Radiative Flux in the Atmosphere

The radiant flux density, or irradiance, on the surface dS is obtained by integrating the radiance over all angles  

FIGURE 4.6 Relation between radiance and radiant flux density. 

When the radiance L is independent of direction, the radiative field is called isotropic. In this case, (4.13) can be integrated over the half-space, fi = 2n, and the relation between the irradiance and the radiance is E = nL. 

The irradiance on a horizontal surface is obtained from the incoming radiance by integrating the radiance over the spherical coordinates 0 and j) 

The spectral radiant flux density, F(X), is the radiant flux density per unit wavelength interval, expressed in watts per square meter per nanometer (W m 2 nm ). Equivalently, when considering radiation incident upon a surface, the spectral irradiance, E(X), is expressed as W m-2 nm 1. 

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Table 19-1 Radiatively important trace species in the atmosphere. Percent change in flux measured relative to the pre-industrial time...

Cynar F. J. and Yayanos A. A. (1993) The oceanic distribution of methane and its flux to the atmosphere over Southern California waters. In Biogeochemistry of Global Change Radiatively Active Trace Gases (ed. R. S. Oremland). Chapman and Hall, New York, pp. 551-573. 

Today, the anthropogenic emissions of SO, primarily from fossil fuel combustion, largely dominate the sulfur flux into in the atmosphere on the global scale. Climate models have determined the corresponding direct and indirect impacts on radiative forcing, but large uncertainties remain in these estimates. In fact, predictions of future climate need to account not only for the effects of sulfate aerosols, but also for the contributions of mineral dust, black carbon, organic carbon, and sea salt. The current view is that atmospheric particles should be treated as multicomponent, mul-... 

The incoming solar energy absorbed by the Earth is —44 units this is balanced by the net upward flux of infrared radiation of — 15 units, plus —6 unit loss by sensible heat conduction, and —23 unit loss by latent heat. The Earth emits — 115 units of infrared radiation to the atmosphere, whereas the atmosphere emits —170 units of infrared radiation, a net deficit of —55 units. Since the atmosphere absorbs —26 units of solar radiation, the net radiative loss from the atmosphere is —29 units this is made up for by the sensible and latent heat fluxes. The net radiative cooling of the atmosphere is thus balanced by the latent heat of condensation released in precipitation processes and by the convection and conduction of sensible heat from the surface. 

Dimethyl sulfide (DMS), through its oxidation to sulfate in the troposphere, acts as a source of cloud condensation nuclei, thus potentially influencing the radiative balance of the atmosphere. DMS is formed in sea water through the microbial decomposition of dimethyl sulfonioproprionate (DMSP), a compound believed to act as an osmolyte in certain species of marine phytoplankton. The flux of DMS to the atmosphere is controlled by its concentration in surface sea waters, which is controlled in turn by the rate of its decomposition. Estimates indicate that 7-40% of the total turnover of DMS in the surface waters of the Pacific Ocean is due to the photosensitized destruction of this compound, illustrating the potential importance of this pathway in controlling the flux of DMS to the atmosphere. 

In the forced convection layer, wind shear plays a dominant role, and the Monin-Obukhov similarity l pothe-sis applies. To develop their similarity theory Monin and Obirkhov idealized the field of motion that is frequently used in the atmosphere near the ground. It was assumed that all statistical properties of the temperature and velocity fields are homogenous and do not vary with time. The steady mean motion was considered to be unidirectional at all heights in the x-direction. Second-order terms in the equations of the field were considered to be negligible. The scale of motion was considered to be small enough to omit the Coriolis Force, and the radiative heat inflow was neglected. In the surface layer the turbulent fluxes are approximately constant at their smface values. 

The radiative flux terms (in S f, ) are typically separated into short-wave and long-wave fluxes. The short-wave fluxes, also called solar fluxes, are separated into direct and diffuse irradiance. The direct irradiance is the non-scattered flux, while the difflise irradiance is the scattered radiative flux from the sun. The direct irradiance is sometimes further separated into visible and near-infrared components. In cloudy model atmospheres, parametrizations based on cloud liquid water content, or more crudely on arbitrary attenuation based on relative humidity, are used. Typically only diffuse irradiance is permitted for overcast model conditions. Some models weight the fluxes for partly cloudy skies, using separate parametrizations for clear and overcast sky conditions. Polluted atmospheres also require parametrization of their effect on solar irradiance, although only a few mesoscale models have explored this issue. 

There are a number of estimates of the actinic flux at various wavelengths and solar zenith angles in the literature (e.g., see references in Madronich, 1987, 1993). Clearly, these all involve certain assumptions about the amounts and distribution of 03 and the concentration and nature (e.g., size distribution and composition) of particles which determine their light scattering and absorption properties. Historically, one of the most widely used data sets for actinic fluxes at the earth s surface is that of Peterson (1976), who recalculated these solar fluxes from 290 to 700 nm using a radiative transfer model developed by Dave (1972). Demerjian et al. (1980) then applied them to the photolysis of some important atmospheric species. In this model, molecular scattering, absorption due to 03, H20, 02, and C02, and scattering and absorption by particles are taken into account. 

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