1. Problem: Brad noticed that lake ice is not being flagged in the AOD retrievals in this Feb 12, 2018 case on the west side of Lake Michigan. As a result, AOD is retrieved over ice when it shouldn’t be.
Shobha said that the upstream cryosphere algorithm says no snow/ice.

2. VIIRS RGB

3. 250m true color image from MODIS Terra over Lake Michigan on February 12, 2018.
It appears the west coast is covered by very thin lake ice.

4. IMS

5. Further investigation by Yinghui Liu
High AOD retrievals over Lake Michigan case over Lake Michigan on February 12, 2018.
The high AOD seems over pixels where lake ice exists but not identified by the ice algorithm.
I investigated if lake ice exists over those pixels, and if current ice algorithm is capable of detecting those pixels.
Conclusion:
Those pixels seem to be covered by lake ice but it is very thin.
There are three tests: visible (actually, µm), Normalized Difference Snow Index (NDSI), and temperature. All must be met.
Our current ice algorithm cannot identify those pixels as lake ice due to very low visible channel reflectance, e.g. at µm. Even lowering the visible threshold by 10-20% wouldn’t identify this area as ice.

6. RBG over Lake Michigan on February 12, 2018.
Ice on the west coast of the Lake Michigan are vague. Note: this is not high-resolution image.

7. Reflectance at 0.865 µm over Lake Michigan on February 12, 2018.
Values lower than 0.08, which is the threshold in ice algorithm, is in purple color

8. NDSI over Lake Michigan on February 12, 2018.
NDSI values lower than 0.6, which is the threshold in ice algorithm, is in purple color

9. BT at 10.763 µm over Lake Michigan on February 12, 2018.
Values lower than 275K, which is the threshold in ice algorithm, are in purple color