This section compares trends in the data for monthly differences from the mean value over the six-year period. Positive values indicate greater UV exposure than the mean and negative values indicate reduced UV exposure. Comparing such sequences associated with both data sources may enable us to identify the general trend and can also characterize how the patterns they detect differ. Figure 10.3 summarizes such an analysis as a whole using a series of time series plots. Within each plot, the y axis represents the UV-I difference relative to the mean and the x axis represents the month over the study period. On a monthly time scale, UV-I peak values agree between the two datasets. The mean square values of the UV-I deviation from the monthly means at the WA01 station are 9.75 and 9.69 for the TOMS and ground-based measurements, respectively. Obviously, similar fluctuations of these two data sources are confirmed. Yet the negative slope of TOMS time series data at station WA01 indicates a decrease of the deviation relative to the monthly means of the multi-year UV-I time series trend, which is quite different from the long-term trend of the ground-based measurements at this station. At station CA01, the two datasets differ substantially in terms of both the locations of the peak values and the overall trend. The mean square values of the UV-I deviation from the monthly means at station CA01 are 9.43 and 11.39 for the TOMS and ground-based measurements, respectively. The unique finding is that TOMS data show a decreasing trend in the ground-based measurements. At station CO01, the negative slopes of both deviations relative to the monthly means show a decreasing trend associated with both data sources at this station. The mean square values of the UV-I deviation from the monthly means at station CO01 are 7.43 and 10.61, associated with TOMS and ground-based measurements, respectively. As for the analysis at station AZ01, the deviations relative to the monthly means indicate stronger monthly variation in both data sources. The mean square values of the UV-I deviation from the monthly means at station AZ01 are 11.56 and 18.88, associated with TOMS and ground-based measurements, respectively, which are much larger than those for the three other stations. The small positive slope of deviations relative to the monthly means of ground-based measurements shows an increasing trend but the TOMS data appear to be slightly decreasing at this station.
Figure 10.3 Trend analysis of the UV-I deviations relative to the monthly means
(d) Ground-based measurements at AZ01 ;TOMS lime series data al AZOI
Figure 10.3 Trend analysis of the UV-I deviations relative to the monthly means associated with these two data sources at four stations
By comparing the four cases above, it can be concluded that the variability in the TOMS data fluctuations are changing with time at a greater rate than are the fluctuations in the USDA data. Because they are located at high elevation with less impact from clouds, ground-based measurements at stations COOl (i.e., elevation 1641 m) and AZ01 (i.e., elevation 2073 m) are more consistent with the satellite observations. At the other 2 stations, WA01 (i.e., elevation 804 m) and CA01 (i.e., elevation 18 m), the corresponding impacts from surface clouds and climate factors become obvious. Time series longer than six years might help clarify such discrepancies in the future.
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