Sea salt aerosol (SSA) exerts a major influence over a broad reach of geophysics. It is important to the physics and chemistry of the marine atmosphere and to marine geochemistry and biogeochemistry generally. It affects visibility, remote sensing, atmospheric chemistry, and air quality. Sea salt aerosol particles interact with other atmospheric gaseous and aerosol constituents by acting as sinks for condensable gases and suppressing new particle formation, thus influencing the size distribution of these other aerosols and more broadly influencing the geochemical cycles of substances with which they interact. As the key aerosol constituent over much of Earth's surface at present, and all the more so in pre-industrial times, SSA is central to description of Earth's aerosol burden.

Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models - A Critical Review

Images of the ocean surface taken with active microwave sensors often contain much information on near-surface and subsurface processes. However, their interpretations may depend on a detailed understanding of the physics of electromagnetic scatter. In scattering theory, the surface hydrodynamics enters the equations via (1) the probability distribution function for either wave heights or slopes, and (2) the two-dimensional wave height/slope autocovariance or its Fourier transform, the wave vector spectrum. This paper advances an improved model for the ocean surface wave vector spectrum based on recent work by M. A. Donelan, by M. L. Banner, and by B. Jahne and their collaborators. The model addresses the range of surface wavelengths from fully developed wind waves to the gravity-capillary region. For gravity-capillary waves, the spectral equation satisfactorily represents the observational data of Jahne et al. taken in tanks at large fetches, in the range from approximately 50 to 1500 rad/m to within the accuracy of the data. From the spectrum, the two-dimensional autocovariance of the sea surface is computed and correlation lengths and curvatures obtained. When used with a modification of Holliday's formulation of microwave radar backscatter from a Gaussian sea, it quantitatively reproduces observational cross section data taken at vertical polarization from aircraft and spacecraft over the open ocean, with differences from the field data having a mean of −0.2 dB and a standard deviation of 1.7 dB, The range of parameters for which satisfactory fits are obtained includes: wind speeds from 1.5 to 24 m/s; frequencies from approximately 5.5 to 35 GHz; and incidence angles from 0° to greater than 60°. For horizontal polarization, the scattering calculations fail rather badly for larger incidence angles, as do all theories based on the Kirchhoff approximation. Additionally, in spite of the incorporation of an anisotropic angular distribution of wave energy, the observed azimuthal variation of radar scatter is not captured, indicating that the source of that variation lies elsewhere.

An improved model of the ocean surface wave vector spectrum and its effects on radar backscatter

Publisher Summary The basic concept of satellite altimetry is to measure the range from the satellite to the sea surface. The altimeter transmits a short pulse of microwave radiation with known power toward the sea surface. The pulse interacts with the rough sea surface and a part of the incident radiation reflects back to the altimeter. The chapter emphasizes on the correction algorithms applied to the dual-frequency altimeter onboard the TOPEX/POSEIDON (T/P) satellite. This state-of-the-art altimeter sets the standard for future altimeter missions as it is significantly more accurate than any of the other altimeters that have been launched to date. To provide assurance that the performance requirements for altimeter measurement accuracy are met or exceeded, extensive calibration and validation (cal/val) are important elements of altimeter missions. Cal/val embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors to the validation of the fully corrected sea-level estimates using in situ data. The chapter concludes with a summary of the T/P mission design and an assessment of the performance of the T/P dual-frequency altimeter in addition, as well as an overview of future altimeter missions.

Chapter 1 Satellite Altimetry

Two-dimensional wave slope spectra have been measured in the large Delft wind-wave facility using an imaging optical technique and digital image processing. The data cover wavelengths from 0.4 to 24cm and wind speeds (U10) from 2.7 to 17.2 ms−1. The spectral densities of small gravity waves at higher wind speeds are proportional to k−3.5 and u*. Capillary-gravity and capillary waves show features which clearly manifest that the energy balance for these waves is much different from that for gravity waves. The degree of saturation is approximately constant at a given wind speed, but strongly increases with friction velocity (∝ u*2.5). A sharp cutoff, which is almost independent of the wind speed, occurs at a wavelength of about 7 mm.

Two-dimensional wave number spectra of small-scale water surface waves

Numerous case reports and regional studies on swell and wind sea events have been documented during the past century. The global picture of these common oceanic phenomena, however, is still incomplete in many aspects. This paper presents a feasibility study of using collocated wind speed and significant wave height measurements from simultaneous satellite scatterometer and altimeter sources to observe the spatial and seasonal pattern of dominant swell and wind wave zones in the world's oceans. Two energy-related normalized indices are proposed, on the basis of which global statistics of swell/wind sea probabilities and intensities are obtained. It is found that three well-defined tongue-shaped zones of swell dominance, termed “swell pools,” are located in the eastern tropical areas of the Pacific, the Atlantic, and the Indian Oceans, respectively. Regions of intensive wave growth are observed in the northwest Pacific, the northwest Atlantic, the Southern Ocean, and the Mediterranean Sea. Seasonal...

A Global View of Swell and Wind Sea Climate in the Ocean by Satellite Altimeter and Scatterometer

For equilibrium and near-equilibrium sea states, the wave slope variance is a function of wind speed U and of the sea maturity. The influence of both factors on the altimeter measurements of wind speed, wave height, and radar cross section is studied experimentally on the basis of 1 year's worth of Geosat altimeter observations colocated with in situ wind and wave measurements by 20 NOAA buoys. Errors and biases in altimeter wind speed and wave height measurements are investigted. A geophysically significant error trend correlated with the sea maturity is found in wind-speed measurements. This trend is explained by examining the effect of the generalized wind fetch on the curves of the observed dependence. It is concluded that unambiguous measurements of wind speed by altimeter, in a wide range of sea states, are impossible without accounting for the actual degree of wave development.

Stuart H. Pilorz

Papers

Effects of sea maturity on satellite altimeter measurements