P-3 aircraft in both clear-air low-wind testing environments and in Hurricane Nigel (2023). The pressure, temperature, humidity (in clear air), and derived wind velocity collected by the StreamSonde compare favorably to the widely used RD41 dropsonde that was developed at the National Center for Atmospheric Research (NCAR) and is manufactured by Vaisala. At coreleased drops in Hurricane Nigel, mean absolute differences between RD41 dropsondes and StreamSondes are generally below 1°C for air temperature, 1.5 m s⁻¹for wind speed, and 6° for wind direction. The benefits of using the StreamSonde instrument along with planned improvements to the platform are discussed. Significance StatementThis study presents proof of concept for operational deployment of a new, lightweight atmospheric profiler called the StreamSonde in a tropical cyclone. It uses advanced positioning technology to accurately measure three-dimensional wind velocity, has an adjustable terminal velocity, and can be deployed in “swarms” of sensors that have up to eight (50 in the future) instruments simultaneously active. The versatility of this emerging technology makes it useable for many meteorological applications. 

Abstract Cyclostrophic rotation in the core region of tropical cyclones (TCs) imprints a distinct signature upon their turbulence structure. Its intensity is characterized by the radius of maximum wind, , and the azimuthal wind velocity at that radius, . The corresponding cyclostrophic Coriolis parameter, /, far exceeds its planetary counterpart, , for all storms; its impact increases with storm intensity. The vortex can be thought of as a system undergoing a superposition of planetary and cyclostrophic rotations represented by the effective Coriolis parameter, . On the vortex periphery, merges with . In the classical Rankine vortex model, the inner region undergoes solid‐body rotation rendering constant. In a more realistic representation, is not constant, and the ensuing cyclostrophic ‐effect sustains vortex Rossby waves. Horizontal turbulence in such a system can be quantified by a two‐dimensional anisotropic spectrum. An alternative description is provided by one‐dimensional, longitudinal, and transverse spectra computed along the radial direction. For rotating turbulence with vortex Rossby waves, the spectra divulge a coexistence of three ranges: Kolmogorov, peristrophic (spectral amplitudes are proportional to ), and zonostrophic (transverse spectrum amplitude is proportional to ). A comprehensive database of TC winds collected by reconnaissance airplanes reveals that with increasing storm intensity, their cyclostrophic turbulence evolves from purely peristrophic to mixed peristrophic‐zonostrophic to predominantly zonostrophic. The latter is akin to the flow regime harboring zonal jets on fast rotating giant planets. The eyewall of TCs is an equivalent of an eastward zonal jet. 

Abstract Convective cold pools (CPs) are inherent to mesoscale convective systems and have been identified in tropical cyclone (TC) eyewalls and rainbands. However, their distribution within TCs and their impacts on the TC enthalpy balance are not well understood. This gap is due to the scarcity of high-frequency observations over the ocean. By comparing 1-min data from Saildrone uncrewed surface vehicles to 10-min ocean moored buoy data, we demonstrate that the latter can detect CPs effectively. The analysis of the combined mooring-Saildrone dataset, associated with 241 TCs in the North Atlantic over the period 1998–2023, reveals that the frequencies of occurrence of CPs in the motion-right and shear-left quadrants are 50% and 30% higher than in the motion-left and shear-right quadrants, respectively. This indicates that there is enhanced convection in the motion-right and shear-left quadrants, and TC motion is more important than vertical wind shear in organizing CPs. Although, on average, CPs occur only about 6% of the time in TCs, their contribution to tropospheric latent heat release from their uplifting effect could be comparable to the total surface enthalpy flux in TCs under non-CP conditions. In addition, we found that CP gust fronts can boost surface sensible and latent heat fluxes by 65% and 11%, respectively, which can help low-enthalpy downdraft boundary air recover more quickly, increasing the readiness of the boundary layer for new convection under TC conditions. These findings suggest that properly resolving CP dynamics in TC models could improve the accuracy of TC intensity forecasts. Significance StatementConvective cold pools are bursts of cool, dry air near the surface, often originating from thunderstorms. As they travel, they uplift surface moist air to higher altitudes, which helps form new thunderstorms. As thunderstorms are an integral part of tropical cyclones, the purpose of this study is to investigate the distribution of cold pools inside tropical cyclones and how much they impact tropical cyclone energy. We found that cold pools are more common on the right side of tropical cyclone paths, suggesting stronger thunderstorms in that part of the storm. Despite a low frequency of occurrence of 6%, the amount of energy contributed by cold pools’ uplifting effect in a hurricane can match the total energy released by that hurricane. 

