## 3 Results
### Regional Assessment of _I_SBACH-Modelled Start of Season against Satellite Observations
The upper panel shows estimates for evergreen needle-leaf forest: (**a**) determined from satellite observation, and (**b**) determined from JSBACH simulations. The lower panel shows estimates for deciduous broad-leaf forest: (**c**) determined from satellite observations and (**d**) from JSBACH simulations. The application of the method used for detection of ENF_SOSsat (Section 2.5.2) relating to the JSBACH-simulated snow cover was not possible for many years (2005, 2007, 2008 and 2009) in the hemiboreal, southern boreal zone and partly also in the middle boreal zone, because the simulated scaled FSC was below 1 during the whole spring period. For the remaining areas with sufficient snow cover, the modelled decrease of FSC occurred in mean earlier than ENF_SOSsat (8 days) for the period 2003-2010, which is also confirmed by local comparison to snow depth observations in Sodankyla (Figures 4c and 5c). Figure 8: Grid cell-wise bias between start of season derived from the JSBACH model and remote sensing in Finland for the period 2003–2010. **(a)** in evergreen needle-leaf forest; and **(b)** in deciduous broad-leaf forest; **(c)** Bias in JSBACH-modelled starting date of the “growth phase” of DBF because of the driving temperature data. The bias was calculated as the difference in the start date of the “growth phase” simulated by running the LoGro-P-model with regional climate model data and with gridded temperature observations. Red colours show areas with a late bias and blue areas indicate an early bias of the modelled date. Figure 7: Scatterplots of mean satellite- and JSBACH model-derived start of season for forested areas in Finland for the period 2003–2010 for: **(a)** evergreen needle-leaf forest (ENF_SOS); and **(b)** deciduous broad-leaf forest (DBF_SOS). The dotted line gives the 1:1 line and the black line the linear fit between JSBACH-modelled and satellite-observed dates. We verified whether the driving temperature contributed to the differences between DBF_SOS\({}_{\text{sat}}\) and DBF_SOS\({}_{\text{mod}}\). Correlations between the multi-year average dates of the start of the \"growth phase\" of DBF for the period 2003-2010 from LoGro-P simulations with two temperature drivers (regional climate model and FMI gridded temperature) were very high and the remaining bias for the whole of Finland was very small (R\({}^{2}\) = 0.98, RMSE = 1.66, Bias = \(-\)0.05 days). Nevertheless, the bias varied spatially (Figure 8c); late biases (1-2 days) occurred in central Finland and early biases were observed in north-eastern Finland (\(-\)2---4 days). The last part of this Section focuses on the inter-annual variations in the start of season dates. On the whole, inter-annual variations of \(\mathrm{JSBACH}\) model-derived dates were in agreement with satellite observations (Figure 10), except in 2007, when DBF_SOS\({}_{\text{mod}}\) was 8 days delayed compared to DBF_SOS\({}_{\text{sat}}\). The deviation from the respective mean date (period 2003-2010) at individual locations (grid cell) ranged in average between \(-\)14 and 10, and \(-\)12 and 8 days for ENF_SOS\({}_{\text{sat}}\) and ENF_SOS\({}_{\text{mod}}\) throughout Finland. Regarding DBF_SOS, inter-annual variations at one location were slightly smaller than for ENF_SOS, ranging in mean from \(-\)8 to 9, and \(-\)8 to 7 days for DBF_SOS\({}_{\text{sat}}\) and DBF_SOS\({}_{\text{mod}}\), respectively. The inter-annual variations of mean air temperature for Finland in May correlated with the inter-annual variations in DBF_SOS\({}_{\text{mod}}\); that is, higher May temperatures lead to an earlier start of season, (R\({}^{2}\) = 0.71). Instead, for the evergreen forest, April temperatures are more important: the mean air temperature in April was correlated with the ENF_SOS\({}_{\text{mod}}\) time series (R\({}^{2}\) = 0.56). The inter-annual differences between DBF_SOS\({}_{\text{mod}}\) and ENF_SOS\({}_{\text{mod}}\) can be largely explained by the differences in the mean air temperature in April and May (R\({}^{2}\) = 0.70).