The airborne Lyman-α photofragment fluorescence hygrometer FISH (Fast In-situ Stratospheric Hygrometer) is a well-established closed-path instrument for measuring water vapor in the range of 1 to 1000 ppmv . FISH is specially designed for measuring the low water vapor mixing ratios prevailing in the upper troposphere and lower stratosphere. It is regularly calibrated on the ground to a reference frost point hygrometer (MBW DP30) and had an accuracy of ±(6 % + 0.4 ppmv) during the TACTS and ESMVal campaigns (). The air supply for the measuring cell is provided by a forward-facing inlet, which also samples ice crystals if present (H2Ocond). Together, the evaporated crystals and the gas-phase water (H2Ogas) sum up to a total water measurement.

The Hygrometer for Atmospheric Investigation (HAI) is a fast, airborne, in situ hygrometer capable of multi-phase water detection that employs direct tunable-diode laser absorption spectroscopy (dTDLAS) in combination with a special data retrieval method which allows absolute water vapor measurements without any sensor calibration . This data retrieval was recently successfully validated via a direct side-by-side comparison with the German metrological water vapor scale . HAI is a 2×2, multi-channel TDLAS spectrometer which realizes, in a unique concept, a simultaneous gas-phase and total water measurement via parallel water detection in an open-path cell, outside the aircraft fuselage, and in extractive (i.e., gas sampling) closed-path cells inside the aircraft. To enlarge the dynamic range to 1–40 000 ppmv, both cell types are simultaneously probed by two laser wavelengths, at 1.4 µm for the lower to mid-troposphere and at 2.6 µm for the upper troposphere and the stratosphere. The ranges of the 1.4 and 2.6 µm spectrometer channels overlap and therefore provide a redundant, independent detection between 10 and about 5000 ppmv. The metrologically calculated spectrometer uncertainties are ±(4.3 % + 3 ppmv) at 1.4 µm and ±(5.9 % + 0.4 ppmv) at 2.6 µm. In this study, data from the two closed-path spectrometers are used, i.e., total water measurements similar to FISH.

The TRIHOP instrument is a three-channel quantum cascade laser infrared absorption spectrometer capable of the subsequent measurement of CO, CO2, N2O and CH4. During TACTS/ESMVal the instrument was calibrated in situ against secondary standards of compressed ambient air which are traceable against NOAA standards. Integration time for each species was 1.5 s at a duty cycle of 8 s, which finally limits the temporal resolution of the measurements. For the ESMVal flight on 13 September 2012, TRIHOP CH4 data achieved a precision (2σ) of 9.5 ppbv and accuracy of 13.5 ppbv without any corrections applied.

FAIRO is a new accurate ozone instrument developed for use onboard the HALO aircraft. It combines two techniques, the UV photometry (light absorption of O3 at λ=250–260 nm) with high accuracy and chemiluminescence detection with high measurement frequency. A UV-LED is used as a light source for the UV photometer, which can be controlled well (in contrast to Hg lamps) for constant light emission. The 1σ precision is 0.08 ppbv at a measurement frequency of 4 s and a cuvette pressure of 1 bar and the total uncertainty is 2 %. The chemiluminescence detector shows a measurement frequency of 12.5 Hz and a high precision of 0.05 ppbv (at 10 ppbv absolute, a measurement frequency of 5 Hz, and a pressure of 1 bar) .

