Evaluation of JULES-SIF against TROPOMI data

The use of SIF to evaluate land surface models shows considerable promise to help constrain estimates of, and elucidate the processes that control Gross Primary Productivity (GPP) on large spatial scales. To use SIF effectively for this purpose, we argue that forward modelling of the observations from the land surface model – as opposed to, say, relying on empirical relationships with the modelled GPP – is desirable if we wish to understand structural deficiencies in the land surface model.

This presentation describes the prediction of SIF from JULES, the Joint UK Land Environment Simulator, which is the land surface scheme of the Hadley Centre climate models, and the UK Earth System Model (UKESM). We explain how we couple leaf-level SIF models to the biochemistry routines in JULES, and how we scale the emitted SIF to the canopy level using a vegetation radiative transfer scheme (L2SM) that is consistent with the physics inside JULES but also allows for radiative emissions within the canopy. The SIF scheme includes attenuation within the leaf, utilizing either modelled or observed leaf reflectance and transmittance spectra and can make predictions of the canopy leaving SIF at arbitrary wavelengths. Downregulation of fluorescence by water stress is also included.

We show results from JULES-SIF at regional and global scales, and make comparisons against TROPOSIF data. The results show generally good agreement and are sufficiently aligned with the observations that they are able to highlight areas where JULES is not correctly modelling the relevant environmental processes. Future directions for the JULES SIF module are explained, including accounting the directional component of the canopy leaving SIF.