Jakob J. Stamnes

Accurate and self-consistent ocean color algorithm: simultaneous retrieval of aerosol optical properties and chlorophyll concentrations

A new algorithm has been developed for simultaneous retrieval of aerosol optical properties and chlorophyll concentrations in case I waters. This algorithm is based on an improved complete model for the inherent optical properties and accurate simulations of the radiative transfer process in the coupled atmosphere-ocean system. It has been tested against synthetic radiances generated for the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) channels and has been shown to be robust and accurate. A unique feature of this algorithm is that it uses the measured radiances in both near-IR and visible channels to find that combination of chlorophyll concentration and aerosol optical properties that minimizes the error across the spectrum. Thus the error in the retrieved quantities can be quantified.

Pitfalls in atmospheric correction of ocean color imagery: how should aerosol optical properties be computed?: reply to comment

Gordon [Appl. Opt.42, 542 (2003)] argues that use of external rather than internal mixing when aerosol optical properties are computed will not seriously affect atmospheric correction of ocean color imagery, in spite of the fact that top of the atmosphere reflectances computed with the two approaches differ significantly as shown by Yan et al. [Appl. Opt.41, 412 (2002)]. We apply an algorithm for simultaneous retrieval of aerosol optical properties and chlorophyll concentrations to demonstrate that use of the internal-mixing approach leads to atmospheric corrections that differ significantly from those obtained with the more realistic external-mixing approach. For relative humidities of 90% or more, the differences in retrieved aerosol optical properties and chlorophyll concentrations, incurred by application of the internal-mixing approach, become unacceptably large.

Shortwave Radiative Transfer in the Atmosphere and Ocean

Introduction Two problems in atmospheric and environmental science have received much attention: the occurrence of widespread ozone depletion and global warming. Ozone depletion has been related directly to the release of man-made trace gases, notably chlorofluorocarbons used in the refrigeration industry and as “propellants” in spray cans. Since ozone provides an effective shield against damaging ultraviolet radiation from the Sun, there is indeed good reason to be concerned, because a thinning of the ozone layer has serious biological ramifications. The most harmful ultraviolet (UV) radiation reaching the Earths surface, commonly referred as UV-B radiation, lies in the wavelength range between 280 and 320 nm (see Table 1.1). UV-B radiation, which has enough energy to damage the DNA molecule, is strongly absorbed by ozone. Radiation with wavelengths between 320 and 400 nm, referred to as UVA radiation, is relatively little affected by ozone. UV-A radiation can mitigate some of the damage inflicted by UV-B radiation (a phenomenon known as photo-repair), but it causes sunburn and is therefore believed to be a partial cause of skin cancer. In addition to the harmful effects on humans, too much UV radiation has deleterious effects on terrestrial animals and plants, as well as aquatic life forms. Ozone is a trace gas, whose bulk content resides in the stratosphere. Its abundance is determined by a balance between production and loss processes. Chemical reactions as well as photolysis are responsible for the destruction of atmospheric ozone. Its formation in the stratosphere relies on the availability of atomic oxygen, which is produced by photodissociation of molecular oxygen. Ozone is then formed when an oxygen atom (O) and an oxygen molecule (O 2 ) combine to yield O 3 . It is produced mainly high in the atmosphere at low latitudes where light is abundant, and subsequently transported to higher latitudes by the equator-to-pole circulation (Brasseur and Solomon, 2006). Thus, the distribution of ozone in the atmosphere, vertically and globally, is a result of a subtle interplay between radiation, chemistry, and dynamics. Ozone absorbs ultraviolet/visible radiation as well as thermal infrared (terrestrial) radiation in the 9.6 μ m band. A thinning of the ozone layer renders the stratosphere more transparent in the 9.6 μ m region, thereby allowing more transmission, and less surface backwarming.

Discrete ordinate and Monte Carlo simulations of radiative transfer in coupled atmosphere-ocean systems

We present comparisons between deterministic solutions of the vector radiative transfer (RT) equation based on the discrete ordinate (DISORT) method and probabilistic simulations based on the Monte Carlo (MC) method for an atmosphere comprised of either a size distribution of spherical aerosol particles with an average size of 0.3 μm or a size distribution of spherical cloud particles with an average size of 5 μm. Also, we discuss preliminary deterministic results for a coupled atmosphere-ocean system consisting of two turbid media separated by a plane interface across which the refractive index changes abruptly.