Introduction

The atmosphere lies between the ocean surface and the satellite sensor, and greatly affects the transmission of radiation. The presence of fixed concentrations of atmospheric gases such as oxygen, carbon dioxide, ozone and nitrogen dioxide, plus the variable concentrations of water vapor, means that only a few windows exist in the visible, infrared and microwave for Earth observations. Even within these windows, the atmospheric absorption varies with the concentration of water vapor and with the liquid water droplets and ice particles that make up clouds. The absorption is also affected by atmospheric aerosols, which include the water droplets and salt nuclei in the marine boundary layer, and the particulate matter generated over land by urban pollution, biomass burning and volcanic eruptions that is advected over the oceans.

In the following, Section 4.2 describes the vertical structure of the atmosphere and the molecular and aerosol constituents that affect the transmission of radiation. Sections 4.3 and 4.4 describe the propagation, absorption and scattering of a narrow beam of radiation. For the different atmospheric constituents, Section 4.5 discusses the dependence of transmissivity on wavelength and the role of these constituents in defining the atmospheric windows. To prevent the chapter from becoming overly long, the discussion is restricted to the visible/infrared; Chapter 9 extends it to the microwave. Section 4.6 applies these results to the ideal instrument. Section 4.7 discusses the radiative transfer equation (RTE) and the atmospheric emission and scattering source terms. Section 4.8 derives two limiting solutions of the RTE, one for the infrared and microwave windows where absorption and emission dominate; the other for the visible wavelengths where absorption and scattering dominate. Section 4.9 concludes with a discussion of diffuse attenuation and skylight.

Since the publication of the first edition a decade ago, the variety and use of ocean observing satellites has continued to grow. Combined with a similar expansion in computer resources and in surface receiving and distribution networks, this growth has greatly increased our knowledge of the properties of the upper ocean and the overlying atmosphere.

Ten years ago, many satellites were large, managed by single countries and carried multiple sensors. Now, by international agreement, different countries collaborate on constellations of smaller satellites that fly in complementary orbits and focus on a single oceanic or atmospheric feature such as biology, winds or sea surface temperature (SST). Many of these data sets such as SST from the constellations are available in a common format from public archives that also provide software tools for working with the data. These constellations and their archives greatly improve research opportunities for students and professionals.

Because remote sensing involves many disciplines, the book provides under one cover the necessary background in electromagnetic theory, atmospheric and seawater properties, physical and biological oceanography, physical properties of the sea surface and the properties of satellite orbits. The contents range from the reflective and emissive properties of clouds and foam to the radar-scattering properties of ocean waves, to the optical properties of plankton-associated pigments. It also includes many examples. The book describes the development of satellite oceanography from 1975 to 2013, and outlines pending missions. The book requires only an introductory knowledge of electromagnetic theory and differential equations.

Introduction

The importance of the multifrequency passive microwave imagers is that, irrespective of cloud cover, they can retrieve a large variety of surface and atmospheric variables. Figure 9.1 shows the atmospheric and surface variables that affect the transmissivities and emissivities, and are retrievable by passive microwave. In the atmosphere, these include columnar water vapor, the cloud liquid water and the surface rain rate. At the surface, they include sea ice extent and type, sea surface temperature (TS) and salinity (SS) and the scalar and vector wind speeds.

As this chapter shows, because the emissivity or transmissivity associated with each atmosphere or oceanic constituent has a different frequency dependence, the variables are retrieved from a set of multifrequency, multivariable simultaneous equations. The disadvantage of this formulation is that in most cases the solutions are not separable, so that if, for example, the only variable of interest is TS, then by necessity many of the other variables must also be retrieved. An advantage of the microwave is that except in regions with heavy rain the retrievals are independent of cloud cover. In the following, Section 9.2 discusses the frequency dependence of the atmospheric absorption and transmission, and shows that, for frequencies less than 10 GHz, the effects of clouds and water vapor are negligible. Section 9.3 discusses the microwave form of the radiative transfer equation, the problem of sun glint, radio-frequency interference and Faraday rotation. Section 9.4 describes the effect on the surface emissivity of ocean waves, surface roughness and foam and shows how the azimuthal distribution of capillary and gravity waves relative to the wind direction permits retrieval of the vector wind speed. Section 9.5 describes the effect of sea surface temperature and salinity on the emissivity; Section 9.6 describes the multichannel algorithms for retrieval of the different oceanic and atmospheric variables; Section 9.7 describes the WindSat retrieval of vector winds; Section 9.8 discusses the sea ice algorithms.