Abstract : The demand for more accurate tropical cyclone (TC) forecasts with longer lead times is greater than ever due to the enormous economic and societal impact of these storms. There has been spectacular improvement in TC track prediction; a three-day hurricane track forecast today is as skillful as a one-day forecast was just 30 years ago. However, there has been little progress in improving TC intensity and structure forecasts due to a variety of reasons, ranging from a lack of critical observations under high wind conditions and in the TC environment, to inaccurate representations of TC physical processes in numerical weather prediction (NWP) models. Advances in high-resolution TC modeling and data assimilation are thought to be necessary to significantly improve the performance of intensity and structure prediction. To this end, the Naval Research Laboratory in Monterey, California, has developed the Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones (COAMPS-TC ), a new version of COAMPS designed specifically for highigh-resolutionopical cyclone prediction.
Using GPS signals reflected from the ocean surface is developing into a simple technique for measuring sea‐state and inferring surface wind speeds. Theoretical models have been developed which are considered valid to approximately 24 meters per second. The GPS reflection technique has an obvious extension to extremely high sea states, cyclones and extra‐tropical storms. In October of 2000 a GPS system mounted in a NOAA Hurricane Hunter research aircraft, was flown into Hurricane Michael off the South Carolina coast. The first acquisition of GPS signals reflected from the sea surface inside tropical cyclones was accomplished. This paper presents some examples of the data sets as well as early wind speed retrieval results using direct extensions of current models. Data from the GPS wind speed retrievals as well as from direct aircraft measurements are compared and discussed.
The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.
A new microwave radiometric ocean surface emissivity model has been developed to support the analysis and design of the new airborne Hurricane Imaging Radiometer, HIRAD. This radiative transfer model extends current ocean surface emissivity capabilities to higher wind speeds and incidence angles. This model utilizes a variety of empirical data sources many of which were collected in hurricanes.
Abstract : Major goals for this project are three-fold: 1) provide a quality-controlled WC-130J airborne observation data set for the flights conducted during the Tropical Cyclone Structure 2008 field program (TCS-08), 2) diagnose the interaction of mature Western Pacific (WPAC) tropical cyclones (TCs) with their underlying ocean features and provide new observations simultaneously, both within the TC itself as well as the ocean below, to the developing Naval Research Laboratory (NRL) coupled TC modeling effort (COAMPS(tm)1-TC) and 3) utilize TCS-08 field program datasets to improve understanding of TC life cycle, especially genesis stage and rapid intensification mature-stage episodes associated with oceanic and large-scale atmospheric environmental changes.
Abstract The National Oceanic and Atmospheric Administration’s (NOAA) Sensing Hazards with Operational Unmanned Technology (SHOUT) project evaluated the ability of observations from high-altitude unmanned aircraft to improve forecasts of high-impact weather events like tropical cyclones or mitigate potential degradation of forecasts in the event of a future gap in satellite coverage. During three field campaigns conducted in 2015 and 2016, the National Aeronautics and Space Administration (NASA) Global Hawk, instrumented with GPS dropwindsondes and remote sensors, flew 15 missions sampling 6 tropical cyclones and 3 winter storms. Missions were designed using novel techniques to target sampling regions where high model forecast uncertainty and a high sensitivity to additional observations existed. Data from the flights were examined in real time by operational forecasters, assimilated in operational weather forecast models, and applied postmission to a broad suite of data impact studies. Results from the analyses spanning different models and assimilation schemes, though limited in number, consistently demonstrate the potential for a positive forecast impact from the observations, both with and without a gap in satellite coverage. The analyses with the then-operational modeling system demonstrated large forecast improvements near 15% for tropical cyclone track at a 72-h lead time when the observations were added to the otherwise complete observing system. While future decisions regarding use of the Global Hawk platform will include budgetary considerations, and more observations are required to enhance statistical significance, the scientific results support the potential merit of the observations. This article provides an overview of the missions flown, observational approach, and highlights from the completed and ongoing data impact studies.
On 24 august 1998, the NASA/Goddard Space Flight Center Scanning Radar Altimeter (SRA) provided the first documentation of the sea surface directional wave spectrum in all quadrants of a hurricane in open water when Bonnie, a large Category 3 hurricane, was east of the Bahamas. The SRA flew into Bonnie again on 26 august 1998, when she was making landfall near Wilmington, NC, documenting the wave field in the region between Charleston, SC and Cape Hatteras, NC.
The RADARSAT synthetic aperture radar (SAR) acquired C-band HH polarization images over four 1998 hurricanes: Bonnie, Danielle, Georges, and Mitch. The authors present the SAR images and discuss their quantitative use in understanding hurricane morphology. The SAR provides a complimentary "view from below" that is most beneficial when considered in the context of more conventional hurricane observations.
