The Development of Arctic Air Masses in Northwest Canada and Their Behavior in a Warming Climate

Abstract

Surface observations, soundings, and a thermodynamic budget are used to investigate the formation process of 93 arctic airmass events. The events involve very cold surface temperatures-an average of 242.88C at Norman Wells, a centrally located station in the formation region-and cooling in the 1000-500-hPa layer. A multistage process for their formation in northwestern Canada is proposed. This process is contrary to the classical conceptualization of extremely shallow, surface formations.In the first stage of formation, snow falls into a layer of unsaturated air in the lee of the Rocky Mountains, causing sublimational cooling and moistening the subcloud layer. Simultaneously, the midtroposphere is cooled by cloud-top radiation emissions. In the second stage, snowfall abates, the air column dries, and clearsky surface radiational cooling predominates, augmented by the high emissivity of fresh snow cover. The surface temperature falls very rapidly, up to a maximum of 188C day 21 in one event. In the final stage, after near-surface temperatures fall below the frost point, ice crystals and, nearer the surface, ice fog form. At the end of formation, there is cold-air damming, 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.There have been secular changes in the characteristics of the arctic air masses over the period 1948-2008. The surface temperature during the events has become warmer, and the air masses are deeper and moister. The 1000-hPa diabatic cooling during events, which includes latent heat and radiative processes, has decreased by 2.28C day 21 .

A Thermodynamic Analysis of an Intense North American Arctic Air Mass

“…This definition also selects events throughout the winter (DJF) rather than only during January, the month of maximum variability. Turner and Gyakum (2011) found that these arctic air masses were, contrary to the classic conception, deeplayer events with cooling extending up to 500 hPa, rather than being solely shallow, surface radiation-driven events. They were found to form in multistages, beginning with snow falling into a drier surface layer, increasing lowlevel moisture and cooling the upper atmosphere through radiation from cloud tops.…”

“…However, it tended to overestimate wintertime cloud fraction. Turner and Gyakum (2011) defined arctic airmass events as 3 or more consecutive days of surface minimum temperature anomaly that is at least one standard deviation colder than the 30-yr running-mean monthly climatology for that station and month. This criterion must be satisfied in at least 5 of the 10 stations.…”

“…Turner and Gyakum (2011) studied the composite characteristics of their creation and intensification. The air masses formed at an average rate of 1.5 yr 21 over the period 1948-2008, with an average surface temperature at 10 stations in northwest Canada during the events of 239.68C: 1.65 standard deviations colder than the climatological daily winter [December-February (DJF)] temperature at these stations.…”

“…Arctic cloud processes are important in the energy budget and climate of the region (Sorteberg et al 2007;Vavrus et al 2008) and they are poorly parameterized in reanalysis datasets and other models (Intrieri and Shupe 2004;Intrieri et al 2002;Liu et al 2005;Town et al 2005;Walsh et al 2009). To better understand the processes involved in the creation of these events, this paper examines the dynamics and thermodynamics associated with the formation and maintenance of an intense, but characteristic arctic air mass, selected from 93 events in Turner and Gyakum (2011). The event chosen, spanning 2-13 February 1979, is notable for its large vertical depth of cooling and areal coverage.…”

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

“…More recently, Emanuel (2009) found that the rates and depth of cooling in his model were sensitive to the amount of water vapor and clouds present. Turner and Gyakum (2011), in their composite study of 93 Arctic air mass formations in Northwest Canada, found that the cold air mass lifecycle is a multi-stage set of processes. During the first stage, snow falls into a layer of unsaturated air in the lee of the Rockies, causing moisture increases in the sub-cloud layer.…”