For the retrieval, averages over three spectral bands within the O2A band are used. Therefore, in order to reduce data storage requirements, we decided to store only the 3 values of each spectrum.

The REPTRAN absorption parameterization included in libRadtran should be sufficient for this purpose. If it can be applied (instead of full line-by-line simulations), this would also save a significant amount of computational time.

Huan suggested the following:

Average both radiance and solar irradiance over 758-759nm (6points: 758, 758.2, 758.4, 758.6, 758.8, 760)/760-761/765-766nm as rad_aver, irr_aver. Finally to get normalized radiance = rad_aver/irr_aver

Question: Is it not possible to obtain the normalized radiance from LIDORT, so that you do not need to average the irradiance?

MYSTIC simulation

US standard atmosphere, 1e6 photons (quite accurate, for test calculation with LES data 1e5 where used), albedo 0.5, sza 40, vza 60, phi 196.3

cloud from 3-4km altitude, tau=2, reff=10um

MYSTIC calculation gives normalized radiance from 755-775nm at different spectral resolutions. The parameterization is set up in such a way, that if one integrates over a spectral interval, the result is approximately the same as if integrated over line-by-line simulations. Therefore I integrated the results over the 1nm intervals to obtain the following values:

Simulated values for the 3 spectral bands proposed by Huan calculated using REPTRAN (fine spectral resolution) are very close to the values obtained from line-by-line simulation. Similar statistical error to be expected when 1e5 photons are used. REPTRAN (medium spectral resolution) not sufficiently accurate for 765-766nm band.

CPU time can be decreased by a factor of approx. 7 ⇒ Use REPTRAN fine for O2A band simulations? CPU time can further be decreased, because simulation not necessary outside these bands.

Next step: Intercomparison setups (clear and cloudy) for the 3 bands?

O2A band simulation, clear-sky, line-by-line vs. REPTRAN absorption parameterization.

O2A band simulation, clear-sky, line-by-line vs. REPTRAN absorption parameterization, integration over three 1nm-wide bands.

O2A band simulation, cloud, line-by-line vs. REPTRAN absorption parameterization.

O2A band simulation, cloud, line-by-line vs. REPTRAN absorption parameterization, integration over three 1nm-wide bands.

Layer-AMF calculated at 756nm. Check whether the same result is obtained independent of absorption parameterization.