A large-scale comparison of water-vapour vertical-sounding instruments took place over central Europe on 17 October 2008, during a rather homogeneous deep stratospheric intrusion event (LUAMI, Lindenberg Upper-Air Methods Intercomparison). The measurements were carried out at four observational sites: Payerne (Switzerland), Bilthoven (the Netherlands), Lindenberg (north-eastern Germany), and the Zugspitze mountain (Garmisch-Partenkichen, German Alps), and by an airborne water-vapour lidar system creating a transect of humidity profiles between all four stations. A high data quality was verified that strongly underlines the scientific findings. The intrusion layer was very dry with a minimum mixing ratios of 0 to 35 ppm on its lower west side, but did not drop below 120 ppm on the higher-lying east side (Lindenberg). The dryness hardens the findings of a preceding study (“Part 1”, Trickl et al., 2014) that, e.g., 73 % of deep intrusions reaching the German Alps and travelling 6 days or less exhibit minimum mixing ratios of 50 ppm and less. These low values reflect values found in the lowermost stratosphere and indicate very slow mixing with tropospheric air during the downward transport to the lower troposphere. The peak ozone values were around 70 ppb, confirming the idea that intrusion layers depart from the lowermost edge of the stratosphere. The data suggest an increase of ozone from the lower to the higher edge of the intrusion layer. This behaviour is also confirmed by stratospheric aerosol caught in the layer. Both observations are in agreement with the idea that sections of the vertical distributions of these constituents in the source region were transferred to central Europe without major change. LAGRANTO trajectory calculations demonstrated a rather shallow outflow from the stratosphere just above the dynamical tropopause, for the first time confirming the conclusions in “Part 1” from the Zugspitze CO observations. The trajectories qualitatively explain the temporal evolution of the intrusion layers above the four stations participating in the campaign.
The complexity of stratospheric air intrusions into the troposphere has been investigated with lidar systems in great detail. A lot of information was obtained from airborne transects (e.g. Browell et al., 1987, 1996, 2001; Flentje et al., 2005) and ground-based time series (e.g. Ancellet et al., 1991, 1994; Lamarque et al., 1996; Langford et al., 1996; Langford and Reid, 1998; Eisele et al., 1999; Stohl and Trickl, 1999; Galani et al., 2003; Zanis et al., 2003; Trickl et al., 2003, 2010; Di Girolamo et al., 2009; Kuang et al., 2012). Ozone is an excellent tracer for mapping intrusion layers, but does not allow for the erosion of these layers within the troposphere to be quantified because one cannot easily resolve the mixing of tropospheric air into the descending layer. Water vapour is a much better choice for such investigations, because of the low stratospheric volume mixing ratio of about 5 ppm (e.g. Scherer et al., 2008) and only slightly higher values just above the tropopause.
Turbulent mixing has been identified as an important source of tropospheric air in tropopause folds (Shapiro, 1976, 1978, 1980). About half of the air mass in a fold has been estimated to be of tropospheric character (Shapiro, 1980; Vogel et al., 2011). Nevertheless, the tropospheric input had never been quantified along the entire path of the air mass eventually reaching the lower troposphere. An open question has been how much of the tropospheric air originates already from the so-called “mixing layer” around the thermal tropopause (e.g. Danielsen, 1968; Lelieveld et al., 1997; Hintsa et al., 1998; Zahn et al., 1999, 2014; Fischer et al., 2000; Hoor et al., 2002, 2004; Pan et al., 2004, 2007; Brioude et al., 2006, 2008; Sprung and Zahn, 2010; Vogel et al., 2011) prior to the descent and how much of the admixture occurs during the descent of an intrusion layer into the lower troposphere. In some cases mixing of polluted or convectively lifted air into intrusions within the free troposphere has been reported (e.g. Parrish et al., 2000; Brioude al., 2007; Homeyer et al., 2011; Sullivan et al., 2016).
