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One on One Comparisons

During the time period of these intercomparisons, differences normalized to remove the annual cycle, should remain constant. When they do not is another indication of a calibration shift in the instrument.

Figure 8 shows the observations and UV Index forecasts for Davis, CA. Davis characteristically has very clear skies throughout the May to August time period.

Visual inspections of the time series suggests a change in the differences of the obserations and forecast values around day 190. Ratios of the normaled observations and forecasts are shown in Figure 9. Ratios are centered near the zero line to about day 160. The ratios following consistenly lower implying a calibration shift.

Figure 9

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