Unrealistic jump in computed absorption for main isotope of water near 2729.5 nm

[jmmelko]

Hi,

When computing the absorption spectrum of water in the 2725-2732 nm range, spectreatm.py displays a jump at 2729.5 nm

• The problem does not occur with a custom-made Matlab program reading .par files and computing Voigt profiles
• I have checked that the Hitran lines download with HAPI have a 100% match with the ones I am using in my other program
• Increasing eps or the resolution does not solve the problem
• This is not due to missing line data, because it works with my other program and hey, we're dealing with the main isotope of water here

I think it may have something to do with the Voigt code not taking into account enough neighboring lines, i.e. as if the default values for the 'OmegaWing' and/or 'IntensityThreshold' were incorrect.

We were on the verge of migrating our matlab code to Python but it gave m a cold chill.

Please tell me if you can reproduce the issue. Thanks.

My code is the following:

import spectreatm
spectres = spectreatm.Spectra([H2O], [2725,2732] , zenith_angle=0,
                   layers_nb=1, temperature=300, pressure=1.01325, target_dist=1, round_trip=False, eps=10)
spectres.compute_absorption()
spectres.plot_all()
spectres.save_all()

My colleague told me that the error happens within this piece of code:

nu, coef = hapi.absorptionCoefficient_Voigt(
self.components, self.name, Environment=environment,
WavenumberRange=self.wn_range,
WavenumberStep=wn_delta, HITRAN_units=True,
Diluent={'air': 1-vmr, 'self': vmr})

spectreatm / HAPI result:

Matlab (in-house code) result:

[jmmelko]

ok, so i cornered the problem:

this is because the default value for DefaultOmegaWingHW = 50 is too low

setting it at 100 fixes the problem

So here's what I suggest:

• change 50 to 100 and update HAPI, OR
• create a custom parameter in compute_absorption() that will be passed to absorptionCoefficient_Voigt, that will be passed to absorptionCoefficient_Generic, so that OmegaWingHW=DefaultOmegaWingHW is overriden by a user-provided value, or multiplied by a user-provided factor (no unit)