In contrast to the idea of strong tropospheric mixing Bithell et al. (2000) found in a case study that an extremely dry layer of presumable stratospheric origin survived in the troposphere without resolvable change for at least 10 days. Trickl et al. (2014, 2015) verified this behaviour based on water-vapour measurements during about 80 intrusion cases: in 59 % of the deep intrusion cases with subsidence times up to 6 days the minimum relative humidity (RH) was 1 % or less, 1 order of magnitude smaller than the typical results from in situ measurements with the dew-point-mirror instrument at the nearby Zugspitze summit (2962 m a.s.l.). The corresponding mixing ratio of roughly 50 ppm or less is typical of values found in the “mixing layer” that extends a few kilometres into the stratosphere.
Despite this evidence of low free-tropospheric mixing, the ozone number densities in the same intrusion layers stay significantly below full stratospheric values. Trickl et al. (2014) conclude that the ozone values are mostly determined by how far the intrusion layer initially extends into the stratosphere. They found that CO mixing ratios in deep intrusions rarely strongly differ from tropospheric values. This implies that the descending layers depart from the lowest few kilometres above the dynamical tropopause since fully stratospheric CO values are substantially smaller.
Trickl et al. (2014) discussed three cases with rather filamentary structure in order to demonstrate that exceptionally low mixing prevails even for thin layers. In the follow-up paper presented here, sharing the main part of the title, we extend that study by analysing a much more homogenous intrusion layer over a rather large area: The observations took place over a major part of central Europe during LUAMI (Lindenberg Upper-Air Methods Intercomparison, in the evening of 17 October 2008; Wirth et al., 2009b). Quantitative three-dimensional mapping with the DLR (Deutsches Zentrum für Luft und Raumfahrt) airborne lidar system WALES (Water Vapour Lidar Experiment in Space) (Wirth et al., 2009a) around a major part of central Europe is combined with measurements of ground-based lidar systems, balloon-borne sensors at four stations forming the four corners of the flight track. Atmospheric transport modelling shows the development of the descending dry layer between the stratospheric source region over northern Canada and the Alps and clearly confirms the ideas of the previous investigation. The campaign constitutes one of the largest-scale comparisons of water-vapour profiling instrumentation and verifies a very high quality of all the instruments contributing. In particular, the first comparison of an airborne and a ground-based differential-absorption (DIAL) system in the entire free troposphere was made. Detailed results are given in the Appendix.
For the validation flight the DLR Falcon F20 aircraft was equipped with
WALES, a four-wavelength water-vapour DIAL. The name WALES was chosen in analogy to the core instrument proposed
by DLR for a satellite mission (ESA, 2004). The new instrument design, which
is described in more technical detail in Wirth et al. (2009a), features a
robust, highly compact, and efficient transmitter system, which fulfils all
spectral requirements for a water vapour DIAL. The instrument simultaneously
emits radiation at three wavelengths resonant with H
The HITRAN 2008 data base (Rothman et al., 2009) was used as the source of
spectroscopic parameters. The high accuracy of the line parameters for the
lines selected for the LUAMI flight of 1 to 2 % is verified by the
comparisons presented here. A linear combination of the water-vapour
profiles retrieved for the three “on” wavelengths, weighted by the squared
reciprocal uncertainties is obtained from a statistical analysis of the
H
The density profiles along the flight path were obtained by interpolation of meteorological analysis data (T799L91 resolution; Untch et al., 2006) of the European Centre for Medium-Range Weather Forecasts (ECMWF) for the respective location and time. The T799L91 horizontal grid spacing at mid-latitudes is roughly 25 km, and a 91-level vertical grid up to 0.01 mbar (about 50 levels up to 200 mbar) is used.
WALES provided a transfer standard for comparing the performance of the instruments at the four sites participating in that effort, particularly the lidar systems. The lidar approach makes possible an improved volume matching that is an important prerequisite due to the frequently extreme spatial inhomogeneity of water vapour (Vogelmann et al., 2011, 2015).
