Recent Coupled Boundary Layer Air-Sea Transfer (CBLAST) Observations in Atlantic Tropical Cyclones

Peter G. Black¹

¹ NOAA/AOML/Hurricane Research Division, Miami, FL USA Peter.Black@noaa.gov

INTRODUCTION

The hurricane CBLAST experiment of 2004 deployed an array of 38 surface drifting buoys and 16 subsurface ocean profiling floats with U.S. Air force WC-130J aircraft ahead of Hurricane Frances 3 days prior to landfall on the U.S. East Coast and 1 day prior to landfall on the Bahamas. These observations together with those from an array of 12 drifting buoys deployed prior to the season by the Navy provided unprecedented observations of the evolution of the ocean sea surface temperature and surface current fields during hurricane passage. The float data provided observations of the evolution of the ocean mixed layer, surface wave and wind field, and sub-surface density and current fields during hurricane passage. Additional observations were obtained in Hurricane Jeanne about 10 days later in the same region.

Two NOAA WP-3D aircraft were flown in tandem into Frances concurrent with its passage over the buoy/float array. More than a dozen flights were also flown in Hurricanes Ivan and Jeanne in 2004. These observations complemented CBLAST observations from 2003 and helped further define the surface wind and wave fields in developing hurricanes. Concurrent high spatial resolution dropsonde observations further refined peak eyewall air-sea flux estimates in high winds using budget methods. Data sets were obtained that are suitable for testing initialization schemes and developing modified air-sea parameterization schemes for the upcoming NOAA HWRF and Navy WRF operational coupled tropical cyclone forecast models.

Figure 1. Float array trajectories during Hurricanes Frances and Jeanne (2004) together with TOPEX/Poseidon satellite altimeter sea height anomalies showing anticyclonic ocean gyre along the Track of Frances.

LONG-TERM GOALS

Our primary goal is to improve our understanding of air-sea surface flux processes in high winds, specifically in the complex conditions of tropical hurricanes where swell, sea spray and secondary boundary layer circulations play a role. Our ultimate goal and prime motivation for this work is to parameterize these new observations and improve the accuracy of hurricane intensity prediction.

GOALS ACHIEVED

There are two major goals that have been achieved as of mid-2005. The first is the specification of the surface drag coefficient and enthalpy exchange coefficient to hurricane force, i.e. ~32 m/s. This result has shown that at least in certain hurricane quadrants that Cd reaches a maximum at 25 m/s and then rolls off above that speed. Further the enthalpy exchange coefficient, Ce, is constant with wind speed to 32 m/s, an extension of the HEXOS work in the North Sea. Figs 2 and 3 illustrate these results.

Figure 2. Drag coefficient as a function of winds speed. Figure 3. Enthalpy coefficient.

There is some suggestion from CBLAST wave tank experiments on the effect of sea spray, that the enthalpy exchange coefficient used to parameterize this effect could increase dramatically above a wind speed threshold of 50 m/s.

This result contrasts starkly with the exchange coefficients used in many current hurricane dynamical prediction models, such as the GFDL model, and also with recent observational and wave tank estimates of Cd, which do suggest a roll-off at high winds. However, the estimates of this threshold from recen Powell and Donelan studies is on the order of 35-40 m/s. Modelers initially attempted to compensate for their poor Cd and Ce parameterizations by specifying a wave age dependent formulation. More recently a modified formulation has been attempted (Moon et al., 2005) to more closely simulate the Powell (2003) and Donelan (2004) results, as shown in Fig. 4

Fig. 4. Roughness length (a) and drag coefficient (b) verses wind speed at 10 m. Thick blue line, the new formula for Z_o; red circles and gray plus symbols are the results of hurricane Ivan forecast (initial time: 00Z 12 Sep. 2004) from the operational GFDL model and the URI coupled wind-wave model, respectively; black solid line, according to Large and Pond [1981]; dashed dot line, according to Donelan et al. [2004]; squares, averaged values over four layers from data of Powell et al. [2003].

