Abstract
Abstract. Dehydration in the Antarctic winter stratosphere is a well-known phenomenon that is annually observed by satellites and occasionally observed by balloon-borne measurements. However, in situ measurements of dehydrated air masses in the Antarctic vortex are very rare. Here, we present detailed observations with the in situ and GLORIA remote sensing instrument payload aboard the German aircraft HALO. Strongly dehydrated air masses down to 1.6 ppmv of water vapor were observed as far north as 47° S in an altitude between 12 and 13 km in the lowermost stratosphere. The dehydration can be traced back to individual ice formation events above the Antarctic Peninsula and Plateau, where ice crystals sedimented out and water vapor was irreversibly removed. Within these dehydrated stratospheric air masses, filaments of moister air reaching down to the tropopause are detected with the high-resolution limb sounder, GLORIA. Furthermore, dehydrated air masses are observed with GLORIA in the Antarctic lowermost stratosphere down to 7 km. With the help of a backward trajectory analysis, a midlatitude origin of the moist filaments in the vortex can be identified, while the dry air masses down to 7 km have stratospheric origins. Antarctic stratosphere–troposphere exchange (STE) and transport of dehydrated air masses into the troposphere are investigated. Further, it is shown that the exchange process can be attributed to several successive Rossby wave events in combination with an isentropic exchange of air masses across the thermal tropopause. The transport into the troposphere is caused by air masses that are detached from the potential vorticity (PV) structure by Rossby wave breaking events and subsequently transported diabatically across the dynamical tropopause. Once transported to the troposphere, air masses with stratospheric origin can reach near-surface levels within several days.
Highlights
Antarctic stratospheric dehydration occurs regularly every winter and spring in the very isolated and very cold southern hemispheric polar vortex (e.g., Kelly et al, 1989; Schoeberl et al, 1992; Vömel et al, 1995; Nedoluha et al, 2002; Jimenez et al, 2006)
This paper focuses on one measurement flight directly into the Antarctic vortex that was performed as part of the ESMVal (Earth System Model Validation; Schlager, 2014) campaign in September 2012 with the German research aircraft HALO (High Altitude and LOng Range; Krautstrunk and Giez, 2012)
This indicates that the water vapor filaments observed by GLORIA were not generated by rehydration but are likely the signatures of moister midlatitude air masses that were mixed into the vortex during the preceding 5 days
Summary
Antarctic stratospheric dehydration occurs regularly every winter and spring in the very isolated and very cold southern hemispheric polar vortex (e.g., Kelly et al, 1989; Schoeberl et al, 1992; Vömel et al, 1995; Nedoluha et al, 2002; Jimenez et al, 2006). Aura-MLS and POAM 3 satellite measurements (Nedoluha et al, 2002; Jimenez et al, 2006; Schoeberl and Dessler, 2011) do not show dehydrated air masses as far north (not beyond 57◦ S) and as low in the stratosphere and upper troposphere as was observed during this ESMVal flight. This is mostly caused by the poor spatial resolution of satellites in the UT/LS region. The dive down to 3.5 km altitude was performed on the poleward side of the air masses with high PV below the thermal tropopause
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