Heterogeneous formation of polar stratospheric clouds – Part 2: Nucleation of ice on synoptic scales

Abstract

Abstract. This paper provides compelling evidence for the importance of heterogeneous nucleation, likely on solid particles of meteoritic origin, and of small-scale temperature fluctuations, for the formation of ice particles in the Arctic stratosphere. During January 2010, ice PSCs (polar stratospheric clouds) were shown by CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) to have occurred on a synoptic scale (~1000 km dimension). CALIPSO observations also showed widespread PSCs containing NAT (nitric acid trihydrate) particles in December 2009, prior to the occurrence of synoptic-scale regions of ice PSCs during mid-January 2010. We demonstrate by means of detailed microphysical modeling along air parcel trajectories that the formation of these PSCs is not readily reconciled with expectations from the conventional understanding of PSC nucleation mechanisms. The measurements are at odds with the previous laboratory-based understanding of PSC formation, which deemed direct heterogeneous nucleation of NAT and ice on preexisting solid particles unlikely. While a companion paper (Part 1) addresses the heterogeneous nucleation of NAT during December 2009, before the existence of ice PSCs, this paper shows that also the large-scale occurrence of stratospheric ice in January 2010 cannot be explained merely by homogeneous ice nucleation but requires the heterogeneous nucleation of ice, e.g. on meteoritic dust or preexisting NAT particles. The required efficiency of the ice nuclei is surprisingly high, namely comparable to that of known tropospheric ice nuclei such as mineral dust particles. To gain model agreement with the ice number densities inferred from observations, the presence of small-scale temperature fluctuations, with wavelengths unresolved by the numerical weather prediction models, is required. With the derived rate parameterization for heterogeneous ice nucleation we are able to explain and reproduce CALIPSO observations throughout the entire Arctic winter 2009/2010.

A multi-wavelength classification method for polar stratospheric cloud types using infrared limb spectra

“…The method still suggests a strong probability for NAT clouds, which is disputable under the assumption that NAT is only formed for T < T ice . However, the observations are in line with recent results of Engel et al (2013) and Hoyle et al (2013), showing evidence for a process of heterogeneous NAT nucleation at T > T ice for Arctic PSC events.…”

“…The new dataset of observations of PSC types for multiple SH and NH winter conditions up to pole may help to better constrain and validate new microphysical models describing the homogenous and heterogeneous formation processes for ice and NAT (e.g. Engel et al, 2013;Hoyle et al, 2013). These models are beyond the status of simple equilibrium schemes, which are still used in most CTMs and CCMs.…”

“…The formation of STS droplets is well understood (e.g. Carslaw et al, 1995), but for NAT and ice particles, new formation mechanisms by heterogeneous nucleation on meteoric smoke well above T ice are under discussion Engel et al, 2013;. These pathways for NAT and ice formation that are new so far are supported by the observation of an unusually large amount of refractory submicron aerosols of most likely meteoric origin in composition measurements of cloud condensation nuclei (Weigel et al, 2014).…”

“…In addition, mesoscale temperature variations caused by gravity waves are crucial for the formation of PSCs under conditions close to temperature threshold conditions (e.g,. Carslaw et al, 1998;Engel et al, 2013), but are missing from the current generation of CCMs (Orr et al, 2015).…”

Balloon-borne match measurements of midlatitude cirrus clouds

“…This model has been applied previously for polar stratospheric cloud investigations and tropospheric aerosol and cirrus studies (Luo et al, 2003;Hoyle et al, 2005;Engel et al, 2013). It is also used as a column model, driven by temperature and pressure data along staggered air parcel trajectories (Brabec et al, 2012), and in the present context we will make use of this column feature.…”