AHI NDVI temporal signatures

We began our analysis by comparing AHI NDVI temporal profiles to those of VIIRS using the PEN-derived in situ phenology information as a reference (see Supplementary Table S2). The extracted AHI and VIIRS NDVI temporal profiles for the three study sites are shown in Fig. 1. All of the observations, including cloud-contaminated ones, were included in these plots. Since cloud contamination lowers the NDVI, cloud-contaminated NDVI data can be seen as those scattering below the upper envelopes of the NDVI temporal profiles which in turn most likely consisted of cloud-free pixels30,31. A small number of AHI NDVI observations that scattered above the upper envelopes and that were seen more frequently in the winter period (e.g., Fig. 1c,e) were those located near cloud edges, where the brightness typically varies significantly over a short distance. Due most likely to very small band-to-band mis-registration of AHI red and NIR bands, AHI reflectance spectra of those pixels were distorted, resulting in unusual NDVI values.

Figure 1 NDVI temporal profiles for the study sites: (a) AHI and (b) VIIRS for Takayama (TKY), (c) AHI and (d) VIIRS for Fujihokuroku (FHK), and (e) AHI and (f) VIIRS for Terrestrial Environment Research Center of University of Tsukuba (TGF). Full size image

Each VIIRS NDVI temporal profile was composed of ~500 observations per year (Fig. 1b,d,f). With its 10-min temporal resolution, in contrast, each of the AHI NDVI temporal profiles consisted of ~13,000 observations per year, 26 times more observations than VIIRS (Fig. 1a,c,e). This larger number of observations apparently made the AHI temporal profiles capture NDVI seasonal changes more clearly, or more certainly, than the VIIRS counterparts. In the VIIRS NDVI temporal profile of the Takayama deciduous broadleaf forest (TKY) site, for example, there was no observation with a high NDVI value for about a month from the middle of September to early October 2016 (Fig. 1b). The AHI NDVI temporal profile of the same site had approximately a 10-day period of continuously very low NDVI values earlier in this one-month period, but then had index values as high as those before the 10-day period for the rest of the period (Fig. 1a). Thus, with the AHI profile, one would be able to consider that the NDVI linearly decreased in this period with no abrupt vegetation change. Similar “gaps” were seen for the other two sites and they were associated with the passage of Typhoon-16 (Malakas) as described later in this section (Fig. 2). For the Fujihokuroku deciduous needleleaf forest (FHK) site, as another example, the VIIRS NDVI varied largely during the 2016 and 2017 summer (June-September) seasons, making it difficult to depict an cloud-free NDVI temporal signature (Fig. 1d). The AHI NDVI also varied largely, but due to the availability of a larger number of observations one could depict its flat signature with a little uncertainty (Fig. 1c). Compared to the above two sites, the VIIRS and AHI NDVI temporal profiles were very similar both in general and in detail for the Terrestrial Environment Research Center (TGF) site (Fig. 1e,f, respectively), a grass field surrounded by complex mosaics of urban, crop and rice paddy fields, grassland, and deciduous broadleaf forest patches.

Figure 2 AHI NDVI temporal profile (middle) compared to the VIIRS counterpart (bottom) for the Takayama (TKY) site. Sample PEN in situ images representing five phenological stages are shown at the top. The numbers on the PEN images are their acquisition year (lower-left) and DOY (lower-right). The vertical dashed lines are phenological transition dates identified with PEN time-lapse images (see Supplementary Table S2), whereas the gray bar on the AHI NDVI plot corresponds to a two-day data gap. Full size image

To examine AHI NDVI temporal profiles in more detail, the TKY AHI NDVI temporal profile of the year 2016 is plotted along with the VIIRS counterpart in Fig. 2. The AHI data had a much larger number of seemingly “cloud-free” observations available throughout the entire year and around each of the four phenological transition dates than the VIIRS data (Fig. 2).