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) combines a two-dimensional focal plane detector array with a Fourier transform spectrometer to capture about 6000 infrared limb spectra simultaneously. This enables remote sensing observations with high vertical and horizontal resolution to resolve small-scale structures see. The spectral sampling can be switched between 0.625 and 0.0625 cm-1 at the cost of an increased acquisition time in the case of higher spectral resolution . The gimbal mount and inertial altitude control system allows GLORIA to maintain a steady pointing on a moving aircraft; it can also be used to point the instrument at a range of azimuth angles with respect to the aircraft, covering about 70∘. This allows for tomographic measurement patterns and 3-D reconstruction of fine-scale filamentary structures . The vertical location of retrieved quantities approximately follows the tangent points of the measurement geometry (parabola curve through the atmosphere). Thus, while the location of quantities retrieved at flight level lies in the vicinity of the aircraft, the locations of quantities at lower altitudes are horizontally several tens to hundreds of kilometers away from the flight path. Assuming a flight altitude of 13 km, the water vapor observation at 12 km is horizontally 113 km away from the aircraft, while at 8 km it is already 250 km. Concerning the water vapor product used in this paper, GLORIA achieved a vertical and horizontal resolution of about 500 m and 100 km, respectively. The water vapor precision and total error (accuracy + precision) amounts to about 0.2 and 1.0 ppmv in the stratosphere. showed that GLORIA water vapor at flight level agrees fairly well with the in situ FISH measurements, within error bars. The deviations to FISH during the ESMVal/TACTS flights are mostly less than 0.4 ppmv.

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite is one of five satellites in the NASA A-train constellation. CALIPSO completes 14.55 orbits per day with an inclination of 98.2∘ and thus delivers good coverage above the polar regions. In addition to one wide-field camera and an imaging infrared radiometer, the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is aboard CALIPSO. The lidar operates with two wavelengths (532 and 1064 nm) with additional polarization sensitivity, providing high-resolution vertical backscatter profiles of aerosols and clouds. In this study we use the CALIPSO Lidar Level 2 Polar Stratospheric Cloud (PSC) data product, which is described in . This data product provides a PSC composition scheme on a daily basis for all nighttime orbits with a resolution of 5 km horizontally by 180 m vertically.

Global meteorological reanalysis ERA-Interim data of the ECMWF (European Centre for Medium-Range Weather Forecasts) are used to interpret the observations. The meteorological fields have a resolution of 1∘ × 1∘ with 60 vertical levels from the surface (1000 hPa) up to 0.1 hPa. Every 6 h a full global data set is available. The trajectories used in Sect.  are based on the horizontal wind fields and diabatic heating rates; additional parameters such as temperature and pressure are added to the trajectories from the ERA-Interim data (for details see Sect. ).

The general dynamical situation of the Antarctic vortex is shown in Fig. a. High PV values of < -9 PVU denote the Antarctic vortex at the 350 K isentrope, which corresponds approximately to the flight level. The vortex edge is marked with a blue line according to the definition of . The vortex itself is shifted about 10∘ towards Africa, indicating a wave-one pattern caused by Rossby wave activity. This results in high PV values up to 47∘ S, where the aircraft in situ measurements confirm the presence of Antarctic vortex air as will be shown later (see Sect. ). In the lowermost stratosphere below the vortex, represented by the 310 K isentrope (see Fig. b), the PV structure with high PV values around -6 to -3 PVU is more jagged but mostly connected. Here, a strong Rossby wave activity is visible and also RWB events associated with isolated PV structures are discernible. A blob with high PV at 60 to 47∘ S and 10 to 30∘ E indicates an ongoing RWB event. These air masses were probed during the ESMVal flight on 13 September 2012. The flight was performed from Cape Town (South Africa) heading towards Antarctica until 65∘ S at an altitude of 12–13 km (see Fig. ). After a dive from 12.5 km down to 3 km to sample the transition from the polar stratosphere to the troposphere, the return flight took place at 12.5 km and rose up to 15 km shortly before descending back to Cape Town. A hexagonal flight pattern was included in the flight path back to Cape Town to enable the tomographic study of trace gases with GLORIA. The ECMWF wind field (black lines), as well as the strongly increasing potential vorticity (PV; see Fig. ), shows that the vortex edge (yellow dots, determined with the Nash criterion) was far north, up to 47∘ S down to the 340 K level. The color code in Fig. a represents the in situ-measured water vapor mixing ratios from FISH along the flight path. Directly south of the vortex edge the water vapor mixing ratios decrease rapidly to below 2.5 ppmv (purple dots along the flight path).