The new parameterization has been tested using the operational GFDL hurricane prediction model for predictions of Hurricane Isabel (2003), Ivan (2004), Frances (2004), Charley (2004), and Jeanne (2004). The results show that the new formula mostly contributes to the increase of the maximum wind speed at very high winds, while does not affect the hurricane minimum pressure prediction appreciably. However, this new parameterization still does not succeed in simulating a roll-off in Cd, but does reflect the lower threshold for a leveling-off of Cd. The model now significantly underpredicts hurricane intensity, whereas earlier with the equality of Cd and Ce, and also with the wave age formulation, the model over-predicted hurricane intensity. Significant work remains to be done including parameterizing exchange coefficients by storm quadrant using the variation of sea state by quadrant.

OBJECTIVES

The objective of this work is to develop a new surface wave-dependent flux parameterization for the high wind hurricane boundary layer containing secondary (roll-vortex) circulations over fetch limited seas in the presence of sea spray and one or more swell components from an airborne platform. This goal is only now beginning to be addressed.

We propose to test the following hypotheses:

I. Surface momentum exchange coefficients increase with wind speed for moderate winds (≤ 25 m/s), are enhanced by fetch-limited waves or opposing swell, but level off or decrease above a high wind threshold, which we now believe to be at or near 25 m/s. This may be especially true in quadrants where swell has a significant downwind component.

II. Compensating mechanisms for enhanced surface air-sea enthalpy fluxes over and above current parameterizations must exist for storm maintenance and growth above some high-wind threshold wind speed.

III. Candidate mechanisms are separable and can be estimated, such as a) enhanced turbulent fluxes due to wave interactions, b) spray evaporation and c) secondary flow circulations (roll-vortex type).

APPROACH

Our approach is to implement a multifaceted observational program among several investigators to simultaneously measure air-sea fluxes using several independent methods while at the same time measuring two-dimensional surface wave spectra, as well as spray droplet spectra , in wind speed regimes ranging from 20-40 m/s, and possibly higher. The secondary goal is to use existing data sets to inter-compare various published wave-dependent bulk flux parameterizations, with and without wave age effects and with and without spray parameterizations, using never-before-available surface inputs from GPS dropsondes, AXBTs, model-generated as well as remotely-measured wave spectra and remotely-measured surface winds in gale- and hurricane-force conditions.

Our strategy is to obtain new parameterizations of momentum, heat and moisture fluxes from these observations including accurate estimates of the exchange coefficients as a function of wind speed and to estimate the modulation of the fluxes by fetch-limited wind waves, long-wavelength swell, sea spray and PBL secondary circulations, i.e.‘roll vortices’.

CBLAST 2003

A Coupled Boundary Layer Air-Sea Transfer (CBLAST) and Ocean Winds experiment was conducted in Hurricanes Fabian and Isabel during the 2003 hurricane season. The purpose of CBLAST is to improve the understanding and parameterization of high-wind, air-sea fluxes and subsequently improve hurricane intensity forecasting. The purpose of the Ocean Winds experiment was to refine algorithms for retrieving surface winds from satellite scatterometer measurements in high-wind and rain environments. The experiment was sponsored by the Office of Naval Research, the NOAA Hurricane Research Division, NOAA Office of Atmospheric Research, United States Weather Research Program and the Ocean Winds program of the NOAA/NESDIS Office of Research and Applications. The experiment utilized two NOAA/Aircraft Operations Center WP-3D Orion aircraft flying in tandem in Fabian on Sept 2, 3 and 4 and in Isabel on Sept 12, 13 and 14. In each flight, two modules were flown: 1) a stair-step flight pattern was flown for the purpose of obtaining in-situ measurements of air-sea fluxes in gale force winds, and 2) a multiple GPS dropsonde deployment was performed from the two WP-3D in the hurricane eyewall to obtain estimates via budget calculations of air-sea fluxes and exchange coefficients in extreme hurricane force winds. This latter goal was achieved with the deployment of 8-12 GPS dropsondes in each eyewall penetration on the 6 flight days into these CAT 4 and 5 storms. A total of 346 GPS dropsondes were deployed during CBLAST/Ocean Winds, 308 in the hurricanes’ eyewall, with 80% of the sondes reaching levels below 50 m, an unqualified success for sondes deployed in high winds.

