B747-400 First Officer reported severe turbulence encounter during cruise. Crew recovered speed and altitude and notified ATC.

Narrative

Approximately 90NM south of ZZZZZ fix at FL360; aircraft encountered severe clear air turbulence which was not forecast; PIREP'd; or identified on aircraft weather radar. Turbulence caused an immediate airspeed increase of approximately 40 kts.; VMO/MMO exceedance; altitude deviation of +600 to +800 feet; and automatic autopilot disconnect. Turbulence encounter lasted approximately 30 seconds. Aircraft was recovered to assigned altitude and airspeed with Autopilot reengaged after another 30 seconds. ATC was then notified of the turbulence and altitude deviation. Severe turbulence encounter and airspeed exceedance documented in aircraft logbook.