## 3 Results  
### Simulation of 'Real' \(f^{esc}\) in Discontinuous Forest Canopies  
Using the variable input parameters listed in Table 2, TOC SIF and total emitted SIF were simulated respectively using the DART model. After that, SIF escape probabilities (i.e.,'real' \(f^{esc}\)) were calculated according to Equation (2) in Section 2.2.1. Figure 2 shows the simulated TOC SIF and total emitted SIF, as well as corresponding SIF escape probabilities. It appears that varying input parameters for a given sun direction greatly impacts the simulated TOC SIF, total emitted SIF, and corresponding SIF escape probabilities in all simulations. The variability of canopy structure (i.e., FVC) has a larger impact on TOC SIF (Figure 2a) than total emitted SIF (Figure 2b), which then induced a large variability of SIF escape probabilities from 0.2 to 0.4 (Figure 2c). Even though leaf chlorophyll content has a considerable impact on TOC SIF and total emitted SIF, respectively, SIF escape probabilities remain relatively stable if leaf chlorophyll content varies from 15 to 60 \(\upmu\)g/cm\({}^{2}\) (Figure 2f). On the other hand, with the increase of the solar zenith angle from 20\({}^{\circ}\) to 70\({}^{\circ}\), TOC SIF and total emitted SIF decrease at most angles, except from 20\({}^{\circ}\) to 30\({}^{\circ}\) where  
Figure 1: Flowchart of the evaluation process of SIF escape probability. the total emitted SIF slightly increases in most cases (Figure 2b,e). At the same time, the corresponding SIF escape probabilities remain relatively stable for all simulations between 30\({}^{\circ}\) and 70\({}^{\circ}\) (Figure 2c,f). Estimation of \(f^{\text{esc}}\) and \(SIF_{\text{tot}}\) Using Simulated Canopy Interceptance and \(fAPAR_{\text{leaf}}\)  
\(i_{o}\) and \(fAPAR_{\text{leaf}}\) were first simulated using methods described in Sections 2.2.2 and 2.2.3, and then used to estimate SIF escape probabilities using SIT described in Section 2.1. Figure 3 illustrates the variation of \(i_{o}\) and \(fAPAR_{\text{leaf}}\) with the increase of FVC, for Cab = 35 \(\upmu\)g/cm\({}^{2}\), background reflectance = Ref-2. Both \(i_{o}\) and \(fAPAR_{\text{leaf}}\) increase with solar zenith angle. It appeared that \(i_{o}\) is usually higher than \(fAPAR\) for a solar zenith angle larger than 30\({}^{\circ}\). Figure 4 shows the relative errors of SIF escape probabilities that are estimated using SIT with DART-simulated values (i.e.,'real' values). It appears that FVC generally has a much larger impact on the estimation accuracy of SIF escape probabilities than leaf chlorophyll content. For example, in Figure 4a, varying FVC greatly influences the estimated SIF escape probability with relative errors up to about 180%. In comparison, also in Figure 4a, the relative errors caused by the variation of leaf chlorophyll content were no more than 50%. Furthermore, the varying background spectrum has a considerable impact on the estimation of SIF escape probabilities. With a non-reflecting ground, the relative errors between estimation and simulation were no more than 10%. As for the effect of the solar zenith angle, it is clear that the SIF escape probabilities estimated at high solar zenith angles always have smaller relative errors than those estimated at lower solar zenith angles. Figure 2: DART-simulated Top of Canopy (TOC) SIF (**a,d**), total emitted SIF (**b,e**), and corresponding SIF escape probabilities (**c,f**). BS means black soil (i.e., non-reflecting background). The label -\(\bullet\)- indicates the default configuration (fractional vegetation cover (FVC) = 0.6, chlorophyll content (Cab) = 35 \(\upmu\)g/cm\({}^{2}\), Ref-2). The top row refers to varying canopy structure, and the bottom row refers to variable leaf and background spectral properties. We compared the SIF escape probabilities estimated using canopy interceptance as input and those using fAPAR\({}_{leaf}\) as input (i.e., comparison between Figure 4a,c, or Figure 4b,d). No apparent differences were observed in most cases.