Where can I find more information about the high-cloud index in the Reanalysis dataset?

I am not familiar enough with the Reanalysis dataset to answer your question and will have to refer you to the Reanalysis project and their documentation.

You might find the following page helpful in some way: http://dss.ucar.edu/datasets/ds090.0/docs/problems/problem-cld. Otherwise, I would direct you here: http://dss.ucar.edu/pub/reanalysis/

If the documentation does not contain the answer to your question, then you might try contacting the support people at the Reanalysis project.

-- Michael Bell