Upscaling Solar-Induced Chlorophyll Fluorescence from an Instantaneous to Daily Scale Gives an Improved Estimation of the Gross Primary Productivity

Abstract

Solar-induced chlorophyll fluorescence (SIF) is closely linked to the photosynthesis of plants and has the potential to estimate gross primary production (GPP) at different temporal and spatial scales. However, remotely sensed SIF at a ground or space level is usually instantaneous, which cannot represent the daily total SIF. The temporal mismatch between instantaneous SIF (SIFinst) and daily GPP (GPPdaily) impacts their correlation across space and time. Previous studies have upscaled SIFinst to the daily scale based on the diurnal cycle in the cosine of the solar zenith angle ( cos ( SZA ) ) to correct the effects of latitude and length of the day on the variations in the SIF-GPP correlation. However, the important effects of diurnal weather changes due to cloud and atmospheric scattering were not considered. In this study, we present a SIF upscaling method using photosynthetically active radiation (PAR) as a driving variable. First, a conversion factor (i.e., the ratio of the instantaneous PAR (PARinst) to daily PAR (PARdaily)) was used to upscale in-situ SIF measurements from the instantaneous to daily scale. Then, the performance of the SIF upscaling method was evaluated under changing weather conditions and different latitudes using continuous tower-based measurements at two sites. The results prove that our PAR-based method can reduce not only latitude-dependent but also the weather-dependent variations in the SIF-GPP model. Specifically, the PAR-based method gave a more accurate prediction of diurnal and daily SIF (SIFdaily) than the cos ( SZA ) -based method, with decreased relative root mean square error (RRMSE) values from 42.2% to 25.6% at half-hour intervals and from 25.4% to 13.3% at daily intervals. Moreover, the PAR-based upscaled SIFdaily had a stronger correlation with the daily absorbed PAR (APAR) than both the SIFinst and cos ( SZA ) -based upscaled SIFdaily, especially for cloudy days with a coefficient of determination (R2) that increased from approximately 0.5 to 0.8. Finally, the PAR-based SIFdaily was linked to GPPdaily and compared to the SIFinst or cos ( SZA ) -based SIFdaily. The results indicate that the SIF-GPP correlation can obviously be improved, with an increased R2 from approximately 0.65 to 0.75. Our study confirms the importance of upscaling SIF from the instantaneous to daily scale when linking SIF with GPP and emphasizes the need to take diurnal weather changes into account for SIF temporal upscaling.

Correcting Clear‐Sky Bias in Gross Primary Production Modeling From Satellite Solar‐Induced Chlorophyll Fluorescence Data

“…These empirical relationships are generally calibrated with clear‐sky GPP and SIF, which could cause uncertainties in GPP estimation under cloudy sky conditions. To improve all‐sky GPP estimation with satellite SIF retrievals, the difference in solar radiation between clear‐sky and cloudy‐sky days has been introduced to SIF‐based GPP estimation (Hu et al, 2018). Moreover, our CFCB approach also considers the enhancement of LUE under cloudy conditions.…”

“…Compared with instantaneous SIF total (SIF inst ), daily SIF total (SIF daily ) can minimize the effect of varying illumination conditions and is more useful in carbon cycle studies. Therefore, SIF inst is converted to SIF daily using cos (SZA)‐based (Frankenberg et al, 2011) or photosynthetically active radiation (PAR)‐based approaches (Hu et al, 2018): where cosSZA daily and PAR daily are the daily averaging values of the instantaneous cosSZA inst and PAR inst , respectively. PAR is calculated as 0.46 × SW (shortwave radiation) from the fifth ECMWF ReAnalysis (ERA5‐land) hourly product (Albergel et al, 2018).…”

Generation of a Global Spatially Continuous TanSat Solar-Induced Chlorophyll Fluorescence Product by Considering the Impact of the Solar Radiation Intensity

“…Since the incoming PAR of instantaneous SIF differs according to the location, the SIF normalized by PAR has been widely used to describe the global photosynthetic capability [46,47]. For satellite SIF observations, which are available only under clear-sky conditions, the PAR can be assumed to be linearly correlated with the cosine value of the solar zenith angle at the transit time of the satellite [8,11,48]. SIF normalized is the SIF normalized to a fixed transit time, based on the central latitude and longitude of each grid cell (Equation (3)):…”

“…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]. Numerous studies have shown that the gross primary productivity (GPP) of vegetation photosynthesis can be directly estimated using satellite, airborne, in-situ and ground level SIF data [4][5][6][7][8][9].…”

Improving the Capability of the SCOPE Model for Simulating Solar-Induced Fluorescence and Gross Primary Production Using Data from OCO-2 and Flux Towers

“…SIF has been incorporated into ecological models such as the 1-D (dimensional) SCOPE (Soil Canopy Observation, Photochemistry and Energy Fluxes) model [15,16] and 3-D models (e.g., DART [17], and FluorFLIGHT [18]) for more complex canopies. The SCOPE model is a state-ofthe-art model integrating radiative transfer, photosynthesis, energy balance, and fluorescence [15] and has been widely used in SIF and photosynthesis studies (e.g., [19][20][21][22]). The model was partly developed for the upcoming Fluorescence Explorer (FLEX) mission that will be able to capture SIF for the full spectral range from 500 to 780 nm with 300-m spatial resolution and bi-weekly global coverage [23].…”