Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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5.2. Comparison of Measured and Calculated Variabilities in UV Irradiance
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UV irradiance at the surface of the Earth depends on several variable factors identified in Section 5.1. These factors include scattering (Rayleigh, aerosol, and cloud)
and absorption (ozone, aerosol, and pollution) processes that occur in the atmosphere, as well as variations in extraterrestrial solar flux and ground reflectivity.
Variability in each of these factors combines to produce large fluctuations in UV irradiance-for example, between corresponding months of different years
(Weatherhead et al., 1997). This large variability makes it difficult to quantify systematic decadal changes in UV irradiance and interpret them in terms of cause and
effect with instruments other than well-maintained spectroradiometers.
It is important to understand and quantify the individual effects of the many variables affecting UV irradiance at the surface of the Earth. The contributions of the
different variables can be studied with the use of radiative transfer models. Several models have been developed for a variety of applications (Dave, 1965; Frederick
and Lubin, 1988; Stamnes et al., 1988; Madronich, 1992; Ruggaber et al., 1994; Forster, 1995; Herman et al., 1996). These models use extraterrestrial solar spectral
irradiance (Mentall et al., 1981; Neckel and Labs, 1984; Kaye and Miller, 1996; Woods et al., 1996) as input and simulate the physical processes that occur as
radiation is scattered and absorbed by the atmosphere and at the surface of the Earth. Model output includes global (direct plus diffuse) and diffuse spectral radiation
at the Earth's surface in a form that can be compared with measurements.
The comparison of UV measurements with model simulations is an important exercise for checking both the accuracy of the model and the quality of measurements.
Once it is demonstrated that the measurements and model results are in good agreement for a wide range of conditions, a reliable simulation of the transfer of UV
radiation through the atmosphere is possible. The model can then be used with confidence to extend a measurement series in time and in space (between groundbased
stations), provided measurements of all variables affecting surface UV irradiance are available. The model can also estimate future levels of surface UV
irradiance by using predictions of variables such as aerosols or ozone that may change as a result of increased air traffic. Models are used in this context in Section
5.4.
Comparisons between models and measurements are best evaluated by two approaches. One is the comparison of irradiance as a function of wavelength normalized
to a certain wavelength-usually in the UV-A, where ozone has a negligible effect. This comparison emphasizes the response of irradiance to variables that are strongly
wavelength-dependent (such as ozone absorption or wavelength error) and minimizes effects that are weakly wavelength-dependent, such as clouds, aerosols, or the
absolute calibration of instrument responsivity. The second approach is the comparison of absolute irradiances at the wavelength of the normalization to quantify
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