Conversion of WL radiance to reflectance in MOBY data

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masudocn02
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Conversion of WL radiance to reflectance in MOBY data

by masudocn02 » Mon Mar 20, 2023 6:23 pm America/New_York

Hi there,

I want to convert the MOBY (in situ buoy data) water leaving radiance () to remote sensing reflectance.
The data can be seen on the website: https://www.star.nesdis.noaa.gov/socd/moby/filtered_spec/M272_2022052323d.txt.

The following is the information given in the text file.
" DscS 1: MOS Calibrated Wavelength (nm)
DscS 2: Water-Leaving Radiance Lw1, Lu1 and Kl1(Lu1-Lu2) (�W/cm�/sr/nm)
DscS 3: Water-Leaving Radiance Lw2, Lu1 and Kl2(Lu1-Lu3) (�W/cm�/sr/nm)
DscS 4: Water-Leaving Radiance Lw7, Lu2 and Kl3(Lu2-Lu3) (�W/cm�/sr/nm)
DscS 5: Surface Irradiance (�W/cm�/nm) for Lw1
DscS 6: Surface Irradiance (�W/cm�/nm) for Lw2
DscS 7: Surface Irradiance (�W/cm�/nm) for Lw7
DscS 8: Modeled KL: Water-Leaving Radiance Lw21, Lu1 and Kl21-m (�W/cm�/sr/nm)

Xdat:
lambda, Lw1, Lw2, Lw7, Ed Sfc, Ed Sfc, Ed Sfc, Lw21
343.8885, 1.158202, , , 78.828, 78.828, 81.033, 1.158202
344.4570, 0.824319, , , 68.706, 68.706, 71.467, 0.824319
-------------------------------"

Is there any way to convert the Lw values into Rrs (unit /Sr)? My plan is to compare the Hawaii time series Rrs (Sr^-1) (MOBY) with SeaHawk L2 Rrs (Sr^-1)?

Can I get the Rrs (Sr^-1) by the following equation?
Rrs = Lw1/Ed Sfc [As this is an insitu mesurement]

I found this site (https://seadas.gsfc.nasa.gov/help-8.2.0/rad2refl/Rad2ReflAlgorithmSpecification.html#:~:text=The%20conversion%20from%20TOA%20radiance,the%20inverse%20of%20this%20equation.) But, I am not sure if it is applicable for the buoy data.

Any help will be highly appreciated.

Best regards,
Masud

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OB SeaBASS - cwproctor
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Re: Conversion of WL radiance to reflectance in MOBY data

by OB SeaBASS - cwproctor » Wed Mar 22, 2023 5:06 pm America/New_York

Can I get the Rrs (Sr^-1) by the following equation?
Rrs = Lw1/Ed Sfc [As this is an insitu mesurement]
You're correct. That approach to the in situ data of dividing water-leaving radiance by surface irradiance will appropriately provide you with Rrs (sr^-1).

You might also wish to consider applying a bidirectional effect (BRDF) correction to the in situ measurement, which is common during validation exercises. However, the effect is likely small for those data.

One other tip: Just FYI, pay close attention to the units of lat & lon in those MOBY in situ files. If I recall correctly the coordinates are provided in the form DD.MMmm (i.e., degrees minutes seconds) which some people might mistake for decimal degrees. To convert to decimal degrees, take the .MMmm portion, multiply by 100 to convert into minutes, then divide by 60 to convert to degrees (then add this fraction to the DD)

masudocn02
Posts: 11
Joined: Tue Jan 24, 2023 11:03 am America/New_York
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Re: Conversion of WL radiance to reflectance in MOBY data

by masudocn02 » Thu Mar 23, 2023 3:01 pm America/New_York

Hi all,

I did convert the degree, mins, and secs into decimal degrees. Yes, this was very easy to make a mistake.
Thank you all for your information and feedback.

Regards,
Masud

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