27th Conference abstracts

The hurricane mixing front

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The hurricane mixing front

Eric A. D'Asaro, Univ. of Washington, Seattle, WA; and R. Harcourt, E. Terrill, P. P. Niiler, and T. B. Sanford

The temperature of the sea surface beneath the hurricane inner core is a key factor controlling the flux of enthalpy from the ocean to the hurricane and thus an important influence on hurricane intensification. Mixing caused by the hurricane winds produces a rapid cooling of the sea surface as cooler water is mixed upward from below. This produces a front in sea surface temperature beneath the storm. The position of this front relative to the eye should thus be related to SST- induced storm intensification. Data from the CBLAST measurements in hurricanes is used to map several examples of this front. The sensitivity of its position to storm properties is explored using simple models of ocean mixing.

An asymmetric hurricane wind model for storm surge and wave forecasting

Shaowu Bao, North Carolina State University, Raleigh, NC; and L. Xie and L. J. Pietrafesa

With the increase in US coastal population, hurricanes become an increasing threat to the lives and properties of the residents living in the vulnerable coastal regions. Storm surge and inundation are the greatest threat to life and property along the immediate coast associated with a landfalling hurricane. Accurate forecast of the storm surge and inundation is critical to hurricane preparedness and evacuation plans. The accuracy of winds is one of the most important factors affecting accuracy of the forecasts of hurricane-caused storm surge, inundation and waves. Although in recent years full physics regional and mesoscale numerical models have been greatly improved, their performance in the hurricane intensity forecasts remains unsatisfactory. Therefore, simple axis-symmetric parametric hurricane vortex models, such as the Holland model, are still widely used to provide the wind forcing input to storm surge, wave and inundation models. These axis-symmetric models are not capable of describing the asymmetric structures in real-world hurricanes, which are rarely axis-symmetric.
To meet the needs of the storm surge and wave modeling community for an asymmetric hurricane wind model, a real-time hurricane wind forecast model is developed by incorporating an asymmetric effect into the Holland hurricane wind model. National Hurricane Center (NHC)'s hurricane forecast guidance is incorporated into the model for prognostic hurricane wind modeling. This model also assimilates the National Data Buoy Center (NDBC) real time buoy data into the model's initial wind field. The method is validated using all 2003 and 2004 Atlantic and Gulf of Mexico hurricanes. The results show that 6 h and 12 h forecast winds using the asymmetric hurricane wind model is statistically more accurate than using a symmetric wind model. Detailed case studies were conducted for four historical hurricanes, namely, Floyd (1999), Gordon (2000), Lily (2002) and Isabel (2003). Although the asymmetric model performed generally better than the symmetric model, the improvement in hurricane wind forecasts produced by the asymmetric model varied significantly for different storms. In some cases, optimizing the symmetric model using observations available at initial time and the forecast mean radius of maximum wind can produce comparable wind accuracy measured in terms of RMS error of wind speed. However, in order to describe the asymmetric structure of hurricane winds, an asymmetric model is needed.
This wind model can be applied in numerical simulations of storm surge and waves induced by hurricanes. An automated real time wind forecast system has been developed using this algorithm.
The Critical Role of Air-Sea Enthalpy and Momentum fluxes in Tropical Cyclone Intensity and Structure
Kerry A. Emanuel, MIT, Cambridge, MA
I review theoretical, modeling and observational evidence that tropical cyclone intensity, and to some extent structure, are strongly modulated by air-sea fluxes of both enthalpy and momentum. In particular, theory and models for which ambient conditional instability plays little or no role are especially sensitive to surface fluxes, with intensity being largely controlled by fluxes under the eyewall. On the other hand, models for which ambient conditional instability plays a larger role are less sensitive to fluxes under the eyewall. The observed maintenance of hurricane intensity in some storms that are largely over land but whose eyewalls remain over the ocean support the former view. The critical need to parameterize surface fluxes in very high wind speed conditions, and CBLASTs attempt to make the measurements needed to formulate such parameterizations will also be discussed.

