You have said that warm air in thunderstorms rising to 50-60 thousand feet eventually runs into warmer air above that level that prevents it from rising further. Isn’t air at those elevations much colder, like below freezing?
It is, but be aware that air temperatures normally decline 3 to 5 degrees per thousand feet of increasing altitude. This is due primarily to decreasing air pressure as one ascends. Temperatures within the rising air currents of a thunderstorm, while decreasing with increasing altitude, are still warmer than surrounding non-thunderstorm air. This creates upward buoyancy within the thundercloud. However, at about 40 to 60 thousand feet aloft (where temperatures, even during the summer, are typically -65 degrees), temperatures remain steady or begin to increase. Storm updrafts, when they encounter this “warmer” air, cease to rise.