## 4 Results
### Comparison and Validation of Modified MOD10A1 Data with Hj-1A/B Data  
HJ-1A/B is a new generation of small, Chinese, civilian, earth-observing optical remote sensing satellites that have medium/high spatial resolution, high temporal resolution, hyperspectral resolution, and wide bandwidth. They are equipped with a wide-coverage multispectral charge-coupled device camera that has a nadir pixel resolution of 30 m and a field-of-view width of 360 km [34]. Multitemporal and multispectral data provided by HJ-1A/B can provide daily snow cover classification maps; thus, we utilized these images in our work as reference materials because of their superior resolution in both space and time.  
Figure 10 shows a graphical comparison of snow-covered areas derived from raw MOD10A1 data, modified MOD10A1 data, and HJ-1A/B data for four consecutive days from 14-17 May 2011. In comparison with data from the original MOD10A1 dataset, visual inspection shows that the modified MOD10A1 data have clearer snow cover outlines and they are more consistent with the snow cover results calculated from the HJ-1A/B data. In particular, the blurred edges of snow cover on the images of 15 and 17 May disappear following our modification, and the minor amount of cloud cover was accounted for on each daily image.  
Daily SCP values, the changes in these SCP values, their overall accuracies, and the calculated rates of increase of the overall accuracy for each of these three datasets are given in Table 2. By taking the HJ-1A/B SCP values as a reference, the decrease in SCP after modification of the MOD10A1 dataset indicated that snow cover overestimation due to a large SZA was especially important on 15 and 17 May, 2011. The overall accuracies of the MOD10A1 data for the four days were 67.75%, 63.28%, 66.68%, and 62.86%, respectively; the overall accuracies of the modified snow cover dataset were 68.14%, 67.96%, 67.20%, and 66.63%, respectively; and the overall accuracies for each improved by 0.39%, 4.68%, 0.52%, and 3.77%, respectively. The minor increases in the overall accuracies on 14 and 16 May were likely due to the occurrence of low SZA values that were below the threshold outlined above, whereas the clear increases in the overall accuracies on 15 and 17 May can be attributed to the modification of the snow cover overestimation pixels that had a large SZA.  
In order to demonstrate more convincingly the effects of this modification, we selected 30 snow-covered areas in 2011 that had SZA values larger than the optimal threshold of 22.37\({}^{\circ}\), and we compared the overall accuracies calculated for raw MOD10A1 data and modified MOD10A1 snow cover data using HJ-1 A/B as a reference. These results are given in Table S1 in the supporting material and they show that the overall accuracies of the modified dataset all increased in comparison with the overall accuracies of the original MOD10A1 data. These data showed an average increase of 4.11% in the accuracy, which we take as confirmation that our modified snow cover dataset is both practical and effective.  
Figure 9: Comparison of average snow cover frequency (SCF) values obtained from modified data (red squares) and only de-clouded data statistics (black circles) on an (**a**) annual and (**b**) monthly basis for the 10 years between 2002 and 2011.