Whitecaps, sea-salt aerosols, and climate Magdalena D. Anguelova Physical Oceanography Dissertation Symposium College of Marine Studies, University of Delaware June 17-21, 2002 Breckenridge, Colorado

What? Outline How? Why? n Work n Background n Results

Aerosol effects IPCC, 2001

n Assessment: Effect of anthropogenic aerosols = Effect of all aerosols – Effect of natural aerosols Aerosol radiative forcing n Defined as… natural

Background atmosphere n Natural aerosols; n Baseline of an unperturbed atmosphere.

Background baseline Sea-salt aerosols are the dominant aerosol species in background atmosphere. Sea-salt aerosols n Natural aerosols; n Baseline of an unperturbed atmosphere.

Formation of sea-salt aerosols n Sea spray; n Droplet sizes:  m; < 20  m; n Sea-salt aerosols : Phase state; Sizes: to 20  m.

Climate effects of sea-salt aerosols n Direct effect n Indirect effect: n Halogen chemistry: -- cooling. Dominate the activation of CCN; Compete with SO 4 2- aerosols. Reactive Cl and Br; Tropospheric O 3 : Sink of S.

Sea-salt aerosol effects must be accounted for in climate models. Motivation

Modeling sea-salt aerosols n Generation; n Transport; n Diffusion and convection; n Chemical and physical transformations: in clear air; in clouds; below clouds; n Wet and dry deposition. Generation

Rate of production of sea spray per unit area per increment of droplet radius, r (s -1 m -2  m -1 ). Sea spray generation function Explicit forms for 4 size regions covering 1.6 to 500  m range. Andreas (2001) (Monahan and O’Muircheartaigh, 1980) Best

Improved generation function? r 0  1.6  m Measurements 0.1 W(U 10,  T, T s, S, f, d, C ) (Monahan and O’Muircheartaigh, 1986) ), C ),  T, T s, S, f, d, C ), C ), Ts, Ts, S, S, f, d, d Whitecap coverage

Need for a database W (U 10,  T, T s, S, f, d, C )

Need of a database W (U 10,  T, T s, S, f, d, C ) points 477 points New method

Outline n Work n Background n Results

n Ocean emissivity is composite e Method concept e  as W  n Emissivity of foam-free ocean is low. n Emissivity of foam-covered ocean is high. e – e s –  e r e f – e s –  e r W = = (e s +  e r )(1-W ) + W e f

The task: calculate emissivities Composite emissivity e : Specular emissivity e s : Foam emissivity e f : Roughness correction  e r : Radiative transfer equation Fresnel formula, Debye equation Fresnel formula, empirical relation Empirical relation T B, V, L T s, S U 10, T s SSM/I AVHRR NOAA SSM/I AVHRR

Valid estimation of W W < 0 e < e s +  e r 2 – 10 %

Error of W

Method accuracy Relative error,  W /W (%) Count

Whitecap coverage 27 March 1998

Validation with in situ data

n Magnitude; n Trend: Suppression at high winds; Enhancement at moderate winds. n Variability!

Outline n Work n Background n Results

Database Use: Investigate spatial and temporal characteristics of global whitecap coverage; Evaluate whitecap contribution to climate processes. Parameterize effects of additional factors on whitecaps; n Content: Daily and monthly estimates of W and  W for the entire 1998; Collocated measurements of U 10, T s, S ;

Spatial distribution n Same magnitude; n Different spatial features: More uniform; 3% instead of 1%. March 1998 W  U 10 3

Effects of additional factors Wind speed, U 10 (m s -1 ) Sea surface temperature, T s ( o C) n Wind fetch and duration; Surface-active material. March 1998

Ocean surface albedo n Natural climate agent; n Average: 0.11 W m -2 ; n Anthropogenic agents: Stratospheric ozone ( 0.18 W m -2 ) Biomass burning ( 0.21 W m -2 ) Land use ( 0.22 W m -2 ) Radiative flux changes,  F  (W m ‑ 2 )

CO 2 transfer velocity n 5 – 150 cm h -1 ; n Average: 56.8 cm h -1 ; n Tropics are source of CO 2 ; n Southern Ocean is sink of CO 2. CO 2 transfer velocity, k CO2 (cm h ‑ 2 ) Flux = k CO2  C

Improved generation function? r 0  1.6  m Measurements 0.1 W(U 10 ) (Monahan and O’Muircheartaigh, 1986) W(U 10,  T, T s, S, f, d, C )

Modified generation function    Assimilating new method estimates    Andreas, 2001 Monahan et al., 1986 Future work mm mm mm

Sea-salt aerosol loading n Magnitude; n Weak wind dependence; T s in places with U 10 = 10 m s -1 Sea surface temperature, T s o C T s in places with U 10 = 15 m s -1

Haywood et al., 1999 Model - Experiment Spatial distribution of sea-salt 1     10 6 Number flux, dF/dr 0 (#  m -1 m -2 s -1 ) Direct effect: 15 W m -2

Conclusions n Whitecap coverage estimation n Whitecap coverage database n Generation of sea-salt aerosols