The Hurricane Component of the CBLAST Departmental Research Initiative (DRI) aims to measure, analyze, understand and parameterize air-sea fluxes in the hurricane environment. Unlike mid-latitude cyclones where baroclinic processes are important, hurricanes, or tropical cyclones, draw their energy supply from the ocean. Fluxes of sensible heat and water vapor enrich the immediate atmospheric boundary layer (ABL); the momentum flux destroys the gradient balance and creates the cross-isobaric inflow converting atmospheric potential energy into kinetic energy. The warm and moist air is then transported into the hurricane inner region or rainbands to fuel the convection and release of latent heat that drives the storm. The significant air-sea flux exchanges greatly modify the near-surface ocean temperature and currents. Under stationary or slow-moving hurricanes, the induced sea-surface temperature (SST) cooling can reach several degrees and the induced current can extend to great depths. The altered oceanic state profoundly modifies the behavior of the overlaying hurricane. Hurricanes are indeed the most interesting and complex nature laboratory for air-sea interaction study.
The air-sea interfacial boundary under hurricane winds is not well defined, and physical processes are not properly quantified. The ocean surface waves and swell are characterized by limited fetch in this strongly forced regime. High winds and strong shear mechanically form ocean spray, which is found to have significant effects on the thermal structure of the ABL and may play an important role in hurricane thermodynamics, dynamics and intensity change. The ocean mixed layer is filled with air bubbles affecting air-sea exchange and form the basis for microwave and acoustic remote sensing of surface wind and stress. Standard boundary layer parameterizations, based on observations mostly taken at wind speeds below 20 m/s, have not been validated for hurricane conditions and highly disturbed sea states. Observation, understanding, and, eventually, modeling of the structure and physical processes in the hurricane-ocean coupled boundary layer are the main objectives of the CBLAST Hurricane Component.
The research effort in the CBLAST Hurricane Component consists of re-examining existing observations of hurricane-ocean boundary layer, wave condition, and hurricane energetics. The effort also includes limited sensor development and calibration, and a refinement of observing strategies. The effort will culminate in a coordinated campaign in the 2003 or 2004 hurricane season of coincident airborne in situ and remote sensing measurements, together with air-deployed, in-situ measurements. The airborne measurements will be conducted with the NOAA WP-3D, equipped with radome and nose-boom mounted turbulence packages for direct measurement of momentum, heat and moisture fluxes. Other onboard measurements include the UMass scatterometers (SCSCAT/KSCAT) with improved horizontal resolution at 15 m and coherency to obtain the ABL wind profiles. A Particle Measurement System (PMS) will be used to measure spray droplet size distribution down to an altitude of 60 m in rain-free, high-wind ABL. The surface-wind measurements will be supplemented with Quikscat and TRMM images. GPS dropsondes and AXBTs will be expended to obtain vertical sounding of atmospheric and oceanic structure below flight level. TOPEX/POSEIDON satellite altimetry will be utilized to analyze ocean heat content during hurricane passage. An additional set of GPS dropsondes will be densely deployed in the inner high-wind core regions of developed hurricanes. These closely spaced measurements will be used to infer surface fluxes, momentum and enthalpy based on the budget technique of Hawkins and Rubsam. The NASA airborne Scanning Radar Altimeter (SRA) will provide measurements of wave topography in all quadrants of hurricanes over open water. Directional wave and swell spectra will be deduced in real-time during the field experiment from SRA wave topography. A laser altimeter will be utilized to measure 1D wave spectra between rainbands in order to estimate the high-frequency portion of the ocean wave field not resolved by the SRA. A wave-following camera system will be utilized to document wave breaking processes and generation of foam and spray. A group of in situ extreme turbulence (ET) probes will be deployed from the aircraft to measure turbulence at the air-sea interface. In addition to the AXBTs, neutrally buoyant, Lagrangian floats will be deployed to measure 3D mean currents and large eddy turbulence properties. Wave spectra and momentum fluxes will be obtained from measurements by ambient noise sensors carried by the Lagrangian floats. Modified SOLO/ARGO floats will be also deployed by USAR WC-130, carrying additional sensors to measure surface wave heights, breaking, voids, heat fluxes, rainfall, wind speed, and the thermal-salinity structure of the upper ocean. Detailed logistics planning and coordination of aircraft operations in order for the multi-sensor, simultaneous, hurricane-ocean measurements to be successful will be conducted at HRD/AOML. The CBLAST Hurricane field measurements will complement experimental design and cross validation with the CBLAST DRI modeling component.
The CBLAST Hurricane field experiment will be coordinated with the USWRP Hurricane Landfall Experiment. At present the CBLAST effort already maintains a close relationship with an NSF-funded airborne hurricane-ocean field study relating ocean heat content changes in the Gulf of Mexico Loop Current and associated warm eddies to changes in hurricane intensity. The CBLAST field measurements will likely be joined by NASA CAMEX series and other NOAA and NSF-funded efforts within the 2003-2004 timeframe.