I do not have the ICAO Report but the following might give you some leads.
The BAC 1-11 accident was published in the Department of Civil Aviation Aviation Safety Digest # 55 November 1968. It mentions that in a previous Aviation safety Digest #52 of September 1967 that there was a discussion from American Aviation publication. It was by Dr. Tetsuya Fujita of Chicago University. His speciality was thunderstorms.
I did some 'Googling' re Dr. Fujita and found numerous hits. Maybe if you follow this line you might find out what you are seeking.
Regards. RV.