Achieved consistency with observed fAPAR

Before assessing the results of the model simulation in detail, it is important to insure that the assimilation scheme has been successful at improving the consistency of modelled fAPAR with the remote sensing data. A x2 statistics is used here as a criterion for the goodness-of-fit, taking into account the assumed error in the satellite-derived fAPAR, C/ = 0.2:

where fm is satellite-derived fAPAR, gm the mean annual cycle of modelled fAPAR, and m the nm months with valid satellite data. This is shown in Fig. 2 for the simulation with BETHY before and after applying the fAPAR assimilation scheme. Simulations with x2> 1 can be considered to deviate significantly from observations, and there appear to be two large regions for which this occurs: the boreal forest belt of Canada, Scandinavia and northern Russia, and the semi-arid tropics, especially the Brazilian, African and Australian savannas and bushlands. Some heavily populated regions also appear, especially in North Africa and the Middle East, Western Europe, Bangladesh, and China.

Figure 2. x2-statistics of the fit between average monthly simulated and satellite observed fAPAR with the BETHY model. Panel (a) was generated using initial model parameters, panel (b) after assimilation of the satellite fAPAR fields

Figure 2. x2-statistics of the fit between average monthly simulated and satellite observed fAPAR with the BETHY model. Panel (a) was generated using initial model parameters, panel (b) after assimilation of the satellite fAPAR fields

After assimilation, consistency clearly improves and is now mostly better than It turns out that in the boreal areas, vegetation cover had been largely overestimated, probably because nitrogen limitation of forest densiti-ty has not been accounted for explicitly. The model also predicts spring-time greening too late in Western Europe, which leads to high values there. In these two cases, it is mainly adjustment of the parameters fc and Tt/ which leads to improved consistency with measured fAPAR.

In most of the semi-arid tropics, however, vegetation cover is most sensitive to the maximum plant-available soil moisture content, wmax. Here, vegetation cover has been underestimated in the initial model run, i.e. before assimilation, with the exception of some of the Brazilian savannas, where the initial value of wmax is very high (see next subsection).

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