====== WP 200: Clear sky radiative transfer model intercomparison ======

In order to make sure that the radiative transfer models MYSTIC (to be used to generate synthetic data) and LIDORT (used in BIRA's NO2 retrieval algorithm) are consistent, we first performed a clear sky model intercomparison.

===== General settings =====

**Surface albedos**: 0, 0.1, 0.3, 0.8

**solar zenith angles [degrees]**: 30,60

**viewing zenith angles [degrees]**: 0,30,60

**viewing azimth angles [degrees]**: 0,90,180

**Rayleigh scattering parameterization:**

MYSTIC uses Bodhaine 1999:
Bodhaine, B. A., Wood, N. B., Dutton, E. G., and Slusser, J. R.: On
Rayleigh optical depth calculations, J. Atmos. Ocean Tech., 16,
1854–1861, 1999.


===== Intercomparison for VIS (400-500nm) =====

** Spectral range:** 400-500nm (0.2 nm resolution)

** Molecular atmosphere:** US-standard with modified NO2 profiles

**Solar irradiance:** {{ :projects:3dcats:wp_200:sao2010_gconv0p5_s0p2.solref |Solar irradiance data.}}
{{ :projects:3dcats:wp_200:solar_spectrum.png?400 |Solar irradiance}}

**NO2 profiles:**
{{:projects:3dcats:wp_200:no2_bmb_ocean.txt|Ocean, low pollution}},  {{:projects:3dcats:wp_200:no2_pacific_polluted.txt|Pacific, medium pollution}},  {{:projects:3dcats:wp_200:no2_european_polluted.txt|European, high pollution}}	
{{ :projects:3dcats:wp_200:no2_profiles.png?400 |NO2 number density profiles used for intercomparison}}

**Absorption cross sections:**
{{:projects:3dcats:wp_200:no2_vandaele_294k_gconv0p5_s0p2.xs|NO2}},
{{:projects:3dcats:wp_200:o3_243_serdyuchenko_highresol_vac_gconv0p5_s0p2.xs|O3}}, {{:projects:3dcats:wp_200:o4_thalman_volkamer_293k_gconv0p5_s0p2.xs|O4}}, {{:projects:3dcats:wp_200:h2o_hitemp_2010_390-700_296k_1013mbar_vac_gconv0p5_s0p2.xs|H2O}}

{{ :projects:3dcats:wp_200:abs_crs.png?500 |Absorption cross section data}}


==== Reflectance spectra for 400-500 nm ====

Overall, we have compared spectra for 216 cases, results at 400 nm are shown in the following figure. MYSTIC results are shown as blue circles, LIDORT as red circles. Relative differences are below 1.5%.

{{ :projects:3dcats:wp_200:clear_comp_400nm.png |Comparison between MYSTIC and LIDORT at 400 nm.}}

The following figures show various spectra. In each column one parameter is varied to see the sensitivity. All other parameters are fixed and correspond to the values in the title of the figure.
{{ :projects:3dcats:wp_200:comp_norm_radiance_0.png |Comparison between MYSTIC and LIDORT}}
{{ :projects:3dcats:wp_200:comp_norm_radiance_1.png |Comparison between MYSTIC and LIDORT}}

MYSTIC Results:

{{:projects:3dcats:wp_200:mystic_400_500_clear_v2.nc|Results for 400-500 nm}}

We find a good agreement between MYSTIC and LIDORT, remaining difference can be attributed to statistical error of Monte Carlo method.

==== Box airmass factors ====


{{ :projects:3dcats:wp_200:comp_amf_0.png |Comparison between MYSTIC and LIDORT}}

We find mostly a good agreement for box AMFs.


===== Intercomparison for O2A band =====

** Spectral range:** 755-775nm (0.005 nm spectral resolution)

** Molecular atmosphere:** midlatitude-summer
{{:projects:3dcats:wp_200:afglms.dat|Midlatitude summer atmosphere}}

** Absorption optical thickness **
{{:projects:3dcats:wp_200:moltau_755_775.nc|Line-by-line absorption optical thickness}}

** Solar irradiance **
{{:projects:3dcats:wp_200:kurudz_0.005nm.dat|Solar irradiance}}



==== Reflectance spectra for 755-775 nm ====

The following figure shows the results and relative differences for 755nm, outside the O2A band.
{{ :projects:3dcats:wp_200:clear_comp_o2a.png |Comparison between MYSTIC and LIDORT at 755 nm.}}
Good agreement (<1.5%).

The following figures show example spectra for the full range and for a zoom showing the ange between 767 and 768 nm.

{{ :projects:3dcats:wp_200:comp_norm_radiance_o2a_755_775_0.png |Comparison between MYSTIC and LIDORT.}}
{{ :projects:3dcats:wp_200:comp_norm_radiance_o2a_767_768_0.png |Comparison between MYSTIC and LIDORT.}}

Agreement between models quite good. Relative differences mostly below 1%. As expected, there are larger relative differences in line centers, where radiance becomes 0 or close to 0.

#Huan, should we compare the spectra convolved with instrument filter function?

MYSTIC Results:
{{:projects:3dcats:wp_200:mystic_755_775_clear_v2.nc|Results for 755-775 nm}}


{{:projects:3dcats:wp_200:mystic_clear_reptran_bands.nc|Averaged values for 3 wavelengths bands.}}