Dear community,
I have measurements of pressure (p), temperature (T) and relative humidity (i.e. water vapour content, RH) and I'd like to know if there's a simple way to calculate atmospheric transmissivity from these quantities, and for the transverse (i.e. horizontal, same altitude) direction. I've looked at several atmospheric radiation models such as Modtran but these are all for depth-integrated calculations, and it seems to me that there must be a simple way to do this if we don't need to take into account the varying optical properties of the different atmospheric layers. Are these three measurements (and assumptions for the concentrations of other atmospheric gases such as 400 ppm CO2) sufficient? I'm specifically interested in transmission in the thermal infrared band (7.5-13.5 microns). Thanks for any help that you can provide.
Regards,
David Jessop
Yes, you can use SpectralCalc here: http://www.spectralcalc.com/calc/spectralcalc.php
Thank you Alastair, though I would rather be able to "see" what's going on behind the scenes in any software that I use. Plus SpectralCalc is pretty limited if one doesn't want to subscribe (which I don't for now).
Well you might find something here: http://www.realclimate.org/index.php/data-sources/#Model_code but I think most models are designed for vertical radiation.
If you are prepared to modify the code then these might be useful: http://www.realclimate.org/index.php/data-sources/#GCM_code
SpectralCalc is the only one I know which is not. If you work out exactly what you are planning to do, then you can take out a subscription for a month, which is not too expensive.
Getting behind the scenes means having access to absorption coefficients for the molecules with bands in that region, plus water "continuum" absorption (generally accumulated line tails from more distant bands I believe). You also haven't mentioned aerosol extinction - perhaps you have access to local visibility data, though most airports don't report any meteorological range greater than 20 km... Aerosols aren't a single radius so a simple Mie calc isn't sufficient. Sorry that I haven't looked for simple models since I've used Modtran and predecessors for 40 years... You might also look into SBDART.
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