LAI remote sensing products and simulated LAI: an intercomparison over FRANCE

International audienceThe leaf area index (LAI) is an important vegetation variable. It is a good measure of the amount of active vegetation and it is linked to canopy conductance, water and carbon fluxes at the interface between the atmosphere and the vegetation. In this study, we compare four different estimates of LAI over France: two LAI products derived from visible remote sensing, and simulated LAI from two land surface models. The simulated LAI is produced by the ISBA-A-gs model (developped by CNRM) and by the ORCHIDEE model (developped by LSCE). The same atmospheric forcing is used by two models: the SAFRAN dataset, a high resolution (8-km) operational product over France. The same vegetation map is used: the ECOCLIMAP2 dataset. The LAI remote sensing products are: MODIS (Collection 5), derived from the MODIS sensors, and CYCLOPES, derived from the SPOT4/VEGETATION sensor. These products were re-projected on the grid used by the model. The study period ranges from 2000 to 2007 and allows to study the average seasonal cycle as well as the interannual variation of LAI. We analyse monthly values of LAI over the period. The average seasonal cycle over the seven years starts earlier in the ORCHIDEE simulations than in the ISBA-A-gs ones. ORCHIDEE simulates a maximum interannual variability of LAI in spring (and a small one in summer), whereas in the ISBA-A-gs simulation the maximum is in summer (with small values in spring). These differences can be explained by contrasting levels of phenology constraints used in the two models. The interannual variability of both satellite products is more complex, with a weaker seasonal cycle and contrasted spatial patterns. Finally, we compare the water and carbon fluxes simulated by the two models