## 3 Results
### Comparison with Field Measurements
#### 3.2.2 Evaluation of Seasonal Variability  
Figure 7 shows the seasonal variabilities in the C5 8-day CI (red), C6 daily CI (blue), and field CI (black) data for the 7 field sites with seasonal profiles of field-measured CIs (Table 2). All valid MODIS CI values from the field sites during the field measurement year are shown, including the main algorithm (circles and squares denote retrievals of QA = 0 and 1, respectively) and backup algorithm (triangles) retrievals. Corresponding scatter plot for comparison of C5 8-day CI and C6 daily CI data with the field CI data can be found in Supplementary Materials (Figure S1). Figure 6: Comparison between the monthly C5 and C6 CI retrievals and field measurements. Details of the field CI data are shown in Table 1. The red and blue dots indicate the C5 and C6 CIs, respectively. Solid regression lines represent the linear fits of the field-measured CIs with the MODIS CIs, which shows a higher accuracy acquired by using the updated version of upstream MODIS BRDF/albedo products. In general, seasonal variations in the C5 and C6 MODIS CI data agree with the field-measured CIs, which reflect the states of vegetation growth and its intrinsic structures to some extent. The C6 CI data are closer overall to the field-measured CIs than the C5 CI data, while the C6 CI data present more temporal details and fluctuations due to their higher temporal resolutions. Both versions of the MODIS CI data and field-measured CI data show lower CI values (0.4-0.8) and small seasonal variations at evergreen conifer forest sites (i.e., TP39, TP74, Yatir, and RAMI pine) and higher CI values (0.5-1.0) and strong seasonal variations at cropland sites (i.e., Honghe and Hailun) and woody savanna sites (i.e., Tonzi), which are consistent with previous studies [30,47,89,100]. The relatively small seasonal change in the CIs foriferous forests may be attributed to the low needle turnover rate [47,101]. For the TP39, TP74, and RAMI pine sites, the field-measured CIs are stable at approximately 0.55, while the C5 and C6 CIs fluctuate at approximately 0.55, and the C6 CIs are closer to the field-measured CIs than the C5 CIs. For the two cropland sites, field data were mainly collected in the growing season (days of year (DOYs) 170-240), and both the C5 and C6 CIs showed seasonal variations similar to the field-measured CI data, which exhibited lower CI values during the peak growing period and higher CI values at the beginning and end of the growing season [30]. At the Honghe site, paddy rice is usually  
Figure 7: Comparison of seasonal variabilities in the C5 8-day CI and C6 daily CI with the field CI data. Details of the field CI data are shown in Table 2. Black dots denote the field CI data. Red and blue symbols indicate the C5 and C6 CIs, respectively. Circle, square, and triangle symbols represent the quality of the MODIS CIs (i.e., C5 and C6 CIs), which are the best quality (QA = 0), good quality (QA = 1), and backup algorithm retrievals (QA = 2/3), respectively. transplanted in May (DOYs 120-150) with higher CI values, and subsequently, the CI values decrease with vegetation growth and return to higher values after the rice harvest in late August (DOYs 230-240). At the Hailun site, crops are generally sown in May with relatively high CI values, and then, the CI values decrease in June (DOYs 150-180) with vegetation growth and increase in July and August (DOYs 200-230) during the tasseling and milking stages; finally, they slightly increase in the mature stage in September (DOYs 240-270). In the fallow period, except for the freeze period (late October to April), the two versions of the MODIS CIs show relatively strong fluctuations before (DOYs 70-100) and after (DOYs 260-300) the growing season, which may be associated with the uncertainties in the corresponding MODIS BRDF data. The quality of MODIS BRDF data may be influenced by the increased heterogeneity of surface reflectivity before and after the growing season [108] and the poor angular sampling away from the principal plane in winter [109].