Magdalena D. Anguelova

Production flux of sea spray aerosol

Knowledge of the size- and composition-dependent production flux of primary sea spray aerosol (SSA) particles and its dependence on environmental variables is required for modeling cloud microphysical properties and aerosol radiative influences, interpreting measurements of particulate matter in coastal areas and its relation to air quality, and evaluating rates of uptake and reactions of gases in sea spray drops. This review examines recent research pertinent to SSA production flux, which deals mainly with production of particles with r 80 (equilibrium radius at 80% relative humidity) less than 1 m and as small as 0.01 m. Production of sea spray particles and its dependence on controlling factors has been investigated in laboratory studies that have examined the dependences on water temperature, salinity, and the presence of organics and in field measurements with micrometeorological techniques that use newly developed fast optical particle sizers. Extensive measurements show that water-insoluble organic matter contributes substantially to the composition of SSA particles with r80 < 0.25 m and, in locations with high biological activity, can be the dominant constituent. Order-of-magnitude variation remains in estimates of the size-dependent production flux per white area, the quantity central to formulations of the production flux based on the whitecap method. This variation indicates that the production flux may depend on quantities such as the volume flux of air bubbles to the surface that are not accounted for in current models. Variation in estimates of the whitecap fraction as a function of wind speed contributes additional, comparable uncertainty to production flux estimates.

Dielectric and Radiative Properties of Sea Foam at Microwave Frequencies: Conceptual Understanding of Foam Emissivity

Foam fraction can be retrieved from space-based microwave radiometric data at frequencies from 1 to 37 GHz. The retrievals require modeling of ocean surface emissivity fully covered with sea foam. To model foam emissivity well, knowledge of foam properties, both mechanical and dielectric, is necessary because these control the radiative processes in foam. We present a physical description of foam dielectric properties obtained from the foam dielectric constant including foam skin depth; foam impedance; wavelength variations in foam thickness, roughness of foam layer interfaces with air and seawater; and foam scattering parameters such as size parameter, and refraction index. Using these, we analyze the scattering, absorption, reflection and transmission in foam and gain insights into why volume scattering in foam is weak; why the main absorption losses are confined to the wet portion of the foam; how the foam impedance matching provides the transmission of electromagnetic radiation in foam and maximizes the absorption; and what is the potential for surface scattering at the foam layers boundaries. We put all these elements together and offer a conceptual understanding for the high, black-body-like emissivity of foam floating on the sea surface. We also consider possible scattering regimes in foam.

Global distribution and seasonal dependence of satellite-based whitecap fraction

We present the first study of global seasonal distributions of whitecap fraction, W, obtained from satellite-based radiometric observations. Satellite-based W incorporates variability from forcings other than wind speed and can capture differences in W in initial and late lifetime stages. The satellite-based Wis more uniform latitudinally than predictions from a widely used wind speed-dependent parameterization, W(U10), formulated from in situ observations, being on average higher than the W(U10) predictions at low latitudes and lower at middle and high latitudes. This difference provides an explanation for the consistent geographical biases in sea spray aerosol concentration found in a number of large-scale models. Satellite estimates of W would benefit air-sea interaction and remote sensing applications that use parameterizations in terms of W such as sea spray flux, gas transfer, and surface winds.

Passive Remote Sensing of Sea Foam using Physically-Based Models

Demonstrating the high variability of sea foam fraction (whitecap coverage) on the ocean surface, we justify the need for estimating foam coverage with space-based passive remote sensing as an alternative to conventional photographic measurements. We outline the concept and prove the feasibility of a method for deriving foam coverage from satellite measurements. The encouraging results of an initial implementation motivate further work addressing its drawbacks. We describe data and modeling improvements to the initial algorithm and report results on satellite-derived whitecap coverage using physically based models.

Aerial Radiometric and Video Measurements of Whitecap Coverage

This paper presents the results of high-altitude microwave radiometric and video measurements in the presence of breaking waves made during the passage of Hurricane Dean on August 21, 2007, over the Gulf of Mexico. Previous measurements of foam fraction and radiometric brightness temperature have focused on the small scale, in which individual foam patches were of the same scale as the radiometer footprint. To work with data from spaceborne microwave radiometers, which have footprints on the scale of tens of kilometers, the knowledge of how the foam fraction sensitivity of brightness temperature scales when footprints increase from meters to kilometers is necessary. Video images of the sea surface recorded with a high-resolution monochrome digital camera were used to determine the foam fraction. Ocean-surface brightness temperature was measured with the Airborne Polarimetric Microwave Imaging Radiometer (APMIR) of the Naval Research Laboratory at frequencies of 6.6 [vertical and horizontal (VH) polarizations], 6.8 (VH), 7.2 (VH), and 10.7 GHz (V), with full polarimetric brightness temperatures measured at 19.35 and 37.0 GHz. Collocated nearly contemporaneous brightness temperatures were available from WindSat, Special Sensor Microwave Imager/Sounder, and Special Sensor Microwave/Imager satellite radiometer overpasses. Oceanographic and meteorological data were taken from buoys located along the flight track. There was good correlation between brightness temperatures measured with APMIR and satellite-borne radiometers with absolute differences largely within the expected uncertainty of the data. An analysis of the video imagery provided the fractional area coverage of the actively breaking waves on the ocean surface. The increase in brightness temperature from each of the microwave sensors was correlated with the whitecap coverage measured by the camera. The experiment not only serves as an important bridge between measurements made with spatial scales on the order of tens of meters and data collected from satellites with spatial scales of tens of kilometers but also provides guidance for improving future field measurements on this topic.