A sample of the eyewall boundary layer structure from a single penetration in CAT 5 conditions in Isabel is described in Black, et al (2004). The twelve successful flight level mean profiles obtained by the low-level stepping WP-3D aircraft (43RF) and concurrent GPS dropsonde profiles obtained during over-flights by the higher level WP-3D (42RF) are reported by Uhlhorn and Black (2004).

In addition to the in-storm WP-3D flights, a pre-Fabian deployment of 16 drifting buoys and 6 subsurface oceanographic floats was conducted into the path of Fabian on Sept 3, during the second WP-3D in-storm flight, by an Air Force Reserve WC-130J aircraft, as shown in Fig. 2. Its purpose was to obtain surface current, ocean mixed layer and surface wave observations as well as surface meteorological observations concurrent with a subsequent research flights on Sept 4. Fabian passed directly over the center of the array, despite a last minute jog to the east, thanks to direct communication with the aircraft at the last minute that moved the array eastward. Of the 16 buoys deployed at 8 locations (Fig. 6), 7 survived the storm, one at each location except the furthest west point. One of six floats survived and returned excellent data. As it turned out, the surviving buoys and floats also obtained data in the periphery of Isabel as it passed just to the south of the array during 12-14 Sept. Preliminary results are discussed in Terrill, et al., 2004.

These flights and buoy/float deployments generated an unprecedented air-sea interaction data set for diagnosing surface fluxes at gale and hurricane force wind speeds and as input and ground truth for coupled air-sea numerical model predictions of hurricane intensity change.

TURBULENCE MEASUREMENTS

Unprecedented heat and momentum turbulent flux spectra were obtained from the NOAA/FRD BAT probe during 8 of the stepped descents flown in hurricane Isabel (heat flux shown in Fig. 7), after water ingest problems encountered in Fabian were solved (French and Black, 2004).

In addition, excellent moisture flux data were obtained from the new LICOR fast-response humidiometer probe, a USWRP funded instrument, while a companion IRGA sensor developed problems presently under study. These measurements may significantly alter existing estimates of total moisture flux at high winds. Efforts to measure sea spray spectra with a new CIP particle spectrometer are being investigated. It appears that the required FSSP or new DMT aerosol sampler, which was not flown due to aircraft pylon problems, is a necessity.

BULK SENSOR MEASUREMENTS

Excellent observations of mean variables also were obtained in Fabian and Isabel. Excellent 2-D wave spectra were obtained with the SRA in Fabian which showed a pattern of swell consistent with CAMEX 3 and 4 observations in Hurricanes Humberto and Bonnie (Walsh, et al., 2004). This pattern reveals steep, developing waves in the rear quadrant, highest swell waves moving slightly right of the wind in the right-front quadrant, and swell moving normal to the wind in the left semicircle, creating at least 3 distinctly different roughness regimes, which will presumably lead to different surface fluxes at the similar wind speeds. No wave data were obtained in Isabel due to a failure of the instrument.

Initial analysis of the wave spectra in conjunction with the flux estimates is revealing vertical velocity perturbations that are in phase with the swell at the lowest level (100 m) of the stepped descent in the left front quadrant of Fabian, where swell are moving at right angles to the local wind and sea.

Complementary photo observations of the sea surface at 60 Hz frequency from the Scripps MASS camera system is allowing measurements of surface momentum dissipation due to wave breaking as the velocity of the leading edge of individual breakers is computed (Kleiss, et al., 2004).

Other wave images (Fig. 8) show evidence for secondary circulations in both the ocean mixed layer and atmospheric boundary layer, which will undoubtedly modulate the fluxes.: Langmuir cells producing foam streaks with 50-100 m spacing, and roll vortex cells producing light and dark bands with 1-2 km spacing in capillary waves and spray streaks.

