[bhaskargurram-ai]

Description Of Changes

When the smallest value of xp is repeated, interpolate_1d (and log_interpolate_1d) returns NaN for a point at that value, with a "invalid value encountered in divide" warning. The search for the first point above x gives index 0, which is then bumped to 1, so the interpolation is between two points with the same coordinate and divides 0 by 0. For a sounding this happens when the top pressure level is reported twice, and anything that interpolates to the top of the profile then returns NaN. That's what happens in #1387: precipitable_water is NaN for an observed sounding with repeated pressure levels, and works once the duplicates are dropped.

>>> p = [1000, 950, 900, 850, 800, 700, 600, 500, 500] * units.hPa
>>> td = [20, 18, 15, 12, 9, 3, -5, -15, -15] * units.degC
>>> precipitable_water(p, td)
<Quantity(nan, 'millimeter')>

Changes:

• When the points below and above have the same coordinate, the interpolation weight is 0 (the value of the point below is used) instead of 0/0.
• xp is now sorted with a stable sort, so with repeated values the first of the repeated points in the input is the one used, whatever the input order. The Notes section of the docstring says so.

As far as I can tell, repeated values in the interior and at the top end of xp already gave finite results. This makes the bottom end match. dopplershift suggested comparing with np.interp: like np.interp, there's no NaN now. At exactly a repeated value, np.interp returns the last of the repeated points and this returns the first, which is what MetPy already did for interior repeats.

Tests:

• tests/interpolate/test_one_dimension.py: repeated values at the start and in the middle of xp, increasing and decreasing input, and log_interpolate_1d to a repeated top pressure level
• tests/calc/test_indices.py: precipitable_water with a repeated top level matches the result without the duplicate

Those that hit the bottom-end case fail on main and all pass with this change. tests/interpolate and tests/calc pass. Ruff and flake8 are clean.

Checklist

• Closes Precipitable Water is Nan #1387
• Tests added
• Fully documented

[CLAassistant]

All committers have signed the CLA.