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 −42.8°C 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 clear-sky surface radiational cooling predominates, augmented by the high emissivity of fresh snow cover. The surface temperature falls very rapidly, up to a maximum of 18°C day−1 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 2.2°C day−1.
