Relationship Between UVyitD and UVEry

The relationship between UVVitD and UVEry, as deduced from several years of spectral UV data from Lauder, New Zealand, is shown in Fig. 2.12. At first impression, the plot in the upper panel seems to indicate a close proportionality between the two quantities, with UVVitD being approximately 2 x UVEry (for standard normalization). However, at lower values, which are characteristic of the situation throughout winter months, this proportionality breaks down. For the truncated version of the action spectrum, the plot is very similar (not shown), but the slope is reduced by approximately 3%.

The relationship is examined in more detail in the lower panel of Fig. 2.12, where the ratio UVvitD/UVEry is plotted as a function of SZA. The ratio is ~2 when the SZA is small, and reduces to a value close to unity at SZA ~70°.The ratio reaches a minimum for SZA ~85° and increases thereafter. As expected, the departures from proportionality with UvEry are more marked, and the ratios are slightly lower, with a truncated version of the action spectrum (not shown). There are groupings of the data at 5-degree steps in SZA, which occur because these are the preferred sampling intervals in the spectral measurements. Several spectra are also taken at 15-minute intervals over a 2-hour period around the solar noon. The largest vertical separation in points occurs at intermediate SZAs, where there is a distinct bimodal distribution of data at each SZA. This is a consequence of the seasonal variations in ozone. In the spring, when ozone amounts are larger, the ratio is smaller than in autumn when ozone amounts are smaller.

Although UVVitD is not directly proportional to UVEry, it is possible to estimate UVVltD from UVEry using a radiative transfer model if ozone and SZA are known (McKenzie et al., 2008). The measured ratios in Fig. 2.12 are consistent with those calculated with the TUV radiative transfer model.

In summary,

where the values of their ratio (R) are given as a function of ozone and SZA (see Fig. 2.12, and the appendix in (McKenzie et al., 2008)).

Figure 2.12 Left panel: Relationship between UVVitD and UVEry based on 100,000 spectra measured at Lauder, New Zealand over the period 1998 to 2007. Right panel: Ratio UVVitD/UVEry as a function of SZA

Figure 2.12 Left panel: Relationship between UVVitD and UVEry based on 100,000 spectra measured at Lauder, New Zealand over the period 1998 to 2007. Right panel: Ratio UVVitD/UVEry as a function of SZA

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