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.

In this study, the first of two parts, the planetary boundary layer (PBL) depicted in high-resolution Weather Research and Forecast Model (WRF) simulations of Hurricane Isabel (2003) is studied and evaluated by direct comparisons with in situ data obtained during the Coupled Boundary Layer and Air–Sea Transfer Experiment (CBLAST). In particular, two boundary layer schemes are evaluated: the Yonsei University (YSU) parameterization and the Mellor–Yamada–Janjić (MYJ) parameterization. Investigation of these schemes is useful since they are available for use with WRF, are both widely used, and are based on entirely different methods for simulating the PBL. In this first part, the model domains and initialization are described. For additional realism of the low-level thermodynamic environment, a simple mixed layer ocean model is used to simulate ocean cooling. The YSU and MYJ schemes are discussed, along with some modifications. Standard measures of the accuracy of the hurricane simulations, such as track, maximum surface wind speed, and minimum surface pressure are described for a variety of parameter choices and for the two parameterizations. The effects on track and intensity of increased horizontal and vertical resolutions are also shown. A modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes. Instantaneous wind maxima on the innermost grid with 1.33-km resolution are shown to be an accurate representation of the simulated 1-min sustained winds. The simulated boundary layers are evaluated by direct comparison of the PBL as simulated and as observed by in situ data from the CBLAST experiment in the “outer core” region of the storm. The two PBL schemes and their modified counterparts reproduce the observed PBL remarkably well. Comparisons are also made to the observed vertical fluxes of momentum, heat, and moisture. In Part II, the detailed comparisons of the intensities and structures of the simulated and observed inner-core boundary layers are presented, and the reasons for the differences are discussed.

New multi-lead-time versions of three statistical probabilistic tropical cyclone rapid intensification (RI) prediction models are developed for the Atlantic and eastern North Pacific basins. These are the linear-discriminant analysis–based Statistical Hurricane Intensity Prediction Scheme Rapid Intensification Index (SHIPS-RII), logistic regression, and Bayesian statistical RI models. Consensus RI models derived by averaging the three individual RI model probability forecasts are also generated. A verification of the cross-validated forecasts of the above RI models conducted for the 12-, 24-, 36-, and 48-h lead times indicates that these models generally exhibit skill relative to climatological forecasts, with the eastern Pacific models providing somewhat more skill than the Atlantic ones and the consensus versions providing more skill than the individual models. A verification of the deterministic RI model forecasts indicates that the operational intensity guidance exhibits some limited RI predictive skill, with the National Hurricane Center (NHC) official forecasts possessing the most skill within the first 24 h and the numerical models providing somewhat more skill at longer lead times. The Hurricane Weather Research and Forecasting Model (HWRF) generally provides the most skillful RI forecasts of any of the conventional intensity models while the new consensus RI model shows potential for providing increased skill over the existing operational intensity guidance. Finally, newly developed versions of the deterministic rapid intensification aid guidance that employ the new probabilistic consensus RI model forecasts along with the existing operational intensity model consensus produce lower mean errors and biases than the intensity consensus model alone.

An important outcome from the ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s−1. Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.

As part of the recent ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Departmental Research Initiative, an aircraft was instrumented to carry out direct turbulent flux measurements in the high wind boundary layer of a hurricane. During the 2003 field season flux measurements were made during Hurricanes Fabian and Isabel. Here the first direct measurements of latent heat fluxes measured in the hurricane boundary layer are reported. The previous wind speed range for humidity fluxes and Dalton numbers has been extended by over 50%. Up to 30 m s−1, the highest 10-m winds measured, the Dalton number is not significantly different from the Humidity Exchange over the Sea (HEXOS) result, with no evidence of an increase with wind speed.

The Hurricane Weather Research and Forecasting (HWRF) system was used in an idealized framework to gain a fundamental understanding of the variability in tropical cyclone (TC) structure and intensity prediction that may arise due to vertical diffusion. The modeling system uses the Medium-Range Forecast parameterization scheme. Flight-level data collected by a NOAA WP-3D research aircraft during the eyewall penetration of category 5 Hurricane Hugo (1989) at an altitude of about 450–500 m and Hurricane Allen (1980) were used as the basis to best match the modeled eddy diffusivities with wind speed. While reduction of the eddy diffusivity to a quarter of its original value produced the best match with the observations, such a reduction revealed a significant decrease in the height of the inflow layer as well which, in turn, drastically affected the size and intensity changes in the modeled TC. The cross-isobaric flow (inflow) was observed to be stronger with the decrease in the inflow depth. Stronger inflow not only increased the spin of the storm, enhancing the generalized Coriolis term in the equations of motion for tangential velocity, but also resulted in enhanced equivalent potential temperature in the boundary layer, a stronger and warmer core, and, subsequently, a stronger storm. More importantly, rapid acceleration of the inflow not only produced a stronger outflow at the top of the inflow layer, more consistent with observations, but also a smaller inner core that was less than half the size of the original.