Parameterizations of the Sea-Spray Effects in the Hurricane Atmospheric Boundary Layer or The Role of Air-Sea Interaction in Hurricanes and Storms [Shuyi: please add the authors from U. of Miami here.] Jian-Wen Bao Christopher W. Fairall James M. Wilczak Sara A. Michelson Physical Sciences Division/NOAA-ESRL

Background Improvements to hurricane track prediction over past several decades have been much larger than improvements to hurricane intensity prediction. Hurricanes are heat engines driven by surface enthalpy flux. Surface drag reduces hurricane intensity. The ratio of enthalpy and momentum transfer coefficients (CK/CD) has been found to be key for controlling hurricane intensification in models. Direct measurement of enthalpy and drag in hurricane conditions is extremely difficult.

Sea Spray and CK Thermal and moisture evolution of spray droplets Small droplets Large droplets 1) Thermal conduction and evaporation occur on different time scales. 2) Time scales of both are highly dependent on drop size. Date of Meeting Meeting Title From E. L. Andreas (1995).

Important factors for Sea-Spray Total mass of spray Size distribution of spray droplets (Results from Andreas and Fairall can differ by one order). Mean residence time of each size droplet

Mass and Size Distribution of Spray SPANDEX – SPUME Droplet Study This figure shows some of the instruments used in the data from the tunnel study conducted at the UNSW Water Research Facility. The two black optical devices are part of the Phase Doppler Analyzer (PDA) system, which measures particle size and velocity vector. The gold colored device inside the tunnel is the Cloud Imaging Probe (CIP) manufactured by DMT in Boulder, CO; this device measures the size spectrum of droplets from 12 to 750 microns radius. Other instruments (not shown) include 7 different wave wires and 3 video imaging systems.

Droplet Spectra Profiles from SPANDEX

Physical Droplet Source Model (Fairall et al. 2006) (predicts the size spectrum of sea spray produced by the ocean in terms of wind speed, surface stress, and wave properties ) h: Significant wave height/2 P: energy wave breaking σ: surface tension r: droplet radius η: Kolmogorov microscale f: fraction of P going into droplet production Vf: droplet mean fall velocity

Parameterization for NWP Models Simplification: consider large droplets that are ejected, cool to wet bulb temperature and re-enter ocean with negligible change in mass Stages: Cool from To to Tair = Qs Cool from Tair to Twet = Ql_a Evaporation while at Twet = Ql_b Total droplet enthapy transfer Qse=Qs+Ql_a Enthalpy Bowen ratio = Qs/Ql_a=(To-Ta)/(Ta-Twet) Qs=Qse*bowen/(1+bowen)

Feedback Characterization δTa Effect on the fluxes:

Feedback of sea spray to the mean temperature and moisture profiles

NCEP operational hurricane model with and without spray parameterization Spray No spray No spray Spray Caveats for Interpretation need to be stressed.

Impact on Momentum Acceleration of spray takes momentum from air, directly affecting turbulence. Suppression of near surface drag takes place, which at the same time leads to the reduction of overall enthalpy flux. Tow parameterizations are available: E. Andreas (2004) G. I. Barenblatt et al. (1974, 2005)

Summary Effects of spray on hurricane intensity are strongly dependent on size spectrum of droplets: Small droplets do not add to the enthalpy flux, but cool and moisten the surface layer, and can decrease hurricane intensity. Large droplets increase the enthalpy flux, warm the surface layer, and increase hurricane intensity. A spray-mediated enthalpy parameterization incorporating our latest knowledge demonstrates a significant impact on hurricane intensity in research and operational hurricane models.

Future Work Test improved version of spray parameterization in operational hurricane model. Investigate effects of sea spray on surface drag. Couple spray parameterization with wave model. Interaction between the thermal feedback and kinematic feedback.