Abstract In the marine boundary layer, the exchange of momentum, heat, and moisture occurs between the atmosphere and ocean. Since it is too dangerous for a crewed aircraft to fly close to the ocean surface to directly obtain these measurements, a sUAS (small Uncrewed Aircraft System) is one of the only viable options. On 24 March 2023 a Black Swift Technologies S0 sUAS was deployed from the NOAA P‐3 on a calm clear day off the west coast of Florida. For 23 min at the end of the mission, the sUAS flew 8 straight line legs with an average length of 2.15 km, at roughly 10 m above the ocean surface, with wind speeds between 3.0 and 4.5 m s⁻¹. For the first time over the open ocean using a sUAS, the 4‐Hz wind and thermodynamic data was used to calculate surface momentum flux, sensible heat flux, and latent flux using both direct covariance methods and the bulk aerodynamic formulas. Since all the flux quantities can be found using both direct and indirect methods, we are able to calculate the exchange coefficients of momentum flux (C_D), latent heat flux (C_E), and sensible heat flux (C_H) with results that are generally in good agreement with previous studies over the same wind speed range. This study demonstrates the ability of sUAS to measure air‐sea interactions. Future intention is to use sUAS to obtain similar measurements in high wind events such as hurricanes which could better help understand hurricane intensification and improve model physics. 

Abstract Initially a Category 3 storm, Hurricane Ian (2022) rapidly intensified on the West Florida Shelf reaching Category 5 over the course of about 12 hr. Intensification occurred despite inhibiting factors such as high axial tilt, high vertical wind shear, low atmospheric moisture, and transit over a relatively shallow continental shelf. Using a high‐resolution simulation of Hurricane Ian from the Hurricane Weather Research Forecasting (HWRF) model, we examine the factors that both hindered and supported rapid intensification (RI) by blending various methods. We show that an increase in diabatic heating in the eyewall led to an inward radial advection of momentum, seen in both the absolute angular momentum budget and in the azimuthal wind budget. Analysis of the moist static energy budget indicates that the substantial latent heat flux from the surface was enough to balance heat losses through storm outflow. For instance, surface latent heat fluxes exceeded 1,500 W m⁻²on the West Florida Continental Shelf. As suggested by actual ocean temperature observations that substantially exceeded those in the HWRF simulation, the latent heating may have even been larger. Physical explanations for discrepancies between the simulated Hurricane Ian and observations are provided, particularly those pertaining to the coastal ocean at the time of Ian's passage. This research provides a comprehensive explanation of the RI of a hurricane using momentum budget analyses as part of a coupled air‐sea analysis. Our findings demonstrate the importance of in situ oceanic air‐sea measurements in evaluating the performance of coupled models, especially for hurricanes. 

