What are the physical characteristics of Cumulonimbus mamma clouds as described by the FAA ?

Actually that's Cumulonimbus Mammatus, acronym CBMAM. See pages 3-20 and 3-21 in the AVIATION WEATHER SERVICES Advisory Circular, AC 00-45F here: a href="http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/d6a522c25e53cbf58625776f0050495c/$FILE/AC-0045G_chg1_section3.pdf" rel="nofollow"http://rgl.faa.gov/Regulatory_and_Guidan…/a