Abstract
Abstract. Spaceborne lidar measurements from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) are used to provide a vortex-wide perspective of the 2009–2010 Arctic PSC (polar stratospheric cloud) season to complement more focused measurements from the European Union RECONCILE (reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) field campaign. The 2009–2010 Arctic winter was unusually cold at stratospheric levels from mid-December 2009 until the end of January 2010, and was one of only a few winters from the past fifty-two years with synoptic-scale regions of temperatures below the frost point. More PSCs were observed by CALIPSO during the 2009–2010 Arctic winter than in the previous three Arctic seasons combined. In particular, there were significantly more observations of high number density NAT (nitric acid trihydrate) mixtures (referred to as Mix 2-enh) and ice PSCs. We found that the 2009–2010 season could roughly be divided into four periods with distinctly different PSC optical characteristics. The early season (15–30 December 2009) was characterized by patchy, tenuous PSCs, primarily low number density liquid/NAT mixtures. No ice clouds were observed by CALIPSO during this early phase, suggesting that these early season NAT clouds were formed through a non-ice nucleation mechanism. The second phase of the season (31 December 2009–14 January 2010) was characterized by frequent mountain wave ice clouds that nucleated widespread NAT particles throughout the vortex, including Mix 2-enh. The third phase of the season (15–21 January 2010) was characterized by synoptic-scale temperatures below the frost point which led to a rare outbreak of widespread ice clouds. The fourth phase of the season (22–28 January) was characterized by a major stratospheric warming that distorted the vortex, displacing the cold pool from the vortex center. This final phase was dominated by STS (supercooled ternary solution) PSCs, although NAT particles may have been present in low number densities, but were masked by the more abundant STS droplets at colder temperatures. We also found distinct variations in the relative proportion of PSCs in each composition class with altitude over the course of the 2009–2010 Arctic season. Lower number density liquid/NAT mixtures were most frequently observed in the lower altitude regions of the clouds (below ~18–20 km), which is consistent with CALIPSO observations in the Antarctic. Higher number density liquid/NAT mixtures, especially Mix 2-enh, were most frequently observed at altitudes above 18–20 km, primarily downstream of wave ice clouds. This pattern is consistent with the conceptual model whereby low number density, large NAT particles are precipitated from higher number density NAT clouds (i.e. mother clouds) that are nucleated downstream of mountain wave ice clouds.
Highlights
In spite of more than two decades of research, a detailed understanding of the processes leading to polar ozone depletion has not yet fully emerged
There are several notable differences between the hemispheres: (1) there are 25 times more Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) polar stratospheric clouds (PSCs) observations in the Antarctic; (2) yearto-year variability in PSC composition is much higher in the Arctic; (3) the fraction of ice PSCs is a factor of four smaller in the Arctic; (4) the fraction of Mix 2-enh PSCs in the Arctic (13%) is about half the fraction observed in the Antarctic, but is much larger than the 1–4% of Type 1a-enh clouds reported in the Ny Alesund ground-based lidar PSC climatology (Biele et al, 2001; Massoli et al, 2006)
An intensive field campaign focused on measurements related to PSCs and ozone chemistry was conducted in the Arctic during January–March 2010 as part of the European Union RECONCILE project
Summary
In spite of more than two decades of research, a detailed understanding of the processes leading to polar ozone depletion has not yet fully emerged. Pitts et al.: The 2009–2010 Arctic polar stratospheric cloud season the ozone depletion processes in global climate models and call into question the capability of these models to predict the evolution of the ozone layer in a future stratosphere whose composition differs from the current state. This is of particular concern in the Arctic, where winter temperatures hover near the PSC formation threshold and, future stratospheric cooling could lead to enhanced cloud formation and substantially greater ozone losses (WMO, 2007). The overall goal of this paper is to use spaceborne lidar measurements from CALIPSO to examine the evolution of PSCs during the 2009–2010 Arctic winter on vortex-wide scales and provide context to the more focused measurements obtained during the RECONCILE campaign.
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