Wet spell max length indicator broken

[aulemahal]

Setup Information

• Xclim version: 0.50.0
• Python version: 3.11
• Operating System: Linux

Description

Computing the wet spell max length spell turned out to be incorrect.

Steps To Reproduce

import numpy as np
import xarray as xr
import xclim as xc

a = np.zeros(366)
a[10:13] = 3
a[40:45] = 3
pr = xr.DataArray(a, dims=('time',), coords={'time': xr.date_range('2000-01-01', periods=366, freq='D')}, attrs={'units': 'mm/d'}

xc.atmos.wet_spell_max_length(pr, window=1, op='min')

Returns : 321. The answer should be 5.

Additional context

The issue is here

xclim/xclim/indices/_threshold.py

Line 3520 in 994a5a1

wet = (mask.rolling(time=window).sum() < 1).shift(time=-(window - 1))

.

It looks like the code was directly copied from dry_spell_max_length with all comparison operators inverted. But on that line, the comparison has to do with the number of days, so it should be ">=" instead. The current function completely inverts the run mask, so we get the longest run that is not a wet spell.

I'd note that a fast path for the window=1 case might be of interest here. Also, the documentation was not completely adapted when it was copied from dry_spell to wet_spell. For example, "max" as an "op" here makes no sense. It should be "min".

Contribution

• I would be willing/able to open a Pull Request to address this bug.

Code of Conduct

• I agree to follow this project's Code of Conduct