Observation of mesospheric air inside the arctic stratospheric polar vortex in early 2003

Abstract

Abstract. During several balloon flights inside the Arctic polar vortex in early 2003, unusual trace gas distributions were observed, which indicate a strong influence of mesospheric air in the stratosphere. The tuneable diode laser (TDL) instrument SPIRALE (Spectroscopie Infra-Rouge par Absorption de Lasers Embarqués) measured unusually high CO values (up to 600 ppb) on 27 January at about 30 km altitude. The cryosampler BONBON sampled air masses with very high molecular Hydrogen, extremely low SF 6 and enhanced CO values on 6 March at about 25 km altitude. Finally, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) Fourier Transform Infra-Red (FTIR) spectrometer showed NO y values which are significantly higher than NO y * (the NO y derived from a correlation between N 2 O and NO y under undisturbed conditions), on 21 and 22 March in a layer centred at 22 km altitude. Thus, the mesospheric air seems to have been present in a layer descending from about 30 km in late January to 25 km altitude in early March and about 22 km altitude on 20 March. We present corroborating evidence from a model study using the KASIMA (KArlsruhe SImulation model of the Middle Atmosphere) model that also shows a layer of mesospheric air, which descended into the stratosphere in November and early December 2002, before the minor warming which occurred in late December 2002 lead to a descent of upper stratospheric air, cutting off a layer in which mesospheric air is present. This layer then descended inside the vortex over the course of the winter. The same feature is found in trajectory calculations, based on a large number of trajectories started in the vicinity of the observations on 6 March. Based on the difference between the mean age derived from SF 6 (which has an irreversible mesoCorrespondence to: A. Engel (an.engel@meteor.uni-frankfurt.de) spheric loss) and from CO 2 (whose mesospheric loss is much smaller and reversible) we estimate that the fraction of mesospheric air in the layer observed on 6 March, must have been somewhere between 35% and 100%.

Mean age of stratospheric air derived from AirCore observations

“…They could have been caused by biomass burning from wildfires occurring over western Canada during the observation period. Lowest values of CO in the stratosphere are of the order of 10-20 ppb, in agreement with expected steady-state values (Toon et al, 1999;Engel et al, 2006b). AC-3, which was measured after AC-2 shows an increase in CO mixing ratios above 20 km, which is most probably due to the longer storage time, resulting in more diffusive mixing with remaining fill gas.…”

“…On the other hand, the PG has much higher CO (about 1400 ppb) in order to allow a clear distinction from both tropospheric and stratospheric air. In particular, stratospheric air has much lower CO mixing ratios, which are on the order of 20 ppb (Engel et al, 2006b;Toon et al, 1999). Our measurements showed a gradual decrease of CO values from the high FG values to the significantly lower stratospheric mixing ratios.…”

“…After passing the peak with high CO from the FG, the values drop sharply, showing much lower CO values. These lower CO values are expected in the middle stratosphere (Toon et al, 1999;Engel et al, 2006b) due to the photochemical balance between the production of CO from oxidation of CH 4 and the breakdown of CO due to oxidation with the OH radical. The transition from high CO (remaining FG) to low CO thus marks the beginning of the sampled air at the upper part of the profile.…”

Model evaluation of CO2 and SF6 in the extratropical UT/LS region

“…For this purpose, G is defined in a convenient way as an Inverse Gaussian Distribution (IG) in terms of the mean age G and the width D, used in many different fields [e.g., Chikara and Folks, 1989;Seshadri, 1999]: [34] For the parameterization of the TTD, we apply G 2 /D = 0.7 as suggested by Hall and Plumb [1994] and confirmed by Engel et al [2002]. To prove that the initial mean age field is close to reality, a comparison is made with mean age profiles, derived from balloon-born measurements of CO 2 and SF 6 performed at mid-and high latitudes [Engel et al, 2002[Engel et al, , 2006b]. The good agreement between these profiles and the model profiles derived from the initial stratospheric mean age distribution illustrates the good quality of the KASIMA tracer fields (see Figure 2).…”

Middle atmosphere water vapour and dynamical features in aircraft measurements and ECMWF analyses

“…During winter, dry air from the mesosphere descends into the stratosphere through the polar vortex, as modelled by Le Texier et al (1988) and observed by e.g. Lahoz et al (1994); Aellig et al (1996); Plumb et al (2002); Engel et al (2006).…”