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Pressure levels for heights AGL between 100 and 250 m in HRRR #17

@rthedin

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@rthedin

Working my way through PR #16, I noticed some problems on the HRRR interface.

When getting data from HRRR using wind_velocity_direction_at_altitude, at any height AGL height_above_ground above 100 m, we fall into the following else to get the appropriate pressure level:
https://github.com/NREL/SSRS/blob/a9114c7a65fd1d71c8c9eb8f2f7d53f1685dfccb/ssrs/hrrr/hrrr.py#L350-L363
where ground_level_m is used as an argument to nearest_pressures. However, oftentimes the call to wind_velocity_direction_at_altitude does not include ground_level_m, but rather height_above_ground_m. The ground_level_m is zero by default:
https://github.com/NREL/SSRS/blob/a9114c7a65fd1d71c8c9eb8f2f7d53f1685dfccb/ssrs/hrrr/hrrr.py#L260

According to the nearest_pressures function's docstring, h is the height above sea level:
https://github.com/NREL/SSRS/blob/a9114c7a65fd1d71c8c9eb8f2f7d53f1685dfccb/ssrs/hrrr/hrrr.py#L92-L93
so I'm not sure if it is appropriate to use height_above_ground_m

What should we use here? Not specifying ground_level_m does not work because of the default 0 (and would not even be appropriate). Should we first get the ground level and then combine it with the height AGL?

Right now, in the latest commit from PR #16, I replaced line 359 above with
https://github.com/NREL/SSRS/blob/43a728fe9559ed2d2b668eb6a905de795c4963f6/ssrs/hrrr/hrrr.py#L376
However, I do understand that this is not really appropriate.

What would be appropriate path forward here?

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