Abstract Hurricanes have been the most destructive and expensive hydrometeorological event in U.S. history, causing catastrophic winds and floods. Hurricane dynamics can significantly impact the amount and spatial extent of storm precipitation. However, the complex interactions of hurricane intensity and precipitation and the impacts of improving hurricane dynamics on streamflow forecasts are not well established yet. This paper addresses these gaps by comprehensively characterizing the role of vertical diffusion in improving hurricane intensity and streamflow forecasts under different planetary boundary layer, microphysics, and cumulus parameterizations. To this end, the Weather Research and Forecasting (WRF) atmospheric model is coupled with the WRF hydrological (WRF-Hydro) model to simulate four major hurricanes landfalling in three hurricane-prone regions in the United States. First, a stepwise calibration is carried out in WRF-Hydro, which remarkably reduces streamflow forecast errors compared to the U.S. Geological Survey (USGS) gauges. Then, 60 coupled hydrometeorological simulations were conducted to evaluate the performance of current weather parameterizations. All schemes were shown to underestimate the observed intensity of the considered major hurricanes since their diffusion is overdissipative for hurricane flow simulations. By reducing the vertical diffusion, hurricane intensity forecasts were improved by ∼39.5% on average compared to the default models. These intensified hurricanes generated more intense and localized precipitation forcing. This enhancement in intensity led to ∼16% and ∼34% improvements in hurricane streamflow bias and correlation forecasts, respectively. The research underscores the role of improved hurricane dynamics in enhancing flood predictions and provides new insights into the impacts of vertical diffusion on hurricane intensity and streamflow forecasts. Significance StatementDespite significant recent improvements, numerical weather prediction models struggle to accurately forecast hurricane intensity and track due to many reasons such as inaccurate physical parameterization for hurricane flows. Furthermore, the performance of existing physics schemes is not well studied for hurricane flood forecasting. This study bridges these knowledge gaps by extensively evaluating different physical parameterizations for hurricane track, intensity, and flood forecasts using an atmospheric model coupled with a hydrological model. Then, a reduced diffusion boundary layer scheme is developed, making remarkable improvements in hurricane intensity forecasts due to the overdissipative nature of the considered schemes for major hurricane simulations. This reduced diffusion model is shown to significantly enhance hurricane flood forecasts, indicating the significance of hurricane dynamics on its induced precipitation. 

Abstract The global positioning system dropwindsonde has provided thousands of high-resolution kinematic and thermodynamic soundings in and around tropical cyclones (TCs) since 1997. These data have revolutionized the understanding of TC structure, improved forecasts, and validated observations from remote sensing platforms. About 400 peer-reviewed studies on TCs using these data have been published to date. This paper reviews the history of dropwindsonde observations, changes to dropwindsonde technology since it was first used in TCs in 1982, and how the data have improved forecasting and changed our understanding of TCs. 

Abstract While recent observational studies of intensifying (IN) versus steady-state (SS) hurricanes have noted several differences in their axisymmetric and asymmetric structures, there remain gaps in the characterization of these differences in a fully three-dimensional framework. To address these limitations, this study investigates differences in the shear-relative asymmetric structure between IN and SS hurricanes using airborne Doppler radar data from a dataset covering an extended period of time. Statistics from individual cases show that IN cases are characterized by peak wavenumber-1 ascent concentrated in the upshear-left (USL) quadrant at ∼12-km height, consistent with previous studies. Moderate updrafts (2–6 m s⁻¹) occur more frequently in the downshear eyewall for IN cases than for SS cases, likely leading to a higher frequency of moderate to strong updrafts USL above 9-km height. Composites of IN cases show that low-level outflow from the eye region associated with maximum wavenumber-1 vorticity inside the radius of maximum wind (RMW) in the downshear-left quadrant converges with low-level inflow outside the RMW, forming a stronger local secondary circulation in the downshear eyewall than SS cases. The vigorous eyewall convection of IN cases produces a net vertical mass flux increasing with height up to ∼5 km and then is almost constant up to 10 km, whereas the net vertical mass flux of SS cases decreases with height above 4 km. Strong USL upper-level ascent provides greater potential for the vertical development of the hurricane vortex, which is argued to be favorable for continued intensification in shear environments. 

Abstract Airborne Doppler radar observations of the wind field in the tropical cyclone boundary layer (TCBL) during the landfall of Hurricane Ida (2021) are examined here. Asymmetries in tangential and radial flow are governed by tropical cyclone (TC) motion and vertical wind shear prior to landfall, while frictional effects dominate the asymmetry location during landfall. Strong TCBL inflow on the offshore‐flow side of Ida occurs during landfall, while the location of the peak tangential wind at the top of the TCBL during this period is located on the onshore‐flow side. A comparison of these observations with a numerical simulation of TC landfall shows many consistencies with the modeling study, though there are some notable differences that may be related to differences in the characteristics of the land surface between the simulation and the observations here.