@aukkola Those fractions come from a JSON crosswalking table (we have an old example, which the current one is based on, that came from Mat and Siyuan’s work at the 2024 workshop). I intend to talk about the process in some detail at an upcoming LWG meeting, though unsure which one yet as we have a few things scheduled already.
I forgot to mention that the UKMO’s version of the CCI ancillary suite uses a supplementary dataset for the C4 grasses, currently /g/data/access/TIDS/UM/ancil/atmos/master/vegetation/cover/cci/v3/c4_percent_1d.nc (from ISLSCP II according to the metadata), which is at 1 degree resolution, so nowhere near as resolved as we would like. So suggestions for improved C3/C4 datasets are certainly of interest to us.
I think ideally we would compare to ILAMB, as suggested by @alexnorton? But like you said, it depends on what the precise point of the exercise is. I intend to keep the ancillary suite configurable where possible, so it’s easy to generate ancillaries for different applications.