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Model-Derived Surface UV Radiation
In view of the high spatial and temporal variability of surface UV radiation, and the difficulty of maintaining calibration within networks of instruments, it is unlikely that either a satisfactory global UV climatology or representative long-term UV trends can be derived from ground-based monitoring stations alone. Satellite-based observations of the atmosphere, on the other hand, provide the spatial (global or nearly global) coverage required for climatology development, as well as nearly continuous long-term monitoring. For example, the development of a climatology of UV radiation incident on the oceans will necessarily be based on such satellite-derived estimates. However, the derivation of surface UV irradiance from satellite-based observations is indirect, because satellite instruments see radiation reflected by the atmosphere and surface of the Earth. The determination of radiation transmitted to the surface requires the use of radiative transfer models to relate transmission, reflection, and atmospheric absorption.

Figure 1.3 shows the changes in UV radiation (at 310 nm) reaching the surface, computed for clear skies using satellite-based ozone measurements between 1979 and 1993. As expected from ozone trends (WMO, 1998), UV trends are not significant in the tropics, but increase pole-ward in both hemispheres. The largest changes (percentage and absolute) are seen in the Southern Hemisphere polar regions, but significant inter-annual and shorter variability should be noted at all latitudes, even after considering monthly and zonal averages. Patterns of long-term changes differ also between hemispheres, e.g. with largest changes occurring in the early 1980s at southern mid-latitudes, while northern mid-latitudes show a more persistent long-term trend.
 
Figure 1.3: Changes in daily surface spectral irradiances at 310 nm, computed for cloud-free conditions from satellite-based ozone observations (TOMS, version 7, McPeters et al., 1996, monthly averages over different latitude bands indicated in each panel). Values given are deviations from the 1979-1993 means. Solid curves give absolute deviations (left scale), while dotted curves show percent changes (right scale). Note change of scales for different latitude bands.

Significant progress has been made in recent years, in utilizing satellite-based measurements of cloud cover as well as atmospheric ozone, to derive estimates of surface UV radiation levels (Eck et al., 1995; Herman et al., 1996; Meerkoetter et al., 1997). Recent work also suggests that it may be possible to derive tropospheric aerosol distributions from satellite-based observations (Herman et al., 1997; Krotkov et al., 1997; Hsu et al., 1997). Figure 1.4 shows the type of coverage and geographical detail currently possible with the satellite-based approach. Long-term trends in cloud cover have partly offset or augmented UV trends resulting from ozone changes in some regions, but have been shown by Herman et al. (1996) to have little effect on long term changes when averaged over large geographical scales (zonal means). This type of analysis represents a considerable improvement over earlier analyses of satellite data that considered only ozone changes, with no consideration of clouds (e.g., Madronich, 1992). Table 1.2 shows the trends in surface UV radiation (erythemal weighting) over 1979-1992, derived from measurements of ozone and cloud reflectivity by the Total Ozone Mapping Spectrometer (TOMS, version 7) aboard the Nimbus 7 satellite. Positive trends are statistically significant at the two-standard deviation level over much of the mid-latitudes of both hemispheres. Extension to more recent years is complicated by the use of different instruments aboard different satellites, and analysis is still underway (WMO, 1998).

    The limitations of these satellite-derived surface UV estimates should be recognized. The ozone and cloud determinations at any specific location are based on a single satellite overpass per day, and are estimated for other times by interpolation or, more simply, by assuming constancy over the specific day. Therefore it is essential that comparisons be made between ground-based UV monitoring and the satellite-derived UV levels, in order to have a more complete assessment of the uncertainties inherent in this method. Preliminary results of such comparisons are encouraging (e.g. Eck et al., 1995) but more ground-based validation is needed over longer periods of time and different geographical locations. Even so, comparisons to ground-based UV observations will not be able to account fully for some location-specific biases. For example, optical instruments borne by satellites have difficulty seeing the lower atmosphere (esp. in the presence of clouds) so local conditions (e.g. pollution) are not sampled accurately. Additional local factors, such as surface reflections and elevation gradients, are also problematic. Other promising approaches combine, as above, satellite data with radiative transfer calculations, but also include some ground-based observations by other instruments such as visible and total solar radiation detectors which are more accurate and much more widely deployed than UV detectors (Ito et al. 1994; Bodeker and McKenzie, 1996).


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