Global data: how to set reference height

[BSchilperoort]

For the "global" STEMMUS_SCOPE, data is now being combined from ERA5, ERA5-land, CAMS (CO2), and others (see https://github.com/EcoExtreML/STEMMUS_SCOPE_Processing/pull/63).

However, there is the "reference height" variable which I am sure about. In PLUMBER2, this is the height that the temperature, wind, and fluxes are measured at (AFAIK). The remote-sensing based data has no equivalent.

The main meteorological forcing input comes from ERA5 and ERA5-land, namely the 10 m wind speed and the 2 m temperature and dew point. This measurement heights are of little meaning though, especially considering the spatial resolution of ERA5 (~30 km) and ERA5-land (~9 km).

So to get to the point: @yijianzeng @bobzsu how do we set a reference height? We do have canopy height data available, which we could use to adjust the reference height.

[bobzsu]

Thanks a lot, Bart. This reference height is indeed an issue but we need to determine an value. Ideally we need to convert the values of wind at 10m and air temperature and dew point at 2m to the same height about the canopy, but we need to know the roughness height and make some assumptions.

For consistency, I propose we use MAX(canopy height+2m) and indicate that for wind it should be MAX(canopy height +10m). This assumes that 1 ERA grid covers several land use classes.

Note the nominal height of measurement according to micrometeorological principles should 2 times canopyHeight, or 3 times of roughness height (e.g. 1/8canopyHeight)+ displacement height (which is 2/3canopyHeight).

[BSchilperoort]

Hi @bobzsu, thanks for your reply.

Just to be sure, you mean we take the maximum value between canopy_height and 2?

Additionally, note that the canopy height data we have is at a much higher resolution (10 m!) than the ERA5 data.

[bobzsu]

Sorry Bart - it is a bit confusing. If you have a ERA5-land grid cell of 9km x 9km and a land use map of 1km x 1km, you have then 9 x 9 pixels of land use cells in each ERA5-land grid cell. For each land use we say referenceHeight_i=CanopyHeight_i+2, we take then MAX( refenceHeight_i, for i { 81 ), i.e. the maximum reference height of all land use cells within 1 ERA5-land cell. This is an oversimplification but traceable.