Abstract
Solar-induced chlorophyll fluorescence (SIF) provides a new and direct way of monitoring photosynthetic activity. However, current SIF products are limited by low spatial resolution or sparse sampling. In this paper, we present a data-driven method of generating a global, spatially continuous TanSat SIF product. Firstly, the key explanatory variables for modelling canopy SIF were investigated using in-situ and satellite observations. According to theoretical and experimental analysis, the solar radiation intensity was found to be a dominant driving environmental variable for the SIF yield at both the canopy and global scales; this has, however, been neglected in previous research. The cosine value of the solar zenith angle at noon (cos (SZA0)), a proxy for solar radiation intensity, was found to be a dominant abiotic factor for the SIF yield. Next, a Random Forest (RF) approach was employed for SIF prediction based on Moderate Resolution Imaging Spectroradiometer (MODIS) visible-to-NIR reflectance data, the normalized difference vegetation (NDVI), cos (SZA0), and air temperature. The machine learning model performed well at predicting SIF, giving R2 values of 0.73, an RMSE of 0.30 mW m−2 nm−1 sr−1 and a bias of 0.22 mW m−2 nm−1 sr−1 for 2018. If cos (SZA0) was not included, the accuracy of the RF model decreased: the R2 value was then 0.65, the RMSE 0.34 mW m−2 nm−1 sr−1 and an bias of 0.26 mW m−2 nm−1 sr−1, further verifying the importance of cos (SZA0). Finally, the globally continuous TanSat SIF product was developed and compared to the TROPOspheric Monitoring Instrument (TROPOMI) SIF data. The results showed that the globally continuous TanSat SIF product agreed well with the TROPOMI SIF data, with an R2 value of 0.73. Thus, this paper presents an improved approach to modelling satellite SIF that has a better accuracy, and the study also generated a global, spatially continuous TanSat SIF product with a spatial resolution of 0.05°.
Highlights
Solar-induced chlorophyll fluorescence (SIF) within the wavelengths ranging from 650 to 850 nm, and which has two peaks centered at 685 and 740 nm [1], is well known as a good proxy for photosynthetic activity [2,3]
The results show that the Random Forest (RF) model performed well at simulating TanSat SIF (Figure 6), with R2 values of 0.75, 0.73 and 0.81, RMSE values of 0.32, 0.30 and 0.32 mW m−2 nm−1 sr−1, and RD values of
The results presented in this paper have confirmed that cos (SZA0) can be used successfully as a proxy for solar-radiation-intensity information to produce better modelling based on satellite SIF data
Summary
Solar-induced chlorophyll fluorescence (SIF) within the wavelengths ranging from 650 to 850 nm, and which has two peaks centered at 685 and 740 nm [1], is well known as a good proxy for photosynthetic activity [2,3]. Due to the point observation, spatial sampling frequency and the low accuracy of single observations, the average spatial resolution of monthly satellite products is 2◦ × 2◦ [10,11,12]. The spatial resolution of the SCIAMACHY satellite is 30 × 240 km (30 km × 60 km in nadir mode), whereas the average spatial resolution of daily satellite products is 1.5◦ × 1.5◦ [10,13,14,15]. Because GOME-2 carries out continuous sampling, the same as for SCIAMACHY, the spatial resolution of its synthetic fluorescence products is higher than that of GOSAT, and is about 0.5◦ × 0.5◦ [13,15,16]. TROPOMI, with a spatial resolution of 3.5 km × 7 km and a swath of 2600 km, has produced daily global coverage since February 2018 [17,18]
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