The latitudinal cross section of PV from ECMWF along the flight path is shown in Fig. b. The thermal tropopause (cyan dots) stays almost constant at around 10 km and rises up to 12 km further poleward. The PV and also the equivalent latitude (not shown) reveal high values within the vortex and thus clearly mark the vortex air masses. However, high PV values of -2 PVU show up far below the thermal tropopause down to 5 km altitude in the latitude range from 60 to 45∘ S. The difference compared to a tropopause fold at the midlatitude jet is that the air masses directly originate from the Antarctic vortex and were strongly dehydrated. Thus, the air masses have a different chemical composition than the “normal” air masses in the midlatitude lowermost stratosphere. This indicates that these air masses originate from higher altitudes and were transported through the thermal tropopause and form a stratospheric intrusion with a low PV-based tropopause. Interestingly, the isentropes between 310 and 320 K intersect the thermal tropopause on the poleward side of the jet core. Therefore, air masses can be potentially transported along these tilted isentropes to the low altitudes across the thermal tropopause into the low PV structure. The flight path crosses the air masses with high PV in the stratosphere before and after the dive. 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.

The time series of the in situ measurements are shown in Fig. , where the measurements of the water vapor instruments are visible in the upper panel and the measurements of the tracers for defining stratospheric/vortex air masses are shown in the lower panel. After the ascent of HALO near Cape Town, the water vapor mixing ratio decreases to the typical stratospheric value of 4 ppmv, from 06:30 until 08:15 UTC. During this time, ozone and PV (i.e., the stratospheric tracers with low values in the troposphere) show an increase compared to the values before 06:30 UTC, while methane (tropospheric tracer with low values in the stratosphere) shows a slight decrease from 1790 to 1740 ppbv. Both indicate that the aircraft entered the stratosphere. Thereafter, methane and water vapor further decrease to 1650 ppbv and 3 ppmv, respectively, on the same flight level starting at 08:15 UTC, and simultaneously ozone and PV further increase. This is the time when HALO penetrated the Antarctic vortex (marked in blue in Fig. ). After ascending to a higher flight level (08:35 UTC), water vapor decreases to slightly above 2 ppmv and stays there until the southernmost point was reached and the dive started at 11:00 UTC. This low water vapor in the time between 08:15 and 11:00 UTC coincides well with high PV values (see Fig. b). Subsequently, the strong increase in methane and decrease in ozone and PV indicate the penetration of the aircraft into the Antarctic troposphere at 11:00 UTC. After returning to the previous flight level (11:45 UTC), the vortex signatures with low values of water vapor and methane as well as high values of ozone and PV show up again. However, at this point (∼12:30 UTC) the water vapor mixing ratio reaches the lowest value of 1.6±0.5 ppmv together with a strong maximum in ozone and PV. The low water vapor below 4 and even below 2 ppmv indicates strongly dehydrated air masses.

The saturation mixing ratio with respect to ice within the vortex is 3 to 4 times larger than the measured water vapor mixing ratio. Thus, the sampled air masses were clearly subsaturated with relative humidities of 25–33 % and it is unlikely that ice particles remained in the probed air masses. In addition, HAI and FISH show no peaks in the total water vapor time series (based on 1 Hz data), which would indicate the presence of ice particles on the flight level. In particular, the water vapor mixing ratios around 2 ppmv during the stratospheric flight legs (06:30 to 11:00 and 11:45 to 15:45 UTC) are very smooth and show no significant variations. Thus, both water vapor instruments, FISH and HAI, measured only gas-phase water vapor, and no cloud particles were present in the vortex air. Both FISH and HAI observed these low water vapor mixing ratios independently.

In order to show the fairly good agreement of both FISH and HAI based on 1 Hz data at the observed low water vapor values, we choose two flight legs for the comparison (leg one: 09:45–10:45 UTC; leg two: 13:00–14:00 UTC). During flight leg one, both hygrometers have an absolute difference of 0.21 ppmv and a mean relative difference of -14.9 % (±10.5 %, 1σ). During flight leg two, a difference of 0.26 ppmv and a mean relative difference of -5.9 % (±7.3 %, 1σ) is found. These rather small differences are consistent with the uncertainties of both hygrometers, which are ±0.45 and ±0.5 ppmv at the 2 ppmv level for FISH and HAI, respectively. The HAI data measured before 09:10 UTC (marked with gray in Fig. ) are influenced by an untypical memory effect of the measurement cell during ascent, caused by a valve which was opened too late, but only when flying in the upper troposphere. As a result, the “wet” measurement cells were not sufficiently dried off with the high mass flows during take-off and ascent. This also explains the asymmetry in the deviations to FISH before and after the dive. In addition, the agreement of GLORIA water vapor observations at flight level with the two in situ instruments, FISH and HAI, is remarkably good, considering that GLORIA is a remote sensing instrument.