Comparing in situ and satellite‐based parameterizations of oceanic whitecaps

The majority of the parameterizations developed to estimate whitecap fraction uses a stability-dependent 10 m wind (U10) measured in situ, but recent efforts to use satellite-reported equivalent neutral winds (U10EN) to estimate whitecap fraction with the same parameterizations introduce additional error. This study identifies and quantifies the differences in whitecap parameterizations caused by U10 and U10EN for the active and total whitecap fractions. New power law coefficients are presented for both U10 and U10EN parameterizations based on available in situ whitecap observations. One-way analysis of variance (ANOVA) tests are performed on the residuals of the whitecap parameterizations and the whitecap observations and identify that parameterizations in terms of U10 and U10EN perform similarly. The parameterizations are also tested against the satellite-based WindSat Whitecap Database to assess differences. The improved understanding aids in estimating whitecap fraction globally using satellite products and in determining the global effects of whitecaps on air-sea processes and remote sensing of the surface.

Whitecap lifetime stages from infrared imagery with implications for microwave radiometric measurements of whitecap fraction

Quantifying active and residual whitecap fractions separately can improve parameterizations of air-sea fluxes associated with breaking waves. We use data from a multi-instrumental field campaign on FLoating Instrument Platform (FLIP) to simultaneously capture the signatures of active and residual whitecaps at visible, infrared (IR) and microwave wavelengths using, respectively, video camera, mid-IR camera, and a radiometer at 10 GHz. We present results from processing and analyzing IR images and correlating this information with radiometric time series of brightness temperature at horizontal and vertical polarizations TBH and TBV. The results provide evidence that breaking crests and decaying foam appear in mid-IR as bright and dark pixels clearly distinguishing active from residual whitecaps. We quantify the durations of whitecap lifetime stages from the IR images and identify their corresponding signatures in TB time series. Results show that TBH and TBV vary in phase during the active and in anti-phase during the residual whitecap stages. A methodology to distinguish active and residual whitecaps in radiometric time series without a priori IR information has been developed and verified with corresponding IR and video images. The method uses the degree of polarization P (the ratio between the sum and difference of TBV and TBH) to capture whitecaps as prominent spikes. The maximum and zero-crossing of the first derivative of P serve to identify the presence of active whitecaps, while the minimum of dP marks the transition from active to residual whitecap stage. The findings have implications for radiometric measurements of active and total whitecap fractions. This article is protected by copyright. All rights reserved.

Foam emissivity models for microwave observations of oceans from space

Adequate accounting for the effect of the sea foam in forward models has the potential to improve the accuracy of satellite-based geophysical retrievals of environmental variables from passive radiometric measurements at frequencies from 1 to 100 GHz. Sea foam has a specific mechanical structure which gradually changes within the foam depth. Due to this vertical stratification, the foam characteristics acquire a wide range of values. To model the foam vertical stratification, it is necessary to use a vertical profile for the foam void fraction and to consider scattering between densely packed bubbles. In this study we evaluate two models which address the specifics of a vertically structured foam layer comprising densely packed bubbles in different ways.

Using Energy Dissipation Rate to Obtain Active Whitecap Fraction

Active and total whitecap fractions quantify the spatial extent of oceanic whitecaps in different lifetime stages.TotalwhitecapfractionWincludesboththedynamicfoampatchesoftheinitialbreakingandthestatic foam patchesduring whitecap decay. Dynamic air‐sea processes in the upper ocean are best parameterized in terms of active whitecap fraction WA associated with actively breaking crests. The conventional intensity threshold approach used to extract WA from photographs is subjective, which contributes to the wide spread of WA data. A novel approach of obtaining WA from energy dissipation rate « is proposed. An expression for WA is derived in terms of energy dissipation rate WA(«) on the basis of the Phillips concept of breaking crest length distribution. This approach allows more objective determination of WA using the breaker kinematic and dynamic properties yet avoids the use of measuring breaking crest distribution from photographs. The feasibilityofusingWA(«)isdemonstratedwithonepossibleimplementationusingbuoydataandaparametric model for the energy dissipation rate. Results from WA(«) are compared to WA from photographic data. Sensitivity analysis quantifies variations in WA estimates caused by different parameter choices in the WA(«) expression. The breaking strength parameterb has the greatest influence on the WA(«) estimates, followed by the breaker minimal speed and bubble persistence time. The merits and caveats of the novel approach, possible improvements, and implications for using the WA(«) expression to extract WA from satellite-based radiometric measurements of W are discussed.

Primary Marine Aerosol Fluxes

A number of atmospheric, oceanic, and aerosol scientists gathered at the National University of Ireland in Galway to review the existing research of the production of sea spray at the air-sea interface. The meeting was divided into three sessions, such as micrometeorological methods for measuring the rate of particle production, the enrichment of aerosol with organic matter, and global estimates of sea spray production. The scientists informed the participants that direct eddy covariance measurements and flux-profile techniques were applied to marine aerosol fluxes in a few studies over a period of time despite being well established for other scalar fluxes. Sea sprays role in numerous physical and chemical processes made it essential to assess global production, including the organic fraction and to incorporate spray production in general circulation, chemical transport, and climate models.