Calculating percentile for each gridpoint in xarray

Question

I am currently using xarray to make probability maps. I want to use a statistical assessment like a “counting” exercise. Meaning, for all data points in NEU count how many time

Answer 1

np.nanpercentile by default works on a flattened array, however, in this case, the goal is to reduce only the first dimension generating a 2D array containing the result at each gridpoint. To achieve this, the axis argument of nanpercentile can be used:

np.nanpercentile(NEU.rr, 1, axis=0)

This however will remove the labeled dimensions and coordinates. It is to preserve the dims and coords that apply_ufunc has to be used, it does not vectorize the functions for you.

xr.apply_ufunc(
    lambda x: np.nanpercentile(x, 1, axis=-1), NEU.rr, input_core_dims=[["time"]]
)

Note how now the axis is -1 and we are using input_core_dims which tells apply_ufunc this dimension will be reduced and also moves it to the last position (hence the -1). For a more detailed explanation on apply_ufunc, this other answer may help.