Thanks guys. Will do some more research and give you an update.
I have also found some NASA icing training program for pilot and a few case studies. In the process I got in touch with NASA retired research scientist, who promised to send me books and research paper. I will talk to him and read up on it.
I fnd icing challenging because you can't know for sure what you'll get till you get there since accumulation is different on different aircraft type and de-ice, anti-ice capability.
Be-99 manual supplement that I found from CASA database ( could not find anything on the FAA side yet) said flight in severe icing condition is prohibited. It defines severe as freezing rain, freezing drizzle but said nothing about freezing fog. According to NASA severe= SLD ( supercooled large droplets) and anything more than 50 microns in diameter is considered large. Now freezing drizzles and freezing rain droplets are larger than 50. Fog I am not sure...is not it about the same size as moisture droplets in clouds?