Whitecaps, sea-salt aerosols, and climateMagdalena D. AnguelovaPhysical Oceanography Dissertation SymposiumCollege of Marine Studies, University of DelawareJune 17-21, 2002Breckenridge, Colorado

OutlineWhat?How?Why?WorkBackgroundResults

Aerosol effectsIPCC, 2001

Aerosol radiative forcing Assessment: Effect of anthropogenic aerosols = Effect of all aerosols Effect of natural aerosolsDefined asnatural

Background atmosphere Natural aerosols;Baseline of an unperturbed atmosphere.

Background baselineSea-salt aerosols are the dominant aerosol species in background atmosphere.Sea-salt aerosolsNatural aerosols;Baseline of an unperturbed atmosphere.

Formation of sea-salt aerosolsSea spray;Droplet sizes:0.5-500 m;< 20 m;Sea-salt aerosols:Phase state;Sizes: 0.025 to 20 m.

Climate effects of sea-salt aerosolsDirect effectIndirect effect:

Halogen chemistry:-- cooling.Dominate the activation of CCN;Compete with SO42- aerosols.Reactive Cl and Br;Tropospheric O3:Sink of S.

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

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

Sea spray generation functionRate of production of sea spray per unit area per increment of droplet radius, r (s-1 m-2 m-1).Best

Improved generation function?0.1, C ), T, Ts, S, f, dWhitecap coverage

Need for a databaseW (U10 , T, Ts , S, f , d , C ) 1234567910111213141516

Need of a databaseW (U10 , T, Ts , S, f , d , C ) 1234567910111213141516New method

OutlineWorkBackgroundResults

Method conceptOcean emissivity is compositeee as W Emissivity of foam-covered ocean is high.= (es + er)(1-W ) + W ef

The task: calculate emissivitiesComposite emissivity e:

Specular emissivity es:

Foam emissivity ef :

Roughness correction Der:

Valid estimation of W

W < 0 e < es + er2 10 %

Error of W

Method accuracyRelative error, W/W (%)Count

Whitecap coverage27 March 1998

Validation with in situ data

Validation with in situ dataMagnitude;Trend:Suppression at high winds;Enhancement at moderate winds.Variability!

OutlineWorkBackgroundResults

DatabaseUse:Investigate spatial and temporal characteristics of global whitecap coverage;Evaluate whitecap contribution to climate processes.Parameterize effects of additional factors on whitecaps;

Content:Daily and monthly estimates of W and W for the entire 1998;Collocated measurements of U10, Ts, S;

Spatial distributionSame magnitude;Different spatial features:More uniform;3% instead of 1%.March 1998

Effects of additional factorsWind fetch and duration; Surface-active material.March 1998

Ocean surface albedoNatural climate agent;Average: 0.11 W m-2; 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 (Wm2)

CO2 transfer velocity5 150 cm h-1;Average: 56.8 cm h-1;Tropics are source of CO2;Southern Ocean is sink of CO2.CO2 transfer velocity, kCO2 (cmh2) Flux = kCO2 C

Improved generation function?r0 1.6 mMeasurements0.1W(U10)(Monahan and OMuircheartaigh, 1986)W(U10, T, Ts , S, f , d , C )

Modified generation functionAssimilating new method estimatesAndreas, 2001Monahan et al., 1986Future workmmm

Sea-salt aerosol loadingMagnitude;Weak wind dependence;

Spatial distribution of sea-salt1105 4105 7105 1106Number flux, dF/dr0 (# m-1 m-2 s-1)Direct effect: 15 W m-2

ConclusionsWhitecap coverage estimation

Whitecap coverage database

Generation of sea-salt aerosols

Oceanic whitecaps produce sea-salt aerosols, which play a specific role in the climate system.

Whitecaps and sea-salt aerosols are the subjects of my work

and I am interested in their influence on the climate.

Today Ill present results of my research on this topic.This is a figure from the latest IPCC report on climate change comparing the radiative forcing of atmospheric aerosols with that of other climate agents either natural, like the solar activity, or anthropogenic, like the greenhouse gasses.

The integral effect of diffident types of anthropogenic aerosols is to cool the Earth-atmosphere system. Aerosol cooling could be from 0.5 up to 2.5 W m-2, which is comparable to the warming by the greenhouse gases. But while our scientific understanding of the warming processes is high, we lack an adequate understanding of how aerosols cool the system. Fossil fuel burning, biomass burning, industry, land use, and other human activities release into the atmosphere sulfate aerosols, mineral dust, black carbon, and organic carbon.

