Initial implementation of TOA reflectance is quirky about crtmv3 HOT 8 OPEN

@gthompsnJCSDA
Digging my way through this, solar_irradiance from CRTM column 2, E_sun in Column 3 from Table 6a (GOES ABI ATBD), pg 21 in particular to see the relationship between the expected solar constant values and the Esun values.

ch     solar_irradi, Esun      
--
1       46.8353729  44.1868715   
2       68.4577789  66.3274497
3       73.7946090  72.6721072
4       70.4625507  67.9143879
5       65.0208639  63.4454241
6       40.0745478  38.1148813

Solar_Irradiance is unchanged any changes in geometry information source zenith, azimuth, sensor etc. So that's our reference quantity. Units: (mW/(m^2·cm^{-1})).

In particular, the ATBD notes that bands 1 - 6 has different radiance units than bands 7-16 (pg 20) -- and the equation is what's necessary to convert. The GOES data probably contains updated Esun values, I don't have a quick way to check this.

Next I will sort out where the solar_irradiance values are computed.

from crtmv3.

Solar_Irradiance comes from the SpcCoeff files for each instrument. So we'll have to provide some correction factor for each instrument to accommodate the difference between what CRTM assumes and what the instrument files assume.
Here's what's in the GOES-18 ABI instrument SpcCoeff file:
Solar_Irradiance = 45.249382046603, 66.1395863631183, 71.2956948943939, 68.0764704168738, 62.8190559920795, 38.7174994906341 ; so now I'm re-confused. Tracking through the code to see if solar_irradiance gets modified somewhere, because these are different from what's being output from CRTM. Still digging.

from crtmv3.

To keep the documentation going in this thread, the esun values (in units of W m-2 um-1) directly in GOES (16) L1b files for channels 1-6 are the following:

channel 1 = 2017.165
channel 2 = 1631.335
channel 3 = 957.0699
channel 4 = 360.9018
channel 5 = 242.5404
channel 6 = 76.8999

and did not change to the printed decimal place for a file in Aug 2021 and Feb 2022.

from crtmv3.

Ignore this information: I'll probably delete it later when I sort out the correct conversion.
470 nm = 21276.60 cm–1 : 46.8353729 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 996.497495
640 nm = 15625.00 cm–1: 68.4577789 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 1069.652795
860 nm = 11627.91 cm–1 : 73.7946090 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 858.077072
1370 nm = 7299.27 cm–1: 70.4625507 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 514.325182
1600 nm = 6250.00 cm–1: 65.0208639 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 406.380399
2200 nm = 4545.45 cm–1 : 40.0745478 mW/(m^2 sr cm^{-1})-> W/(m^2 sr um) = 182.156853

from crtmv3.

Updated ATBD: https://www.star.nesdis.noaa.gov/goesr/documents/ATBDs/Enterprise/ATBD_Enterprise_Cloud_and_Moisture_Imagery_Product_v4_2021-01-13.pdf

Most importantly, page 22, equations 3-2 to 3-5

from crtmv3.

@gthompsnJCSDA
Using the converter information provided in 3.4.4 of the above document, I now have converted from the SpcCoeff Spectral irradiance values:

2111.779435
1681.038527
988.641475
373.192299
250.863008
79.606007

Which is roughly consistent with what I expected to see.

from crtmv3.

[ edited by @gthompsnJCSDA ]

A first version of visible reflectance from CRTM and used within UFO appears to work reasonably well. We identified a couple possible improvements to address in next version(s).

1. Update the Esun values to match Table 6 of ATBD document, which are also the Esun values found in L1b source files.
2. Utilize the earth_sun_distance_ratios.txt to improve reflectance produced in CRTM per Eqn. 3-4.
3. Possibly reduce the number of view angles computed in CRTM (the reason we think there are 7 print statements per channel in debug log above).
4. Possibly impose an upper limit of cloud optical depth since the real world has multiple scattering.

from crtmv3.

Graphics showing the observed 1800UTC 15 Feb 2022 GOES-16 data, followed by H(x), followed by obs minus background (C384), using channel 1, 8km sub-sampled obs data:

from crtmv3.