Transcript Slide 1
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?