Parameterizations of the Sea-Spray Effects in the Hurricane Atmospheric Boundary Layer

orThe Role of Air-Sea Interaction

in Hurricanes and Storms

[Shuyi: please add the authors from U. of Miami here.]Jian-Wen Bao

Christopher W. FairallJames M. WilczakSara A. Michelson

Physical Sciences Division/NOAA-ESRL

• 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.

Background

Date of Meeting Meeting Title From E. L. Andreas (1995).

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.

Sea Spray and CK

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 SpraySPANDEX – SPUME Droplet Study

Droplet Spectra Profiles from SPANDEX

4

3

9

41 2

34 3

rS r

f

l rerf

U h V Slopen

f

u

( )P r

exp [ ( ) ] * [ (( ) /

) ] //

h: Significant wave height/2P: energy wave breaking σ: surface tensionr: droplet radiusη: Kolmogorov microscale f: fraction of P going into droplet productionVf: droplet mean fall velocity

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 )

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

H L C U q T q T Tl a e E s o s d d [ ( ) ( ) ]

H L C U T T Ts a e E o a a [ ( ) ]

Q c F T Tse w pw v o w ( )

Q L G U h T q T T q T Tlb w e o s a a s d d ( ) ( )[ ( ) ( )]

T T feed T T feed sa d a w

1 1

1* ( ) * ( )

feedQ

Q H Q H feed tunel

l s se s

( ) / _

Effect on the fluxes:

Feedback of sea spray to the mean temperature and moisture profiles

NCEP operational hurricane modelwith 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.