## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Land-Surface Reservoirs
#### 3.1.4 Vegetation and Carbon Cycle  
The biosphere plays a prominent role in regulating the flux of gases, energy and momentum into the atmosphere, so it is important to properly represent it in models. Parameterisations of the biosphere are simplified representations of the natural processes in which the spatial scales of interest such as the plant, field or watershed remain largely sub-grid in the foreseeable future and can only describe the main feedback mechanisms sometimes marred by sizeable systematic errors. A key characteristic for water vapour and carbon fluxes modeling is the so-called canopy resistance, which is a bulk representation of stomatal resistance, vegetation type and leaf area. The stomata are leaf pores through which the plants absorb carbon dioxide and transpire water vapour. There are continuing efforts to calculate large-scale evapotranspiration using EO data. Globally available products include MODIS [128], GLEAM [129; 130], SEBS [131] and PT-JPL [132], FLUXCOM [133] or WECANN [134]. Since evapotranspiration cannot be observed directly from radiative, observable properties of the surface and atmosphere, typically the models use EO data such as radiation, rainfall and soil moisture to train or derive simple models. Figure 10: Time-series of the land surface (skin) temperature (LST, Celsius) at the south-pole for in situ observations (BRSN), MODIS satellite data (MODIS), numerical experiment with the bulk (single-layer) snow scheme (CTR) and numerical experiment with the thermal depth of the snow reduced by a factor of 10 (DSN), the latter being a proxy for the potential impact of a multi-layer snow scheme. It is clear that DSN has a larger amplitude LST diurnal cycle than CTR, and a better representation of strong cooling events [127]. Refs. [135; 136] focused on evaluating these products using flux tower and river streamflow under the LandFlux-EVAL initiative. They found significant differences between the products, which limited their use for model evaluation. More recently, Refs. [137; 138] have assessed the latest products. They conclude that (i) the models tend to overestimate soil evaporation, (ii) the way the models treat soil moisture stress is important and that (iii) finding ways to evaluate the split of evapotranspiration into its three components (transpiration, soil surface and interception evaporation) is currently not possible but a critical task for the future. For the latter, Ref. [139] showed that the flux attribution is a complex issue. Despite these caveats, it is imperative that developers use observed data to evaluate their models. To that end, iLAMB (integrated Land Atmosphere Model Benchmarking, [140]) was developed as a shared framework for large-scale model evaluation. It was used by the EarthH2 Observe project [141] to evaluate a suite of ten models including the evaluation of evaporation against the GLEAM EO product. As other researchers, they found that the comparison had limited value due to limited accuracy of the EO product. In the ECMWF land surface scheme, the Leaf Area Index (LAI) expresses the phenological phase of vegetation (growing, mature, senescent, dormant). Initially it was kept constant [142] and assigned using a look-up table depending on the vegetation type. Thus, vegetation appeared to be fully developed throughout the year. In November 2010, Ref. [143] modified this and introduced a seasonality of vegetation via a LAI monthly climatology based on the MODIS (collection 5) satellite product by [144]. The new monthly LAI climatology was shown to affect particularly the spring season when the radiative forcing is already strong, but when the vegetation not yet fully developed. The sensitivity generally indicated a warming of the spring as a consequence of the lower LAI and reduced evaporation (and consequently more sensible heat flux). This resulted in a reduction of the systematic 2-m temperature errors in the spring [143].