Altitude interpolation: interpolate over Potential Temperature instead of Pressure

[stefan-voelk]

Type of issue

• feature request

Description

• Some of the implemented responses are of type 2D and values are interpolated over the pressure (in hPa) coordinates. However in atmospheric modeling, it is a common practice to interpolate over the Potential Temperature instead of pressure.
• Interpolating over Potential Temperature instead of pressure is more consistent with thermodynamic principles, e.g. isentropic surfaces are often more significant than isobaric surfaces. The Potential Temperature remains constant when air parcels rise or sink adiabatically.
• Therefore, we want to change the interpolation method over altitude from the pressure coordinate to the Potential Temperature.
• The update on the interpolation routine is relevant for following species: H2O and NOx related species
• Note: Potential Temperature was already used in AirClim. The differences between the two interpolation approaches becomes more relevant at higher altitudes and for a coarser resolution of the response grid in altitude.
• Update should be made such that all species can reuse method easily. interpolate_space.py module is a good candidate for placing the code.