A corporate jet Captain reported an NMAC with an L-39 shortly after takeoff from NEW when the L-39 descended below his cleared altitude to avoid a cloud base.

Narrative

Our aircraft was cleared by ATC to takeoff and assigned to maintain runway heading and 2;000 FT. Immediately after takeoff; while level at 2;000 FT; within three miles of the airport; ATC instructed an 'experimental' L-39 jet aircraft to maintain 3;000 FT and proceed to the airport. At that time; our crew had visual contact with the L-39 aircraft at our 12 o'clock and above position. Immediately thereafter; our aircraft TCAS reported the L-39 was descending from 800 FT above our assigned altitude of 2;000 FT. As the L-39 flew directly overhead our position; our TCAS issued a RA that the aircraft was descending from +800 FT through +400 FT. There was insufficient time to accomplish an evasive maneuver before the L-39 aircraft passed directly overhead and behind our position and the RA was dismissed by our TCAS. I advised ATC of the L-39's close proximity and mentioned that it was too close! I also advised ATC that our TCAS reported a RA but we were unable to react quickly enough before the alert passed and the situation was again safe. ATC reported for a second time that the L-39 was assigned to maintain 3;000 FT. When questioned by ATC; I heard the L-39 pilot advise the Controller that he had descended to remain clear of the cloud base along the L-39's ATC assigned routing. The remainder of the flight was uneventful.