Learjet 55
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Multiengine
Flight Crew Last 90 Days 100; Flight Crew Total 10300; Flight Crew Type 150
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Regained Aircraft Control
Weather
While level at FL390 deviating around numerous thunderstorms; we encountered severe turbulence that required corrective actions on part of the crew.  The aircraft initially descended to about FL380 then violently climbed to FL405; this happening in a period of approximately 25 seconds.  Corrective actions were taken; but unable to maintain constant altitude due to the severity of the turbulence and concern of the crew for the integrity of the aircraft if we would have insisted on maintaining constant altitude.  The Center Controller advised us of our altitude changes and we reported the situation to them.  Once cleared of the turbulence we requested FL410 due to the fact that weather was clear then and we could see and avoid weather.  I'd like to point out that we had circumnavigated a line of thunderstorms which was nearly 40 to our right hand side and our radar did not indicate any storms at our 12 o'clock position and ATC radar did not as well since we had asked them just prior to the mentioned event.  Soon after the event had been reported to ATC they began advising all aircraft in the area of the potential hazard.  I make this report in the interest that it can be used for future reference to enhance safety and awareness of any weather related event.
LJ55 crew experience severe turbulence at FL390; first loosing 1;000 FT then going 1;500 FT above assigned altitude in ZFW Airspace.  The crew was deviating around thunderstorms in IMC at the time; although none were reported or detected within 40 NM of the aircraft during the incident.