The GLORIA instrument measures quasi-vertical profiles along the flight path by viewing the atmosphere on the right side of HALO. Here, we focus on the H2O retrieval product, which is shown in Fig. . We consider only the times from 08:00 to 14:00 UTC to focus on a time period when GLORIA observed vortex air (the whole data set is from 06:30 to 15:30 UTC). The dive is noticeable as the white area between 11:00 and 12:00 UTC, where GLORIA did not measure in order to prevent condensation of tropospheric water vapor on the cold instrument. The dry vortex air masses are clearly visible between 08:00 and 13:00 UTC at flight level (black solid line in Fig. ) but also below, down to altitudes of 7 km. As mentioned in Sect. , the quantities retrieved from GLORIA are approximately placed on a parabola following the tangent points through the atmosphere (e.g., 250 km horizontal distance to flight path at 9 km altitude). The dry region just before the dive was measured by GLORIA west of the flight path on the way towards Antarctica, while the air masses after the dive are measured east of the flight path on the way back to Cape Town. Thus, the dehydrated air masses below the thermal tropopause cover a large region, with a dimension of at least 500 km horizontally at 9 km altitude.

The thermal tropopause is also derived from the GLORIA-retrieved temperature profiles and is marked with black dots. Interestingly, low water vapor mixing ratios around and below 2 ppmv are observed beneath the thermal tropopause. Especially in the time range from 09:00 to 10:30 UTC and from 12:00 to 13:30 UTC, very dry air masses with water vapor mixing ratios of 2–3 ppmv can be found far below the thermal tropopause, down to 7 km. This indicates that dehydrated stratospheric air masses could have been transported through the thermal tropopause. The dynamic tropopause, which is between the -4 and -2 PVU isoline, is somewhat lower than the thermal tropopause at ∼7 km in the time range from 12:00 to 13:30 UTC and indicates a proceeding stratospheric intrusion. In Sect. , we analyze this transport process with the help of air mass backward trajectories. In addition, no water vapor values less than 6 ppmv are found below the -4 PVU isoline in ECMWF data (not shown here). This shows, at least for this situation, that the transport process of dry stratospheric air masses to these low altitudes we observed here is not captured by the ECMWF meteorological analysis (see Sect. ).

Embedded in the dry vortex air masses, small filaments (marked in Fig.  with red boxes) of enhanced water vapor are visible. These filaments may indicate rehydration layers, where sedimented ice particles from upper layers have sublimated and resulted in layers of enhanced water vapor as described in, for example, for the Arctic. However, trajectory reverse domain filling (RDF) (Sect. ) showed that these air masses could not be attributed to rehydration. Also, the in situ measurements of FISH/HAI showed no evidence of a rehydration layer during the dive from ∼12.5 to 3 km and back. We have analyzed these somewhat moister air masses in more detail, as discussed in the next section.

In this section, we investigate the history of the in situ-measured dehydrated air masses. For that purpose, the trajectories are calculated every 10 s along the flight path, each reaching 50 days backwards in time. For further analysis, we selected all trajectories between 12:20 and 12:29 UTC (in total 54), when the lowest water vapor mixing ratios were measured. However, the other trajectories with measured low water vapor mixing ratios between 2 and 3 ppmv show a very similar history. Figure  shows the median saturation mixing ratio with respect to ice (orange line) and the frequency distribution (grayscale color code) of every 1 h trajectory time step of all trajectories calculated by ClaMS along the 50 days back in time. The trajectories show the same history in saturation mixing ratio from 13 September until 14 August, when the trajectories start to diverge. This indicates that the trajectories have the same path with the same temperature history for at least 30 days. After this time, the trajectories split up into two branches, indicating a bifurcation point and associated mixing on 14 August. The trajectories within the branch with lower saturation mixing ratios of 2 to 3 ppmv below the median line stay together on the same path until the beginning of August. The trajectories before 1 August have different saturation mixing ratios indicating the dispersion of air masses, which limits the interpretation of the trajectory calculations.

