ERA5 "coastal" surface values like runoff on, e.g., 0.25 degree grid

ERA5 “coastal” surface values like runoff on, e.g., 0.25 degree grid


I have a couple questions related to this.

1) How are values - such as runoff - computed and included in netCDF arrays near coasts when your model cell is only fractionally over land?  How is this then handled when re-projecting to the .25 degree lat-lon product?

2) I see a lot of land mask values ("lsm" pulled from same surface product I am pulling runoff from) that are < 1 inland; are these values not 1 due to actual very small water bodies and streams/rivers, or is that an artifact of some sort from your interpolation scheme to 0.25 degree?

3) How should I actually compute runoff and other values for cells with land mask < 1?

I need to somehow handle runoff for fractional land cells, but if I discard all "land" cells, I discard a huge fraction of inland points due to land regularly being considered fractional (say, .9972) inland, while it is not clear to me then whether runoff for the same cell is accounting for this .9972 and, so, perhaps runoff is actually something like 1/.9972 or more greater than the reported value for the cell.

Thank you.

If it helps, in other words:

Simply, what is the volume of water for the pixel?  Depth x area of pixel x land fraction, or depth x area of pixel (which would assume that some average depth of runoff for the area was already computed on your end)?

Is runoff of any pixel with any land fraction included?  Is this the case for any product/grid?  (Or, are some products'/grids' files using a threshold of 50% land, e.g.?)

Meanwhile, is seems some products use a large negative value to represent water pixels; for which is this the case?