Increased concentration of anthropogenic aerosols in the atmosphere affects the absorption and scattering of radiation, which in turn, disturbs the normal equilibrated heat budget of the planet.

Aerosol radiative forcing is defined as the perturbation of the climate due to increased concentration of the anthropogenic aerosols.

We assess the aerosol radiative forcing as a difference between the effect of all aerosols present in the atmosphere and the effect of the natural aerosols. The accuracy of this assessment depends on how well we measure and model the concentrations of total and natural aerosols.

My work contributes to the assessment of the effect of natural aerosols.The natural aerosols determine the state of a clean background atmosphere.

This state sets up the baseline of an atmosphere unperturbed by the presence of anthropogenic aerosols.

An accurate assessment of the aerosol radiative forcing requires an accurate knowledge of the baseline state of the background atmosphere.

On a planet covered with more than 70% of ocean surface, ocean-produced sea-salt aerosols are the most numerous naturally emitted aerosols.

In remote pristine marine air, the physical, optical and chemical properties of sea-salt aerosols determine the radiative processes in the atmosphere and with that its background baseline.

The formation of sea-salt aerosols starts with the formation of sea spray droplets.

As the wind blows over the sea surface, waves break forming whitecaps. The bursting of bubbles within the whitecaps forms sea spray droplets.

The sizes of these droplets could be from half micron to more than half millimeter.

But only sea spray droplets with sizes below 20 micrometer stay long enough into the atmosphere to undergo processes that transform them into sea-salt aerosols.

While into the atmosphere, the sea spray droplets evaporate until they equilibrate with the ambient humidity. In this process they shrink in size and change their phase state from liquid to deliquescent or solid particles. These are the actual sea-salt aerosols.Sea-salt aerosols affect the climate directly, indirectly, and through their specific chemistry.

The most prominent role of sea-salt aerosols is in the direct increase of the planetary albedo.

The indirect effect of sea-salt aerosols relates to their influence on clouds. In clean atmosphere, when sea-salt aerosols are the main component of the total aerosol loading, CCN form exclusively on sea-salt aerosols. In polluted atmosphere, most CCN form on sulfate aerosols, but their activation depends strongly on the concentration of sea-salt aerosols.

Sea-salt aerosols provide volume and surface for chemical reactions, which significantly affect the atmospheric chemistry by:providing halogen radicals, such as Cl and Br atoms, in the troposphere;Affecting the formation and destruction of tropospheric ozone;Serving as a sink for atmospheric sulfur.

The importance of sea-salt aerosols for climate motivates this research and establishes the sea-salt aerosols as the main research subject.

In 1997, Gong, Barrie, and Blanchet first included sea-salt aerosols as an active part of a climate model.

The model simulates a suit of processes starting with the generation of sea-salt aerosols, their

My work contributes to the simulation of the generation of sea-salt aerosols. the concept of the method.

Let we have an area of ocean surface with water temperature Ts. At any given moment we have smooth facets, some roughness, and some whitecaps. All these features make up the composite emissivity of this surface.

This emissivity increases as the amount of whitecaps on this surface increases. We can observe this increase in the emissivity as an increase in the brightness temperature of the ocean surface, which is the product of the composite emissivity and the physical temperature of the water, with a microwave radiometer. So. In any given moment this radiometer will give us the real life situation on the surface.

On the other hand we know that the composite emissivity has several contributions, which are emissivity coming from the foam patches only and the emissivity from the smooth and rough water. When we equate these two approaches of seeing the ocean surface, we can get an expressions for the whitecap coverage. The task at hand now is to find a way to compute all these emissivities sea water emissivity, specular emissivity, foam emissivity, and roughness emissivity.

I will show you how I can do that.The result for the whitecap coverage on March 27, 1998 is this. The range for whitecap coverage values is from 0 to 24 %. Here the scale is stretched for 0 to 10% in order to see details better.

The major features in this map can be even better visualized with a black background. We see, the whitecap coverage is high (6 - 8 , up to 10%) in the high latitudes, where the winds are strong, but also in mid latitudes where the wind is not so strong but the water temperature is higher.

How I can validate the results of this new method? I do that in two ways.