A Thermodynamic Analysis of an Intense North American Arctic Air Mass

Abstract

Northwestern Canada is a genesis region of arctic air masses often considered to be formed primarily through radiative processes. However, the details of their life cycle are poorly understood. This paper examines the formation, maintenance, and dissipation of an intense and long-lived arctic air mass, using a thermodynamic budget analysis.The airmass formation is characterized by a deep-layer, multistage process that begins with snow falling into a nascent air mass. Radiative cooling from cloud tops begins the process. After the snow abates and clear skies are observed, the surface temperature drops rapidly, aided by the high emissivity of fresh snow cover, falling 178C in two days, creating an intense but shallow temperature inversion. Once the surface temperature falls below the frost point, ice crystals form. Afterward, although the surface temperature remains constant, the height of the inversion rises, as radiative cooling at the top of the ice fog layer decreases temperatures.During the maintenance phase, a cold-air damming structure is present with an anticyclone in the lee of the Canadian Rockies, low pressure in the Gulf of Alaska, and an intense baroclinic zone parallel to the mountains, separating warmer maritime air from colder continental air. The air mass persists for 12 days, undergoing several cycles of deep-layer weakening and intensification.

Multi‐day valley cold‐air pools in the western United States as derived from NARR

“…This figure shows that short‐lived VCP events of 1–2 days can occur nearly everywhere in the domain except for ridge tops and other high elevation sites. Over flat terrain, the number of occurrences is relatively high in areas of the Northern Plains, where persistent low‐level inversions may form as a result of heating of the air over snow‐covered ground or cold‐air intrusions from the north (Turner et al , ). As the duration of these events increases to 3 days, the number of occurrences drops quickly to near zero across most of the study domain except for large mountain valleys and basins in the West, including the Columbia Basin, Snake River Valley, Lake Lahontan Basin, Bonneville Basin, Uinta Basin, Colorado Plateau Basin, and the Central Valley (Figure ).…”

Synoptic conditions during wintertime temperature extremes in Alaska

“…Light winds on the extreme day favor the development of strong surface inversions that enhance cooling near the surface. These results are consistent with previous work that has shown the importance of radiative cooling in the creation of cold air masses in the Arctic [ Curry , ; Turner et al ., ].…”

North American extreme temperature events and related large scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

“…Surface ridging that builds southeastward along the eastern slopes of the Rockies, particularly during 31 January and 1 February, facilitates cold-air damming and areal expansion of anomalously cold air southward to the US-Canada border. Turner et al (2013) invoke diabatic cooling to explain the observed cooling during this formation period. This cooling may consist of sublimational cooling from precipitation falling into a dry layer and radiational cooling from suspended ice crystals.…”

“…At the end of formation, there is cold-air damming (Forbes et al 1987;Fritsch et al 1992), with a cold pool and anticyclone in the lee of the Rockies, lower pressure in the Gulf of Alaska and an intense baroclinic zone oriented northwest to southeast along the mountains. Figure 5 shows an event having extremely cold surface temperatures and unusual duration (17 days, compared with Turner et al, 2013, 93-case mean of 5 days). SLP anomalies tend to be higher near the coldest air, but are not extreme during this 5-day formation period.…”