|MID-LATITUDE CYCLONE STRENGTHENING
METEOROLOGIST JEFF HABY
What causes a mid-latitude low pressure to strengthen?
(1) Evacuation of mass due to the jet stream. The jet
stream is a high-speed ribbon of air that creates a large amount of upper level
speed shear. This speed shear
(through divergence) forces air
to rise from the lower levels of the troposphere and forces a low pressure to
strengthen. Low pressures coupled with the jet stream become stronger than those not coupled with the jet.
(2) Surface convergence. Surface convergence forces
the air to rise. Since air coming together at the earth's surface
can not go down into the earth's surface, it rises. Rising air creates lower pressure. This is what creates the
pressure troughs in association with cold
and warm fronts. The rising air near the frontal boundary causes pressures
to lower along the front. Convergence at the surface can occur through several ways. Examples include frontal
convergence, orographic convergence, frictional
convergence, and convergence created by a strong synoptic low-level wind flow (low level jet).
(3) Release of latent heat. The release of latent heat
causes air within a low pressure
to be more buoyant than it otherwise would be. Increased buoyancy leads to an increase of rising air and a lowering
of pressure. Low pressures developing in a very dry
air mass tend to not develop as well as those lows that have the potential to release latent heat.
Well developed mid-latitude cyclones will have all three components mentioned:
low level convergence, upper level divergence, and latent heat release. It is important for the upper level divergence
to be strong enough to maintain the low level convergence. When upper level divergence is greater than low level
convergence, the air will continue to rise. If upper level divergence weakens, the low level convergence will
cause the low pressure to fill and weaken (since
mass is no longer being evacuated at an efficient rate above the low).