The Swiss aerological station Payerne is located approximately 40 km west to
south-west of the Swiss capital Bern at 46.8130
Upper air profiles of pressure, temperature, humidity, wind speed, and direction are operationally measured at Payerne twice a day, and include 3-hourly visual weather observations with 24 h staffed operation. Ozone profiles are measured 3 times per week. In situ radiosonde profiling has been expanded in recent years with ground-based remote sensing profiling techniques, such as wind profilers, microwave radiometers, a Raman lidar system, and a GNSS (Global Navigation Satellite System, using GPS, Global Positioning System) receiving antenna to measure continuously the integrated water-vapour column. All surface and remote sensing instruments are in close vicinity to the radiosonde station.
The Raman Lidar for Meteorological Observations (RALMO) is a custom-designed instrument and has been operated at MeteoSwiss Payerne since August 2008. It was developed by the Swiss Federal Institute of Technology (EPFL) for the needs of MeteoSwiss (for details see Dinoev et al., 2013; Brocard et al., 2013). While other lidar groups (e.g. Leblanc et al., 2008, 2012; Whiteman et al., 2010) have successfully taken the approach of using large integration times during night-time (thus avoiding any daytime sunlight interferences) in order to produce profiles up to the upper troposphere and lower stratosphere, the aim in Payerne is to make continuous measurements of tropospheric water vapour at a high temporal resolution during both day and night. The lidar system uses a frequency-tripled Nd:YAG laser that emits laser pulses (< 8 ns duration) at a repetition rate of 30 Hz. The typical energy per pulse at the (vacuum) wavelength of 354.8 nm is around 0.3 J, resulting an average power of approximately 9 W. Before being emitted in the atmosphere the beam is expanded to a diameter of 140 mm. This ensures an eye-safe laser beam and reduces beam divergence to 0.1 mrad. Four telescopes with 0.3 m parabolic mirrors are arranged symmetrically around the vertical outgoing beam to receive the backscattered photons. The telescope system has a total aperture equivalent to a telescope of 0.6 m diameter and a field of view of 0.2 mrad. The narrow field of view together with narrowband spectral filtering in the receiver allows for daytime operation. Optical fibres connect the telescope mirrors with a grating polychromator used to isolate the rotational–vibrational Raman signals of nitrogen and water vapour (wavelengths of 386.8 and 407.6 nm, respectively). The optical signals are detected by photomultipliers and acquired by a transient digitizer. The data are stored at half-hour intervals.
An ECC ozone sonde was launched at 13:00 CET (12:00 UTC; Central European Time, i.e. UTC
CAELI (CESAR (Cabauw Experimental Site for Atmospheric Research;
The instrument provides profiles of backscatter and extinction coefficients
(
The lidar data are ingested at 10 s time resolution and 7.5 m vertical sampling. The water-vapour profiles are averaged over 15 min, one of them coinciding with the Falcon overpass on 17 October 2008. The water-vapour mixing ratio is calculated from the ratio of the 407 and 387 nm signals and calibrated against the noon radiosonde at De Bilt. Smoothing is applied to the profile with a range-dependent smoothing length going from high resolution at low altitudes and progressively lower resolution to the far range.
The water-vapour Raman lidar RAMSES (Raman lidar for atmospheric moisture
sensing; Reichardt, 2012, 2014; Reichardt et al., 2012, 2014) was installed
at the Richard Aßmann Observatory of the German Meteorological Service in
Lindenberg (east of Berlin) in 2005 (52
At Lindenberg water-vapour profiles are also measured using balloon-borne in situ sensors. In all, 4 times daily balloon launches with Vaisala RS92 radiosonde take place as well as twice monthly additionally with cryogenic frost-point hygrometers (CFH; Vömel et al., 2007a). Lindenberg is the Lead Center for the GCOS Reference Upper-Air Network (GRUAN) of the World Meteorological Organization. All radiosonde data are processed with special GRUAN algorithms developed there (Immler et al., 2010; Dirksen et al., 2014). In addition to the routine ascents a dedicated balloon with an RS92, CFH, and an EnSci ECC ozone sonde was launched during the campaign to coincide with the Falcon overflight and the horizontal flight path of the aircraft was chosen to match the trajectory of the balloon (Fig. 2a).
