Negative rhos in 2020 and 2021 OLI images in a mountain lake

[oneLaker]

Hi Quinten,

I am using ACOLITE to process the Landsat series images in a mountain lake (Mono Lake). However, I found that there are a lot of negative rhos values in Landsat-8 OLI from May to July in 2020-2021. Other images in the period of 1984-2023 are normal.

For those pixels with negative rhos, I found rhorc values are positive. I also try to update the codes to the lastest version from the Github, but the same issue still exists.

Below is the specific setting and an example of output L2R is avaiable from Google drive. I also attached the corresponding log file. Could you provide some suggestions?

Code: Select all

inputfile=Z:\level1_data\Landsat\mono\LC08_L1TP_042034_20200523_20200607_01_T1
output=E:\Satellitedata\mono
l2w_parameters=rhos_*,rhorc_*
rgb_rhot=False
rgb_rhos=True
l1r_delete_netcdf=True
l2t_delete_netcdf=True
ancillary_data=True
elevation=1945.0
limit=37.9,-119.2,38.1,-118.8

Best,
Zhigang

[quinten]

Hi Zhigang

Do you mean negatives are retrieved in the NIR band? For the scene you sent (I got the collection 2 equivalent LC08_L1TP_042034_20200523_20200820_02_T1) I get some slight NIR negatives in part of the lake, marked in red. From the spectrum view you can see they are very close to 0.

zhigang_mono_lake.jpg (552.94 KiB) Viewed 17622 times

The aerosol optical depth at 550 nm is on average 0.15 for tiled processing, which is not unreasonable. For this relatively small region of interest you can also use dsf_aot_estimate=fixed which uses a single aerosol optical depth over the lake. This gives a lower aerosol optical depth of around 0.09 and results in no negatives at least for this scene.

I hope this helps!

Quinten

[oneLaker]

Hi Quinten,

Thanks for your suggestion. Yes, your suggestion also works fine for other images in 2020-2021.

I am confused that ACOLITE with defaulting settings work fine for Landsat except for data in May-July of 2020 and 2021. I might need to use dsf_aot_estimate=fixed to reprocess all images from 1984-2023 to keep the consistent AC processing.

Thanks again!

Zhigang