mean_pressure_weighted unit behavior

[jrleeman]

This would product an expected value of around 19 degrees, but there appears to be a unit issue.

mpcalc.mean_pressure_weighted([1000, 900, 800] * units.hPa,
                              [20, 18, 16] * units.degC,
                              height=[10, 1000, 2000] * units.meters)

produces 292.2026315789474 <Unit('degree_Celsius')>

Now if we try

mpcalc.mean_pressure_weighted([1000, 900, 800] * units.hPa,
                              [20, 18, 16] * units.kelvin,
                              height=[10, 1000, 2000] * units.meters)

produces 19.05263157894737 <Unit('kelvin')>

so it appears that for temperatures this is potentially broken as inappropriate units are being reattached after the calculation is done correctly in base units.

[kgoebber]

Doing a bit of testing, this appears to be an issue with the use of temperature units. If you substitute in different units (e.g., m/s) then you get the expected result. Due to differences being taken in the process, that is likely where something is going awry. What is coming out is the average value in Kelvin (likely getting to Kelvin somewhere in the process to get to a base unit), but then just attaching the original units to this.

It appears that the trapz function accepts units at this point and removing the magnitude calls in the function yields the following results:

mpcalc.mean_pressure_weighted([1000, 900, 800] * units.hPa,
                              [20, 18, 16] * units.degC,
                              height=[10, 1000, 2000] * units.meters)

produces 292.2026315789474 <Unit('kelvin')>

Whether we wish have the result return the original units can be open to discussion, although with this current behavior it would act similarly to the parcel_profile function to return the temperatures in Kelvin. An initial PR fixing this issue will be submitted shortly.