Latent heat fluxes in the hurricane boundary layer

William M. Drennan, Univ. of Miami/RSMAS, Miami, FL; and J. Zhang, J. R. French, and P. G. Black

As part of the recent ONR-sponsored "Coupled Boundary Layers Air-Sea Transfer" (CBLAST) Defense Research Initiative, a NOAA P3 Hurricane Hunter 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 within Hurricanes Fabian and Isabel. We report here the first direct measurements of latent heat flux measured in the hurricane boundary layer. The previous wind speed range for humidity fluxes and Dalton numbers has been extended by over 50%. Up to 32 m/s, the highest 10m winds measured, the Dalton number is not significantly different from that found during HEXOS, with no evidence of an increase with wind speed.

Direct Airborne Measurements of Momentum Flux in Hurricanes

Jeffrey R. French, NOAA/OAR/ARL, Oak Ridge, TN; and W. M. Drennan, J. Zhang, and P. G. Black

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 20 and 32 m/s. Analyses of data from forty-two flux legs flown within 400 m of the surface in two storms are presented. Our results indicate 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 highly uncertain underscoring the need for additional measurements.
CBLAST Wind-Wave Parameterization for Coupled Atmosphere-Wave-Ocean Models in Hurricane Research and Prediction
Shuyi S. Chen, Univ. of Miami/RSMAS, Miami, FL; and W. Zhao, M. Donelan, J. F. Price, E. J. Walsh, and H. Tolman
Improving intensity forecast is the most important issue for hurricane prediction today. The lack of skill in the intensity forecast may be attributed in part to deficiencies in the current prediction models: insufficient grid resolution, inadequate surface and boundary layer formulations, and no full coupling to the ocean. The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in hurricanes push the surface-exchange parameters for temperature, water vapor, and momentum into untested new regimes. The CBLAST-Hurricane program is aimed to develop new coupling parameterizations, using the observations collected during the CBLAST-Hurricane field program, for the next generation hurricane prediction models. Hurricane induced surface waves (that determine the surface stress) are highly asymmetric, which can affect storm structure and intensity significantly. The stress is supported mainly by waves in the wavelength range of 0.1-10 m, which are unresolved by wave models. The CBLAST modeling team developed a wind-wave parameterization that includes effects of the wave spectral tail on drag coefficients using a fully coupled atmosphere-wave-ocean model with 1-2 km resolution that can resolve the extreme high winds in the eyewall. The coupling parameterization has been tested in a number of storms including Hurricane Frances (2004) that is one of the best observed storms during the CBLAST-Hurricane 2004 field program. Model simulations are evaluated with observations of directional wave spectra, air-sea fluxes, profiles of atmospheric boundary layer, ocean temperature and salinity, and SST from various in-situ, airborne, and satellite data during CBLAST-Hurricane. The fully coupled model with the new wind-wave parameterization improves the overall storm intensity forecast and produces a realistic surface wind-pressure relationship, which is sensitive to the treatment of surface stress, whereas the uncoupled atmospheric model over-predicts the minimum sea-level-pressure and under-predict the surface wind.
Synthesis of major results from the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST) in hurricanes (2003–2004)
Peter G. Black, NOAA/AOML/HRD, Miami, FL; and E. A. D'Asaro, J. R. French, and W. M. Drennan
The purpose of Hurricane CBLAST was to investigate the mechanisms for air-sea transfer in the high wind environment of hurricanes and to extend the range of observations for exchange coefficients of momentum and enthalpy to hurricane force winds and beyond. The experimental design consisted of two major components: 1) an aircraft component and 2) an air-deployed drifting buoy and float component. The aircraft component had two modules: a) an aircraft stepped descent module and b) a survey/eyewall multisonde deployment module. The former was to focus on in-situ flux and spray measurements, while the latter was to focus on large-scale structure and eyewall flux budget measurements. Both modules were complemented with an array of airborne remote and in-situ sensors. Buoy/float air-deployments consisted of arrays of measurements of surface and upper ocean conditions before, during and after hurricane passage. Together the aircraft and buoy/float array provided a unique description of air-sea fluxes, surface wave and upper ocean conditions in hurricane conditions never before achieved.
