Downwelling longwave radiation and atmospheric winter states in the western maritime Arctic

Hourly time-series of several atmospheric state variables and downwelling longwave radiation (LWd) collected through the extended winter (mid-December 2007 to March 2008) of the International Polar Year-Circumpolar Flaw Lead (IPY-CFL) ship-board campaign in Amundsen Gulf were analysed. A histogram derived from the hourly LWd time-series revealed a positively (right) skewed frequency distribution. The cold, dry, and mainly clear weather or mean atmospheric states concurrent with the modal and adjacent classes showed that LWd observations within +/- 30Wm(-2) of the modal class mark (160Wm(-2)) identify the presence of the semi-permanent cold-core high pressure centre which appears on average winter mean sea-level pressure maps for the western maritime Arctic. The warmer, moister, and cloudier weather or mean atmospheric states associated with LWd bins in the right tail of the positively skewed frequency distribution, and the weather coincident with the deepest mean sea-level pressure minima, showed that observations of LWd in the 210-280Wm(-2) range identified the passage of baroclinic disturbances. While a right skewed LWd frequency distribution usually occurs, the class mark of the major modal class, and the class marks of the bins in the right tail of the distribution show inter-annual variability of +/- 10 to 30Wm(-2). Owing to its inclusive nature, validation of modelled LWd may provide a comprehensive verification statistic for Arctic climate simulations.