Cold-air damming
Cold air damming, or CAD, is a meteorological phenomenon that involves a high-pressure system accelerating equatorward east of a north-south oriented mountain range due to the formation of a barrier jet behind a cold front associated with the poleward portion of a split upper level trough. Initially, a high-pressure system moves poleward of a north-south mountain range. Once it sloshes over poleward and eastward of the range, the flow around the high banks up against the mountains, forming a barrier jet which funnels cool air down a stretch of land east of the mountains. The higher the mountain chain, the deeper the cold air mass becomes lodged to its east, and the greater impediment it is within the flow pattern and the more resistant it becomes to intrusions of milder air.
As the equatorward portion of the system approaches the cold air wedge, persistent low cloudiness, such as stratus, and precipitation such as drizzle develop, which can linger for long periods of time; as long as ten days. The precipitation itself can create or enhance a damming signature, if the poleward high is relatively weak. If such events accelerate through mountain passes, dangerously accelerated mountain-gap winds can result, such as the Tehuantepecer and Santa Ana winds. These events are seen commonly in the northern Hemisphere across central and eastern North America, south of the Alps in Italy, and near Taiwan and Korea in Asia. Events in the southern Hemisphere have been noted in South America east of the Andes.
Location
Cold air damming typically happens in the mid-latitudes as this region lies within the Westerlies, an area where frontal intrusions are common. When the Arctic oscillation is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the equator, which brings cold air into the mid-latitudes. Cold air damming is observed in the southern hemisphere to the east of the Andes, with cool incursions seen as far equatorward as the 10th parallel south. In the northern hemisphere, common situations occur along the east side of ranges within the Rocky Mountains system over the western portions of the Great Plains, as well as various other mountain ranges along the west coast of the United States. The initial is caused by the poleward portion of a split upper level trough, with the damming preceding the arrival of the more equatorward portion.Some of the cold air damming events which occur east of the Rockies continue southward to the east of the Sierra Madre Oriental through the coastal plain of Mexico through the Isthmus of Tehuantepec. Further funneling of cool air occurs within the Isthmus, which can lead to winds of gale and hurricane-force, referred to as a Tehuantepecer. Other common instances of cold air damming take place on the coastal plain of east-central North America, between the Appalachian Mountains and Atlantic Ocean. In Europe, areas south of the Alps can be prone to cold air damming. In Asia, cold air damming has been documented near Taiwan and the Korean Peninsula.
The cold surges on the eastern slopes of the Rocky Mountains, Iceland, New Zealand, and eastern Asia differ from the cold air damming east of the Appalachians due to the wider mountain ranges, sloping terrain, and lack of an eastern body of warm water.
Development
The usual development of CAD is when a cool high-pressure area wedges in east of a north-south oriented mountain chain. As a system approaches from the west, a persistent cloud deck with associated precipitation forms and lingers across the region for prolonged periods of time. Temperature differences between the warmer coast and inland sections east of the terrain can exceed 36 degrees Fahrenheit, with rain near the coast and frozen precipitation, such as snow, sleet, and freezing rain, falling inland during colder times of the year. In the Northern Hemisphere, two-thirds of such events occur between October and April, with summer events preceded by the passage of a backdoor cold front. In the Southern Hemisphere, they have been documented to occur between June and November. Cold air damming events which occur when the parent surface high-pressure system is relatively weak, with a central pressure below, or remaining a progressive feature, can be significantly enhanced by cloudiness and precipitation itself. Clouds and precipitation act to increase sea level pressure in the area by 1.5 to 2.0 mb. When the surface high moves offshore, the precipitation itself can cause the CAD event.Detection
Detection algorithm
This algorithm is used to identify the specific type of CAD events based on the surface pressure ridge, its associated cold dome, and ageostrophic northeasterly flow which flows at a significant angle to the isobaric pattern. These values are calculated using hourly data from surface weather observations. The Laplacian of sea level pressure or potential temperature in the mountain-normal—perpendicular to the mountain chain—direction provides a quantitative measure of the intensity of a pressure ridge or associated cold dome. The detection algorithm is based upon Laplacians evaluated for three mountain-normal lines constructed from surface observations in and around the area affected by the cold air damming—the damming region. The "x" denotes either sea level pressure or potential temperature and the subscripts 1–3 denote stations running from west to east along the line, while the "d" represents the distance between two stations. Negative Laplacian values are typically associated with pressure maxima at the center station, while positive Laplacian values usually correspond to colder temperatures in the center of the section.Effects
When cold air damming occurs, it allows for cold air to surge toward the equator in the affected area. In calm, non-stormy situations, the cold air will advance unhindered until the high-pressure area can no longer exert any influence because of a lack of size or its leaving the area. The effects of cold air damming become more prominent when a storm system interacts with the spreading cold air.The effects of cold air damming east of the Cascades in Washington are strengthened by the bowl or basin-like topography of Eastern Washington. Cold Arctic air flowing south from British Columbia through the Okanogan River valley fills the basin, blocked to the south by the Blue Mountains. Cold air damming causes the cold air to bank up along the eastern Cascade slopes, especially into the lower passes, such as Snoqualmie Pass and Stevens Pass. Milder, Pacific-influenced air moving east over the Cascades is often forced aloft by the cold air in the passes, held in place by cold air damming east of the Cascades. As a result, the passes often receive more snow than higher areas in the Cascades, which supports skiing at Snoqualmie and Stevens passes.
centered over the Great Basin gives rise to a Santa Ana wind event as the airmass flows through the passes and canyons of southern California, manifesting as a dry northeasterly wind.