The AHI temporal profile contained several distinctive gaps (or temporal signatures) associated with weather events due to its 10-min temporal resolution, which were not clearly discernable in the VIIRS temporal profile. First, a small gap is seen in the AHI profile from the Day of Year (DOY) 130 to DOY 134 (G1 in Fig. 2). This was attributed to 3 consecutive days of very low NDVI values, which were found due to thick cloud cover by visual inspection of AHI false color composite images for the period, followed by 2 consecutive days of very high NDVI values. Second, there was a ~20-day period without high NDVI values around DOY 210 (G2 in Fig. 2). Visual inspection of AHI false color composite images over the period indicated that a large number of patchy clouds persisted over the study site during this period. Finally, there was another period only with low NDVI values (from DOY 259 to DOY 268) (G3 in Fig. 2), which was associated with the passage of Typhoon-16 (Malakas) (see Supplementary Fig. S2 for 2017).

The AHI NDVI temporal profiles also depicted the changes in snow cover much better than the VIIRS counterparts. In Fig. 3, the AHI NDVI temporal profile for the FHK site is plotted along with the VIIRS counterpart and PEN images for the first 5 months of the year 2016. Based on the PEN image inspection, there was no snow cover for the first 17 days of the year 2016, snow covered the forest floor on DOY 18 and remained for the following 10 days until another snow fall accumulated more snow not only on the ground, but also on tree branches on DOY 29. The snow cover gradually melt over the next 30 days until they completely disappeared on DOY 58. It snowed again on DOY 69, but it melted during the following days until it completely disappeared again on DOY 82. Trees remained dormant though a couple of ephemeral snow cover occurred for a day or two until the start of leaf expansion on DOY 103. The AHI NDVI temporal dynamics corresponded exactly to these snow cover changes. The VIIRS NDVI was continuously very low from DOY 82 to DOY 105 when the site was subject only to a couple of ephemeral snow cover, making it difficult to detect/extract the date of the start of leaf expansion from the VIIRS temporal profile (see Supplementary Fig. S3 for 2017).

Figure 3 AHI NDVI seasonal changes (middle) during the first five months of the year 2016 for the Fujihokuroku (FHK) site. Plotted at the bottom is the VIIRS counterpart for comparison. Representative PEN in situ images for every distinctive snow cover condition are shown at the top. The numbers on the PEN images are their acquisition year (lower-left) and DOY (lower-right). Full size image

Frequency analysis

We then examined an improvement in the available number of cloud-free observations with AHI high-temporal resolution data using PEN sky images as a reference. For each AHI observation, the PEN sky image acquired at the same or nearest time to the observation was selected as the reference. In Fig. 4, VIIRS and AHI NDVI data during the 2016 green-up and brown-down periods are compared for the TKY site. Here, the green-up and brown-down periods were defined as the period from the start to end of leaf expansion and that from the start to end of leaf fall, respectively (see Supplementary Table S2). Those observations confirmed as “cloud-free” by PEN in situ sky images are indicated by the red circles in the figure. The red circles are placed only on the maximum NDVI values when multiple “cloud-free” observations were found on the same day.

Figure 4 VIIRS and AHI NDVI data over spring green-up and fall brown-down periods for the Takayama (TKY) site: (a) VIIRS and (b) AHI for the green-up season, and (c) VIIRS and (d) AHI for the brown-down period. Full size image

For the green-up period, whereas VIIRS had 4 cloud-free observation days, AHI had 7 days with at least one cloud-free observation (Fig. 4a,b, respectively). For the brown-down period, VIIRS had 11 days with at least one cloud-free observation, but AHI had 15 days with at least one cloud-free observation (Fig. 4c,d, respectively). Furthermore, VIIRS had only two cloud-free observation from DOY 105 to DOY 132, but AHI had at least one cloud-free observation on 5 days during the same period (Fig. 4a,b, respectively). Similarly, VIIRS had no cloud-free observations from DOY 265 to DOY 288, whereas AHI had 3 days with cloud-free observations (Fig. 4c,d, respectively) (see Supplementary Figs S4–S8 for the 2017 TKY and other two sites).

