Download Cold Air Damming

Cold
Air
Damming
Cold Air Damming
• What is Cold Air Damming?
Cold Air Damming
• Cold Air Damming is when cold air is
locked or “dammed” into a location, often a
valley or side of a mountain chain.
• Causes for this phenomenon include
topography and high pressure systems.
• Cold Air Damming is also known as CAD.
Cold Air Damming
• How does Cold Air Damming occur?
How Does CAD Occur?
• Cold air is stable and dense, so it is not
always easily disrupted or transported.
• Warm air often rides up and over the cold air
that is locked in at the lower levels.
• Hills and valleys work to keep cold air
bottled up, like a dam bottles up a reservoir.
How Does CAD Occur?
• Terrain alone is usually not enough to cause
Cold Air Damming.
• A high pressure system in place or having
an influence is a big factor.
• High pressure funnels in cooler air into
sheltered areas, such as valleys or on sides
of hills or mountains.
Cold Air Damming
a rough sketch of a hypothetical Cold Air Damming situation
CAD Evolution
1. High
pressure
crosses
over into a
sheltered
area.
~Radiational
Cooling
CAD Evolution
2. Cold air is
now in
place
across the
region.
~Cold air is
dense and
stable, so
it will tend
to remain.
CAD Evolution
3. Cold air
remains,
but warm
air is
moving in.
~Cold air
has yet to
be
overcome
by the
warm air.
CAD Evolution
4. Cold air is
bottled up
in the
valley.
~Warmer air
works in
aloft, but
not at the
lower
levels
CAD Evolution
5. Cold,
dense air
remains.
~Warmer air
takes over
aloft and
eventually
works its’
way down
to the
pool.
CAD Graphically
• What does CAD look like closer up on a
weather map?
CAD Graphically
Forecast soundings for same time
Importance of CAD
• Why is Cold Air Damming important?
The CAD Importance
• Cold Air Damming can cause localized
precipitation-type variances.
• A shallow cold layer would promote ZR.
• A deeper cold layer would promote IP.
• If the layer is deep enough and extends
high enough, snow could even fall.
P-Type Example
Recent CAD Event
• 12 to 13 February (weak CAD event)
• Low pressure moves up the coast.
• With high pressure in place prior, cold air
remains and hangs tough.
• Coastal front has trouble moving inland.
• Low pressure tracks SE of the area.
Wednesday Morning 13 Feb
Wednesday Morning 13 Feb
Effects on CAD
• What might effect Cold Air Damming?
Effects on CAD
• High pressure location and intensity.
– If the high is in good alignment, cold air
damming can be maximized.
– If the high is strong, it can also work to drive in
or at least keep in colder air.
Effects on CAD
• Local topography and elevation.
– For the northeast, CAD occurs most effectively
on the east side of hills or mountains.
– If the valley is deep or the mountains are tall,
the depth of the cold pocket can be maximized.
– If the terrain has gaps or wedges, cold air can
escape/warmer air can seep in.
Effects on CAD
• Frontal boundaries.
– If a warm front moves in, the CAD is likely to
diminish as warmer air is advected*.
– CT example, a coastal front establishes a
boundary between the air masses.
*Warm Air Advection*
• Warm Air Advection aloft does not
necessarily or immediately hurt CAD.
• In fact, CAD can be maximized when WAA
occurs aloft and CAA is at the lower levels.
• This causes a capping inversion, which
creates a stable environment.
• Also, cold air is more dense than warm air
and WAA needs to mix and work in.
Effects on CAD
• Mixing.
– When warmer air moves in aloft and mixing
occurs, the cold wedge is disrupted.
– Colder air can be released.
– Warmer air can be filtered and mixed in.
Effects on CAD
• Cloud cover.
– Clouds can work to block insolation and
prevent the cold wedge from warming.
– Also, clouds and precipitation lead to
evaporational cooling to enhance CAD.
CAD Decay
• How might the cold wedge begin to
diminish?
CAD Decay
• Wind flow change.
– If the low pressure system moves too close or
to the northwest of the area, warmer air drives
in. (warm sector)
– Wind flow effects how cold air is dammed into
a location.
– Wind flow may also effect how warmer air can
be driven in or colder air driven out.
CAD Decay
• Mixing.
– When warm and cold air interact, they mix and
the temperature changes.
– Mixing may be due to rising/sinking motion…
– turbulence created by surface roughness…
– strong winds…
– wind shear in the vertical profile.
CAD Decay
• *Cold air advection*
– If cold air advection occurs aloft, it can actually
work to decay the cold air damming.
– CAA favors sinking motion and tends to
diminish cloud-cover and cloud development.
– This would work to allow solar heating to take
place and to warm the layer.
– The capping inversion is disrupted.
CAD in the Northeast
• Discussion on cold air damming in the
northeast and across the tri-state region.
What areas tend to get the best CAD?
What other local factors besides hills and
valleys may affect CAD?
CAD and NWP
• Why do models have difficulty with CAD?
• What models would you expect to forecast
CAD the most effectively?