Origin of Strong Winds in an Explosive Mediterranean Extratropical Cyclone

During 2–3 December 2012, the Black Sea and east coast of Romania were
affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale-model
simulation was used to analyze the evolution of the wind field, to investigate
the physical processes that were responsible for the strong winds and their
acceleration, and to investigate the relative importance of the stability of the
boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated
with a sting jet, a descending airstream from the mid-levels of the cloud head
and the lower part of the cyclonic branch of the warm conveyor belt. The sting
jet started to descend west of the cyclone center, ending at the frontolytic tip
of the bent-back front. The second was a low-level airstream associated with
the cold conveyor belt that originated northeast of the cyclone center and travelled below 900hPa along the cold side of the bent-backfront, ending behind
the cold front. Both airstreams were accelerated by the along-flow pressure
gradient force, with the largest accelerations acting on the sting-jet air be
fore entering into the near-surface strong-wind area. The sensible heat fluxes
destabilized the boundary layer to near-neutral conditions south of the cyclone
center, facilitating downward mixing and allowing the descending air to reach
the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet
formation.