For the first time in a hurricane, continuous boundary layer wind profiles were obtained with the UMASS IWRAP system (Fig. 9) in both storms and at two frequencies (K- and C-band) to complement the tail Doppler observations above the boundary layer (Esteban-Fernandez, et al., 2004), and the extensive dropsonde observations. The C-band profiles in Fig 5 show small-scale perturbations of the boundary layer winds penetrating from the middle of the high wind maximum to almost the surface. These features, never seen before, have a spacing of roughly 2 km, and may account for significant enhancements of surface fluxes as well as explain high wind damage streaks seen in damage surveys of intense storms such as Hurricane Andrew (1992).

REFERENCES

Black, P. G., E. W. Uhlhorn, J. F. Gamache, W. D. Ramstrom, K. Emanuel, D. Esteban, J. Carswell and P. S.Chang, 2004: Eyewall boundary layer structure in Hurricanes Fabian and Isabel. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Donelan, M. A., B. K. Haus, N. Reul, W. J. Plant, M. Stiassnie, H. C. Graber, O. B. Brown, and E. S. Saltzman, 2004, On the limiting aerodynamic roughness of the ocean in very strong winds, Geophys. Res. Lett., 31, L18306.

Drennan, W. M., C. A. McCormick and P. Black, 2004: Measurementsof humidity fluxes in Hurricanes Fabian and Isabel with a modified LICOR humidiometer. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Esteban-Fernandez, D., S. Frazier, J. Carswell, P. Chang, P. Black and F. Marks, 2004: 3-D atmospheric boundary layer wind fields from Hurricanes Fabian and Isabel. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Fairall, C. W., J. E. Hare and A. A. Grachev, 2004: Sea spray droplet measurements in Hurricanes Fabian and Isabel. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

French, J. R, R. Johnson, S. Beard and T. Crawford, 2004: Modification of an airborne gust probe for hurricane boundary layer measurements. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

French, J. R. and P. G. Black, 2004: Turbulent flux measurements within a hurricane boundary layer from an instrumented aircraft. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Kleiss, J. M., W. K. Melville, J. R. Lasswell, P. Matusov and E. Terrill, 2004: Breaking waves in hurricanes Isabel and Fabian. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Moon I.-J., I. Ginis, T. Hara, 2005, Physics-based parameterization of air-sea momentum flux at high wind speeds and its impact on hurricane predictions (submitted)

Powell, M. D., P. J. Vickery, and T. A. Reinhold, 2003, Reduced drag coefficient for high wind speeds in tropical cyclones, Nature, 422, 279–283.

Terrill, E. J. and W. K. Melville, 2004: In-situ measurements of the air-sea interface during Hurricane Fabian. 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Uhlhorn, E. W. and P. G. Black, 2004: Comparison of boundary layer profiles in Hurricanes Fabian and Isabel observed by GPS dropwindsonde and aircraft during CBLAST "stepped-descents", 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Walsh, E. J., C. W. Wright, D. C. Vandemark, S. E. Feuer, P. Black and F. D. Marks, Jr., 2004: Surface gravity wave measurements in Hurricane Fabian (2003) and Humberto (2001). 26th Conf. On Hurr. And Tropical Meteorol., AMS, Miami Beach, FL.

Figure 5. Box containing two Minimet drifting buoys is deployed from an AFRC 53^rd Weather Squadron WC-130J.

Figure 6. Deployment location of drifting buoy and float array ahead of Fabian on Sept 3, 2003.

Figure 7. Heat flux (w’T’) spectra for stepped descent flight legs from 2,500 ft (bottom) to 200 ft (top) in Isabel.

Figure 8. Foam streaks at 35 m/s in Fabian.

Fig. 9. C-band IWRAP continuous vertical profiles of radar reflectivity (top), wind speed (middle) and wind direction (bottom) in Hurricane Isabel, Sept 12, 2003, 1924-1948.