Enthalpy and Momentum Fluxes during Hurricane Earl Relative to Underlying Ocean Features

Using dropsondes from 27 aircraft flights, in situ observations, and satellite data acquired during Tropical Cyclone Earl (category4 hurricane), bulk air‐sea fluxesof enthalpy and momentumare investigated in relation to intensity change and underlying upper-ocean thermal structure. During Earl’s rapid intensification (RI) period,oceanheatcontent(OHC)variabilityrelativetothe268Cisothermexceeded90kJcm 22 ,andseasurface cooling was less than 0.58C. Enthalpy fluxes of ;1.1kWm 22 were estimated for Earl’s peak intensity. Daily sea surface heat losses of 26:560:8, 27:861:1, and 12:360: 7k Jcm 22 were estimated for RI, mature, and weakening stages, respectively. A ratio CK/CD of the exchange coefficients of enthalpy (CK) and momentum (CD) between 0.54 and 0.7 produced reliable estimates for the fluxes relative to OHC changes, even during RI; ar atioCK/CD 51 overestimated the fluxes. The most important result is that bulk enthalpy fluxes were controlled by the thermodynamic disequilibrium between the sea surface and the near-surface air, independently of wind speed. This disequilibrium was strongly influenced by underlying warm oceanic features; localized maxima in enthalpy fluxes developed over tight horizontal gradients of moisture disequilibrium over these eddy features. These regions of local buoyant forcing preferentially developed during RI. The overall magnitude of the moisture disequilibrium (Dq 5 qs2qa )w as determined by thesaturation specifichumidityat sea surface temperature (qs)rather than by thespecific humidity of the atmospheric environment (qa). These results support the hypothesis that intense local buoyant forcing by the ocean could be an important intensification mechanism in tropical cyclones over warm oceanic features.

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abstract: An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.

title: Air-sea exchange in hurricanes : Synthesis of observations from the coupled boundary layer air-sea transfer experiment
abstract: 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 sour...

title: A Study of the Impacts of Vertical Diffusion on the Structure and Intensity of the Tropical Cyclones Using the High-Resolution HWRF System
abstract: AbstractThe 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...

title: Air-Sea Enthalpy and Momentum Exchange at Major Hurricane Wind Speeds Observed during CBLAST
abstract: AbstractQuantifying air–sea exchanges of enthalpy and momentum is important for understanding and skillfully predicting tropical cyclone intensity, but the magnitude of the corresponding wind speed–dependent bulk exchange coefficients is largely unknown at major hurricane wind speeds greater than 50 m s−1. Since direct turbulent flux measurements in these conditions are extremely difficult, the momentum and enthalpy fluxes were deduced via absolute angular momentum and total energy budgets. An error analysis of the methodology was performed to quantify and mitigate potentially significant uncertainties resulting from unresolved budget terms and observational errors. An analysis of six missions from the 2003 Coupled Boundary Layers Air–Sea Transfer (CBLAST) field program in major hurricanes Fabian and Isabel was conducted using a new variational technique. The analysis indicates a near-surface mean drag coefficient CD of 2.4 × 10−3 with a 46% standard deviation and a mean enthalpy coefficient CK of 1.0 × 1...

title: Concentric Eye Walls, Secondary Wind Maxima, and The Evolution of the Hurricane vortex
abstract: Research aircraft observations in recent hurricanes support the model of Shapiro and Willoughby (1982) for the tropical cyclone's response to circularly symmetric, convective heat sources (convective rings). In both nature and the numerical model the tangential wind commonly increases rapidly just inside the radius of maximum wind and decreases inside the eye near the central axis of the vortex. Thus both secondary outer wind maxima and eyewall wind maxima often contract as they intensify. This response is independent of the horizontal spatial scale of the maximum. An outer maximum is frequently observed to constrict about a pre-existing eye and replace it. This chain of events often coincides with a weakening, or at least a pause in intensification, of the vortex as a whole. The concentric eye phenomenon is a common, but by no means universal, feature of tropical cyclones. It is most frequently observed in intense, highly symmetric systems.

title: Sensitivity of tropical‐cyclone intensification to perturbations in the surface drag coefficient
abstract: The recent studies of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al., and Smith et al., are extended to include perturbations of the surface drag coefficient in one of four boundary-layer parametrization schemes: the bulk scheme, the Blackadar scheme, the MRF scheme, and the Gayno–Seaman scheme. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. We find that the intensification rate and mature intensity are essentially unaltered when the drag coefficient is perturbed randomly by variations of up to 60%. The results, in conjunction with an analysis of coherent drag coefficient variations for a moving vortex, question the notion that coupled wind–wave mod els are necessary to accurately forecast tropical-cyclone intensification and mature intensity.

title: How important is the isothermal expansion effect in elevating equivalent potential temperature in the hurricane inner core
abstract: We examine aspects of the thermodynamic structure of mature Atlantic hurricane Earl (2010) based on airborne dropwindsondes released from the upper troposphere during the National Aeronautics and Space Administration (NASA), Genesis and Rapid Intensification Processes (GRIP) experiment. Vertical sounding profiles of the data raise questions concerning the relative roles of isothermal expansion and relative humidity increase in elevating the equivalent potential temperature of air parcels spiralling inwards to the eyewall convection region. The observational results obtained for two successive days of this category 4 hurricane show that the isothermal expansion effect leads to roughly one half of the increment in equivalent potential temperature for boundary-layer air parcels moving between the region outside the eyewall and the eyewall and eye region.

title: Restratification of the upper ocean after the passage of a tropical cyclone
abstract: role of baroclinic instability in the restratification of the upper ocean after the passage of a tropical cyclone (TC) is determined by means of numerical simulations. Using a regional ocean model, the Regional Ocean Modeling System (ROMS), a high-resolution three-dimensional simulation that includes the process of baroclinic instability and is initialized with moderate-amplitude eddy structures reproduces the satellite-observed decay rate of the TC-induced sea surface temperature (SST) anomaly and is also in qualitative agreement with published observations after the passage of Hurricane Fabian in 2003 that showed decaying cold and warm anomalies located in the climatological mixed layer (CML) and upper thermocline, respectively. The model ocean is restratified after approximately one month with a net heat gain in the water column due to anomalous air–sea heat fluxes. The model shows, however, that vertical heat fluxes associated with baroclinic instability dominate over air–sea heat fluxes i...