  There is a clear distinction between simple laminar and complex turbulent
fluids. But in some cases, as for the nocturnal planetary boundary layer, a
stable and well-ordered flow can develop intense and sporadic bursts of
turbulent activity which disappear slowly in time. This phenomenon is
ill-understood and poorly modeled; and yet, it is central to our understanding
of weather and climate dynamics. We present here a simple model which shows
that in stably stratified turbulence, the stronger bursts can occur when the
flow is expected to be more stable. The bursts are generated by a rapid
non-linear amplification of energy stored in waves, and are associated with
energetic interchanges between vertical velocity and temperature (or density)
fluctuations. Direct numerical simulations on grids of 2048^3 points confirm
this somewhat paradoxical result of measurably stronger events for more stable
flows, displayed not only in the temperature and vertical velocity derivatives,
but also in the amplitude of the fields themselves.
