Cold

Air

Damming

Cold Air DammingCold Air Damming

• What is Cold Air Damming?

Cold Air DammingCold 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 DammingCold Air Damming

• How does Cold Air Damming occur?

How Does CAD 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?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 DammingCold Air Damminga rough sketch of a hypothetical Cold Air Damming situation

CAD EvolutionCAD Evolution1.High

pressure crosses over into a sheltered area.

~Radiational Cooling

CAD EvolutionCAD Evolution2. Cold air is

now in place across the region.

~Cold air is dense and stable, so it will tend to remain.

CAD EvolutionCAD Evolution3. Cold air

remains, but warm air is moving in.

~Cold air has yet to be overcome by the warm air.

CAD EvolutionCAD Evolution4. Cold air is

bottled up in the valley.

~Warmer air works in aloft, but not at the lower levels

CAD EvolutionCAD Evolution5. Cold,

dense air remains.

~Warmer air takes over aloft and eventually works its’ way down to the pool.

CAD GraphicallyCAD Graphically

• What does CAD look like closer up on a weather map?

CAD GraphicallyCAD Graphically

Forecast soundings for same timeForecast soundings for same time

Importance of CADImportance of CAD

• Why is Cold Air Damming important?

The CAD ImportanceThe 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 ExampleP-Type Example

Recent CAD EventRecent 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 FebWednesday Morning 13 Feb

Wednesday Morning 13 FebWednesday Morning 13 Feb

Effects on CADEffects on CAD

• What might effect Cold Air Damming?

Effects on CADEffects 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 CADEffects 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 CADEffects 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*

• 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 CADEffects 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 CADEffects 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 DecayCAD Decay

• How might the cold wedge begin to diminish?

CAD DecayCAD 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 DecayCAD 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 DecayCAD 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 NortheastCAD 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 NWPCAD and NWP

• Why do models have difficulty with CAD?

• What models would you expect to forecast CAD the most effectively?