There existed several days that did not include any confirmed cloud-free observations, but whose maximum NDVI values were similar to those of nearby cloud-free observations, particularly in AHI NDVI, e.g., DOY 113, 114, 126, 141, 145, and 150 for the green-up period (see Fig. 4b). In Fig. 5, AHI NDVI data of DOY 150 are plotted as a 10-min time series along with those of DOY 155. The NDVI of DOY 155 changed little from 9:00 to 12:40 all of which observations were confirmed “cloud-free” by the PEN sky images, and the NDVI of DOY 150 was as high as that of DOY 155 for five different time periods during the morning (i.e., 9:00, 9:30, 10:10, 10:40, and 10:50–11:10) (Fig. 5). AHI subset time series images over the TKY site around 11:00 on DOY 150, 2016 showed the passages of several clouds and that there was no cloud cover over the TKY site exactly at 11:00 (Fig. 6a–c). VIIRS data were acquired at 12:35 on the day and the subset image over the TKY site showed clouds over the site (Fig. 6d) (See Supplementary Fig. S9 for DOY 113, 114, 126, 141, and 145 in 2016, and three additional days in 2017).

Figure 5 AHI NDVI diurnal time series plots of two different days for the Takayama (TKY) site. The blue open diamonds for DOY 155, 2016 represent “cloud-free” observations confirmed by PEN sky images. The “X” mark represents the VIIRS NDVI value for DOY 150, 2016. Full size image

Figure 6 AHI and VIIRS subset false color composite (top) and NDVI (bottom) images over the Takayama (TKY) site for DOY 150, 2016. The false color composite images were made by assigning the red, NIR, and green bands to the red, green, and blue color planes. The color scale bar placed below the images is for the NDVI. Full size image

The mean numbers of days with cloud-free observations in AHI and VIIRS data confirmed with PEN sky images are summarized for the three study sites in Table 1 (see Supplementary Table S3 for the individual years). The green-up and brown-down periods for the TGF site were determined to enclose the periods of the increasing and decreasing NDVI, respectively, as observed in the AHI and VIIRS temporal profiles extracted over the site (i.e., Fig. 1e,f). For all the five phenological periods for all the three sites, AHI had larger numbers of days with cloud-free observations than VIIRS (Table 1). Whereas VIIRS had 4–5.5 days with cloud-free observations, AHI had 5.5–10 days with cloud-free observations, 1.6–1.8 times higher numbers for the green-up period. For the brown-down period, AHI had 1.4–1.6 times higher numbers of cloud-free observation days than VIIRS. Differences in the number of cloud-free observation days between AHI and VIIRS were the highest for the peak period, followed by the pre-green-up period, and the lowest in the post-brown-down period. Approximately, a day without cloud-contaminated observations can be expected every 4–6 days with AHI, but every 7–10 days with VIIRS for the green-up season, and a day without cloud-contaminated observations can be expected every 4 and 6 days with AHI and VIIRS, respectively, for the brown-down season (Table 1). A day with cloud-free observations can be expected at much higher frequencies with AHI (every 9–35 days) than with VIIRS (every 19–106 days) for the peak period, but at nearly the same frequencies with AHI and VIIRS during the post-brown-down period (every 2–6 days).

Table 1 Number of Days with Cloud-free Observations. Full size table

Lastly, we examined the relationship of the number of AHI cloud-free observations with the time of day for the five phenological periods (Fig. 7). Those days when, at least, one PEN-confirmed, cloud-free AHI observation was found were used in this analysis. Not all AHI cloud-free observations came from any single one-hour period, but across the 6-hour period, except for the peak season at FHK and TGF. For all the three sites, in general, the highest number of AHI cloud-free observations were available in early morning (9–10) for the pre-green-up, green-up, and peak periods (Fig. 7a–i). The same, but weaker trend was observed for the brown-down and post-brown-down periods for the FHK site (Fig. 7k,n). For the TKY and TGF sites, AHI cloud-free observations were more equally distributed across the 6-hour period for the brown-down period (Fig. 7j,l), and came more from the afternoon and from the morning, respectively, for the post-brown-down period (Fig. 7m,o).