Measurements were made primarily in Hurricanes Fabian and Isabel in 2003 and in Frances, Jeanne and Ivan in 2004. Observations for the aircraft component of CBLAST were made by two NOAA WP-3D Orion aircraft flown by the NOAA Aircraft Operations Center (AOC) and manned by personnel from the Hurricane Research Division, AOC, NESDIS/ORA and CBLAST PI's. The observations obtained from the WP-3D aircraft was a collaborative effort between CBLAST PI's and the NESDIS Ocean Winds project. Collaborative operational Synoptic Surveillance flights from the NOAA G-IV and reconnaissance flights from the Air Force Reserve Command (AFRC) WC-130H Hercules aircraft flown by the 53rd Weather Reconnaissance Squadron were conducted together with most of the CBLAST flights, thus documenting environmental impacts and storm evolution. A total of 87 flights were flown in support of CBLAST, with 31 being direct CBLAST/ Ocean Winds flights: Fabian03 (3 days- 13 flights), Isabel03 (3 days-13 flights), Frances04 (5 days- 25 flights), Ivan04 (4 days- 24 flights) and Jeanne (3 days- 12 flights). Observations for the drifting buoy/ ocean float component of CBLAST were provided by air deployments of 16 drifting buoys and four floats in Hurricane Fabian (2003) and by the deployment of 38 drifting buoys (30 Minimet; 8 ADOS) and 16 floats (10 ARGO/SOLO; 2 gas flux; 2 Lagrangian; 2 EM/APEX) in Hurricane Frances. Deployments were carried out by the 53rd Weather Reconnaissance Squadron of AFRC using WC-130J and C-130J ‘stretch' aircraft.
The big picture of the collective impact of CBLAST initial results is presented in this paper. The CBLAST stepped descent flight segments have resulted in a new description of the behavior of the surface drag and enthalpy exchange coefficients for high winds from 18 to 32 m/s, nearly doubling the prior range of wind speeds for which measurements were available. Detailed bulk profiles of PBL structure in this wind regime have also been obtained concurrently with dropsondes and SFMR surface winds. The rapid-deployment sequence of 12 dropsondes (involving coordinated drops between the two P3 aircraft) across the hurricane eyewall in several quadrants of Fabian and Isabel have resulted in budget-based estimates of exchange coefficients at extreme winds speeds over 50 m/s. These observations together with data from the Tail (TA) Doppler radar and the Integrated Wind and Rain Atmospheric Profiler (IWRAP), a vertical Doppler profiler/scatterometer instrument, have provided documentation in finer detail than ever before of the boundary layer structure in hurricane eyewalls.
Airborne directional wave spectra were obtained from the Scanning Radar Altimeter (SRA) which have resulted in defining three sectors of the hurricane which have uniquely different wave spectra and surface roughness conditions. This characterization allows direct flux observations to be stratified by wave regime for the first time. Wave spectra from the ARGO/SOLO floats in the Frances buoy/float array provided high frequency wave spectral measurements in extreme winds for the first time, which were concurrent with the SRA measurements.
A detailed four-dimensional evolution of the upper ocean temperature structure has also been derived from the Frances buoy/float deployment. Detailed mixed layer float measurements of mean currents and mixing processes were achieved concurrently with the SOLO float observations. The mix of buoy and float data provided unprecedented detail in sea surface temperature time/space evolution during hurricane passage. The collective effect of this defining data set on our new view of air-sea transfer processes will be described in detail as well as the initial impact on evolving hurricane coupled modeling efforts.
Concurrent surface winds from the Stepped Frequency Microwave Radiometer (SFMR) were measured throughout all CBLAST/Ocean Winds flights. This together with the unprecedented SST observations and GPS sonde observations of surface air temperature and humidity have also provided data for computation of surface flux variability due to different flux parameterization schemes, including the CBLAST high wind parameterization.

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