To show the presence of ice particles and subsequent dehydration, CALIPSO data (see Sect. ) are used. A distance criterion is used to match the air mass trajectories with the CALIPSO data. Every trajectory at a specific time which coincides with the CALIPSO orbit inside a square of ±5∘ latitude and ±5∘ longitude is checked for the presence of ice in an altitude range of ±0.2 km. If more than 50 % of all trajectories at one time coincide with a CALIPSO ice observation, the corresponding time periods are marked with a blue bar (see Fig. ).

Figure  illustrates the coincidence of low saturation mixing ratios along the trajectories with observed ice formation (CALIPSO) in the considered air masses. The trajectories pass several cold regions associated with low saturation mixing ratio where ice formation is initiated. If the ice cloud and supersaturation exist long enough (typically around 0.5–1 days; ) to allow the ice particles to grow to sizes of several micrometers (diameter of 10 to 20 µm), sedimentation of these ice particles begins and causes irreversible dehydration. The sedimented ice particles with these sizes fall with velocities of around 2.5 kmd-1 (see ) until warmer temperatures in lower regions causes them to evaporate. With each pass of a cold region, more and more water vapor is removed from the air mass. As stated by , air masses must undergo several periods with dehydration before reaching the 1 to 2 ppmv mixing ratio level, which is typically observed by satellites within the vortex in the Antarctic spring. This periodic behavior is confirmed by Fig. , where trajectories undergo four events with low saturation mixing ratios and simultaneous ice cloud observations by CALIPSO. The first and the second period (1 and 9 August) reveal rather high saturation mixing ratios of around 3 ppmv. In every later period, colder temperatures and lower saturation mixing ratios are necessary to initiate additional ice formation and dehydration. This is the case during the third and the fourth period on 16 and 29 August, when CALIPSO observed ice show lower saturation mixing ratios of 1.5 and 1.2 ppmv, respectively. The last two events will have dried out the air masses to the final state of about 1.6 ppmv, which is observed by the FISH and HAI instruments. After this strong dehydration, the temperatures became warmer in the late Antarctic spring and summer and no additional dehydration took place according to the simulations.

The recurring low saturation mixing ratios and corresponding low temperatures are primarily caused by gravity waves, which are induced by the high topography of the Antarctic continent . In particular, the first three ice formation events occurred just as the trajectories passed the Antarctic Peninsula (northernmost part), which is a strong source of gravity waves e.g.,. Only the last freezing event with the lowest supersaturation occurs above the Antarctic Plateau (central part) in the northeast of the continent.

After freezing, the ice particles can even reach the troposphere due to sedimentation, especially if dehydration takes place at low altitudes (11.5–12.5 km) as revealed by the trajectories. The lack of rehydration signatures in the GLORIA measurements (see Fig. ) also indicates the permanent removal of water vapor from the stratosphere, as will be further discussed in Sect. . The air masses from which the ice particles originate and the air masses where they potentially sublimate in the troposphere behave very differently in dynamical perspective (not shown). Due to the length of time since the last dehydration on 28 August (16 days before measurement) it is very unlikely that rehydrated air masses would still be found below the observed dehydrated air masses in the troposphere. In addition, the in situ measurements taken during the dive confirm the absence of any observed rehydration features.

The trajectories for analyzing the transport of dehydrated air masses observed by GLORIA are calculated 70 days backward in time from each tangent point (in total 30 000 single trajectories). In addition, the trajectories are also calculated 14 days forward to show where the air masses are further transported. With this set of trajectories, the GLORIA measurements can be interpreted and the associated transport pathways can be investigated.

In the first subsection hereafter, the dehydrated air masses observed with the in situ instruments and GLORIA in the lowermost stratosphere are analyzed and the origin of moist filaments is investigated. The second subsection contains the trajectory analysis of dry air masses found to be below the thermal tropopause and investigates the further transport.