The situation during Tehuantepecers and Santa Ana wind events are more complicated, as they occur when air rushing southward due to cold air damming east of the Sierra Madre Oriental and Sierra Nevada respectively, is accelerated when it moves through gaps in the terrain. The Santa Ana is further complicated by downsloped air, or foehn winds, drying out and warming up in the lee of the Sierra Nevada and coastal ranges, leading to a dangerous wildfire situation.
The wedge
The effect known as "the wedge" is the most widely known example of cold air damming. In this scenario, the more equatorward storm system will bring warmer air with it above the surface. This warmer air will ride over the cooler air at the surface, which is being held in place by the poleward high-pressure system. This temperature profile, known as a temperature inversion, will lead to the development of drizzle, rain, freezing rain, sleet, or snow. When it is above freezing at the surface, drizzle or rain could result. Sleet, or Ice pellets, form when a layer of above-freezing air exists with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze, and freezing rain will be the result at the surface. A thicker or stronger cold layer, where the warm layer aloft does not significantly warm above the melting point, will lead to snow.Blocking
occurs when a well-established poleward high-pressure system lies near or within the path of the advancing storm system. The thicker the cold air mass is, the more effectively it can block an invading milder air mass. The depth of the cold air mass is normally shallower than the mountain barrier which created the CAD. Some events across the Intermountain West can last for ten days. Pollutants and smoke can remain suspended within the stable air mass of a cold air dam.Erosion
It is often more difficult to forecast the erosion of a CAD event than its development. Numerical models tend to underestimate the event's duration. The bulk Richardson number, Ri, calculates vertical wind shear to help forecast erosion. The numerator corresponds to the strength of the inversion layer separating the CAD cold dome and the immediate atmosphere above. The denominator expresses the square of the vertical wind shear across the inversion layer. Small values of the Richardson number result in turbulent mixing that can weaken the inversion layer and aid the deterioration of the cold dome, leading to the end of the CAD event.Cold advection aloft
One of the most effective erosion mechanisms is the import of colder air—also known as cold air advection—aloft. With cold advection maximized above the inversion layer, cooling aloft can weaken in the inversion layer, which allows for mixing and the demise of CAD. The Richardson number is reduced by the weakening inversion layer. Cold advection favors subsidence and drying, which supports solar heating beneath the inversion.Solar heating
Solar heating has the ability to erode a CAD event by heating the surface in the absence of a thick overcast. However, even a shallow stratus layer during the cold season can render solar heating ineffective. During breaks of overcast for the warm season, absorption of solar radiation at the surface warms the cold dome, once again lowering the Richardson number and promoting mixing.Near-surface divergence
In the United States, as a high-pressure system moves eastward out to the Atlantic, northerly winds are reduced along the southeast coast. If northeasterly winds persist in the southern damming region, net divergence is implied. Near-surface divergence reduces the depth of the cold dome as well as aid the sinking of air, which can reduce cloud cover. The reduction of cloud cover permits solar heating to effectively warm the cold dome from the surface up.Shear-induced mixing
The strong static stability of a CAD inversion layer usually inhibits turbulent mixing, even in the presence of vertical wind shear. However, if the shear strengthens in addition to a weakening of the inversion, the cold dome becomes vulnerable to shear-induced mixing. Unlike solar heating, this CAD event erosion happens from the top down. Mixing occurs when the depth of the northeasterly flow becomes increasingly shallow and strong southerly flow makes a downward progression resulting in high shear.Frontal advance
Erosion of a cold dome will typically first occur near the fringes where the layer is relatively shallow. As mixing progresses and the cold dome erodes, the boundary of the cold air – often indicated as a coastal or warm front – will move inland, diminishing the width of the cold dome.Classifying Southeastern United States events
An objective scheme has been developed to classify certain types of CAD events in the Southeastern United States. Each scheme is based on the strength and location of the parent high-pressure system.Classical
Classical CAD events are characterized by dry synoptic forcing, partial diabatic contribution, and a strong parent anticyclone located to the north of the Appalachian damming region. A strong high-pressure system usually is defined as having a central pressure over. The northeastern United States is the most favorable location for the high-pressure system in classical CAD events.For diabatically enhanced classical events, at 24 hours prior to the onset of CAD, a prominent 250-mb jet extends from southwest to northeast across eastern North America. A general area of troughing is present at the 500- and 250-mb levels west of the jet. The parent high-pressure system is centered over the upper Midwest beneath the 250-mb jet entrance region, setting up conditions for CAD east of the Rocky Mountains.
For dry onset classical events, the 250-mb jet is weaker and centered farther east relative to the diabatically enhanced classical events. The jet also does not extend as far southwest compared to diabatically enhanced classical CAD events. The center of the high-pressure system is farther east, so ridging extends southward into the south-central eastern United States. Although both types of classical events begin differently, their results are very similar.