Bug: to_radians applied too late in model.py grid_sfc_area?

[spencerkclark]

I hadn't pulled from the develop branch in a while, but I just did this morning. In doing some calculations involving surface area I noticed some things were off. In printing a sum of the surface area returned I got a value of:

<xarray.DataArray 'sfc_area' ()>
array(2.9224513997287056e+16)

This is too large by a factor of 180.0 / pi, hinting that it stems from not converting from degrees to radians.

I think the issue stems from line 194 in model.py:

dlon = to_radians(cls.diff_bounds(lon_bounds, lon))

In my case, but I think in most cases, the width of gridboxes in longitude in degrees is less than 4 pi. This causes utils.to_radians to pass the widths back unchanged. I think a solution here would be to just apply to_radians to lon_bounds (instead of after diff_bounds):

dlon = cls.diff_bounds(to_radians(lon_bounds), lon)

Luckily this shouldn't change any results from area averaging (since it's off by a constant factor), but important to get right nonetheless.

[spencerahill]

Yikes, good catch. I'll fix this right now.

[spencerkclark]

Thanks!

[spencerahill]

02a8a8f should resolve this. I implemented the switch you suggested and also improved the to_radians function itself, both the threshold for conversion and by trying to use the embedded 'units' metadata.

This and the other conversion methods could definitely be improved, potentially even a conversions class or module created.

(Yet another reminder of the need for formal automated tests, both so that we would have caught this in the first place and that I should have added some for the new code in order to be sure it actually works! One day...)

Let me know if there's still any problems, otherwise please close.

[spencerkclark]

This looks great; things are working as expected now. I like the additional improvements to to_radians! Thanks again.