UVI Daily Change Analysis

We produced the daily derived UV-I data from 1999 to 2005 that was associated with the four representative ground-based stations and TOMS data to demonstrate the correspondence between the two data sets by statistical analyses (Table 10.1) and to verify the degree of agreement of the data between these two sources (Fig. 10.2). Table 10.1 provides mean, median, minimum, maximum, std. error, standard deviation, values, and correlation coefficients between the datasets. These summary statistics reveal the degree of correspondence between the two data sources. The maxima of the multi-year time series between these two sources are very close, and the absolute disparity is between 0.2 and 0.8 units. Median values from TOMS were always larger than those from USDA ground-based observations. As for the mean values, TOMS data are 5% to 12% larger than those of the USDA ground-based observations. The standard error of mean at these four stations is between 5.9% and 7.6%. Both standard error of mean and standard deviation are very close at three of the stations—WA01, CA01, and CO01, which shows the

Table 10.1 Daily statistical analysis of UV-I data collected from four ground stations and the TOMS data

WA01 CO01 CA01 AZ01

Table 10.1 Daily statistical analysis of UV-I data collected from four ground stations and the TOMS data

WA01 CO01 CA01 AZ01

USDA

TOMS

USDA

TOMS

USDA

TOMS

USDA

TOMS

Minimum

0.12

0.01

0.11

0.35

0.00

0.03

0.00

0.42

Maximum

10.50

10.70

13.08

12.24

11.20

11.63

14.20

12.86

Mean

3.84

4.03

5.20

5.83

5.26

5.83

6.53

6.95

Median

3.08

3.56

4.44

5.47

5.28

5.97

6.02

7.18

Std. error

0.059

0.060

0.073

0.074

0.063

0.066

0.076

0.067

Std. deviation

2.91

2.96

3.38

3.39

3.13

3.26

3.73

3.28

Correlation coefficient

0.

94

0.

87

0.

96

0.

87

Figure 10.2 Daily UV-I time series variations (left) and the scatter plot along the 45° angle bisector between the TOMS data and the USDA ground-based measurements (right) at 4 stations

similarity in fluctuations. The correlation coefficients associated with WA01 and CA01 are higher than 90% (i.e., significant test P < 0.0001), whereas the correlation coefficients associated with stations CO01 and station AZ01 are about 87% (i.e., significant test P < 0.0001). Such high agreement can also be seen in Fig.10.2, which plots the time-series of UV-I data and also the UV-I values from each data source against each other for all stations. Although there are some outliers that fall out of the 45° angle bisector (i.e., the one-to-one line), these scatter plots highlight the generally good agreement between the two datasets.

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