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

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?