Stratospheric intrusions

Stratospheric intrusions (SI) can be considered as a specific aspect of stratosphere–troposphere exchange (STE): the irreversible downward transport of stratospheric air relatively deep into troposphere.

Such phenomena are highly episodic and can be favoured by a number of different mechanisms, acting on different geographical and temporal scales: tropopause folding and cut-off lows at upper levels, and fronts or high-pressure systems at the surfaces.

Even though it has been assessed that the greatest contribution to tropospheric ozone concentrations today comes from photochemical production, the contribution from STE is far from negligible. For these reasons, at ICO-OV different experimental investigations have been performed with the aim of assessing the frequency of SI and its contribution to troposphere composition. Daily forecast for stratospheric intrusions at ICO-OV are routinely provided by the ETH-Zurich (CH).

3-D visualization of an intrusion of stratospheric air into the troposphere, resulting from a FLEXPART simulation (from Andreas Stohl's home page, http://zardoz.nilu.no/~andreas/)

By analysing six years (1998–2003) of surface ozone and beryllium-7 concentration measurements as well as relative humidity values and three-dimensional backward trajectories, we found that SI can affect Mt. Cimone for about 36 days/year. The obtained 6-year SI climatology showed a clear seasonal cycle with a winter maximum and a spring-summer minimum.The seasonal cycle was also characterized by an interannual variation possibly related with a positive NAO in winter and a negative NAO in autumn.

Mean SI monthly frequency at ICO-OV (1996 - 2007)

The analysis of several case studies (see following picture), clearly points out that SI con significantly affect the ozone variability at ICO-OV. During the most spectacular cases, ozone concentrations up to 100 ppbv (200 μg m−3) were observed at Mt. Cimone during the arrival of dry stratospheric air-masses. As deduced by ICO-OV investigations, SI contribution to background ozone concentrations was largest in winter. On average, an ozone increase of 8% (3%) with respect to the monthly running mean was found during direct (indirect) SI.

SI event as shown by ICO-OV measurements during 30 - 31 May 1996

More recently, we developed the tool STEFLUX (Stratosphere-to-Troposphere Exchange Flux) to obtain a fast-computing and reliable identification of the stratospheric intrusion events. It relies on a compiled stratosphere-to-troposphere exchange (STE) climatology, which makes use of the ERA-Interim reanalysis dataset from the ECMWF, as well as a refined version of a well-established Lagrangian methodology. By exploit ing the fact that the ERA-Interim reanalysis extends back to 1979, the long-term climatology of SI events at Mt. Cimone was assessed. The analysis of the 35-year record at both stations denies the existence of any significant trend in the SI frequency. Thanks to STEFLUX, the tropopause crossings during the whole 35-year for the SI at Mt. Cimone were also assessed.

Box-and-whisker plot of the annual variation in SI frequency at Mt. Cimone. The values obtained by in-situ experimental detection (SIO) and by STEFLUX are presented (from Putero et al., ACP, 2016).

Density of tropopause (TP) crossings for the period 1979–2013, for Mt. Cimone. Values were aggregated on a 1°X1° horizontal grid (from Putero et al., ACP, 2016).

The STFLUX outputs are fully available on request by contacting Davide Putero (d.putero-at-isac.cnr.it).

For further information: p [dot] cristofanelliatisac [dot] cnr [dot] it

Selected references
Putero D, Cristofanelli P, Sprenger M, Škerlak B, Tositti L, Bonasoni P. 2016. STEFLUX, a tool for investigating stratospheric intrusions: application to two WMO/GAW global stations. Atmos. Chem. Phys.

Cristofanelli, P, Calzolari F, Bonafè U, R.Duchi, Marinoni A, Roccato F, Tositti L, Bonasoni P. 2008. Stratospheric Intrusion Index (SI2) from baseline measurement data. Theoretical and Applied Climatology.

Cristofanelli, P, Bonasoni P, Tositti L, Bonafe' U, Calzolari F, Evangelisti F, Sandrini S, Stohl A. 2006. A 6-year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level). J. Geophys. Res.. 111(D03306)

Cristofanelli, P, Bonasoni P, Collins W, Feichter J, Forster C, James P, Kentarchos A, Kubik PW, Land C, Meloen J. 2003. Stratosphere-to-troposphere transport: A model and method evaluation. J. Geophys. Res.. 108(D12)

Bonasoni, P, Evangelisti F, Bonafè U, Ravegnani F, Calzolari F, Stohl A, Tositti L, Tubertini O, Colombo T. 2000. Stratospheric ozone intrusion episodes recorded at Mt.Cimone during the VOTALP project: case studies. Atmospheric Environment. 34(9)

Stohl, A, Spichtinger-Rakowsky N, Bonasoni P, Feldmann H, Memmesheimer M, Scheel HE, Trickl T, Hübener SH. 2000. The influence of stratospheric intrusions on alpine ozone concentrations. Atmospheric Environment. 34(9)