The Zugspitze water-vapour DIAL is operated at the Schneefernerhaus
high-altitude research station (UFS; 47
On the basis of the comparison with the DLR DIAL a minor deficiency in the calculation of the spectral line wings could be detected and was corrected. The choice of spectral line parameters (Ponsardin and Browell, 1997) is justified by the excellent results (Sect. 3.6). A more recent comparison with the Zugspitze Fourier transform spectrometer confirmed this performance and revealed slight discrepancies for some 817 nm lines taken from the HITRAN (Rothman et al., 2009) data base (Vogelmann et al., 2011). Furthermore, in that study, a very high importance of volume matching in comparisons of water-vapour profiling instruments was found (see also Vogelmann et al., 2015).
In addition, in situ data from the monitoring station at the Zugspitze summit are used, namely ozone, carbon monoxide, and relative humidity. Ozone has been measured since 1978 (e.g. Reiter et al., 1987; Scheel et al., 1997; Oltmans et al., 2006, 2012; Logan et al., 2012; Parrish et al., 2012). Recently, ultraviolet absorption instruments have been employed (TE49 analysers, Thermoelectron, USA). Carbon monoxide was measured using vacuum resonance fluorescence (AL5001, AeroLaser, Germany). RH was registered with a dew-point mirror (Thygan VTP6, Meteolabor, Switzerland) with a quoted uncertainty below 5 % RH. However, the instrument has a wet bias of almost 10 % under very dry conditions (Trickl et al., 2014).
The tropospheric ozone lidar at Garmisch-Partenkirchen, Germany (IMK-IFU;
47
In all, 5-day forward trajectories are calculated for the time period from 01:00 CET on 8 October 2008, until 19:00 CET
on 15 October 2010 every 6 h based on the Lagrangian Analysis Tool
(LAGRANTO; Wernli and Davies, 1997; Sprenger and Wernli, 2015). The
three-dimensional wind fields for the calculation of the trajectories were
taken from ERA-Interim data set (Dee et al., 2011) from the ECMWF, which was interpolated to a
longitude–latitude grid with 1
The large set of 5-day trajectories was started in the entire region covering
the Atlantic Ocean and western Europe (20
LAGRANTO 5-day forward trajectories from the full set calculated
that fulfil both the deep-STT criterion and a passage above the blue line
along the 65
The intrusion was first detected in the routine forecast plot daily sent to
former STACCATO (Stohl et al., 2003) partner stations (Zanis et al., 2003).
Here, in Fig. 1 we give a revised version of that plot, now based on ECMWF
re-analysis meteorological data, and based on the all trajectories
calculated for the period between 8 October 2008, and 19:00 CET on 15 October 2008, fulfilling the criteria for deep stratosphere–troposphere
transport (STT) specified in Sect. 2.2. From these trajectories, Fig. 1 shows
those intersecting the 65
The intrusion arrived over central Europe following a frontal system with rain that passed over the eastern Alps to the south-east during the preceding night (not shown). A period of clear weather started, which was associated with the arrival of a high-pressure zone. The water-vapour images of the geostationary satellite METEOSAT show just moderate drying after the frontal passage. A slightly drier, hook-shaped feature arrived over northern Germany in the morning of October 16. It moved eastward to Poland until the following day. No indication of the intrusion is seen further to the south. However, these images are more representative for the upper troposphere. As will be shown below, the dry intrusion layer proceeded well hidden in a rather moist middle and lower troposphere.
The flight path of the DLR Falcon jet is marked in Fig. 1. Colour-coded summary plots of the measurements during the flight on 17 October 2008 are given in the panels of Fig. 2. The flight started at Oberpfaffenhofen (ICAO (International Civil Aviation Organization) code EDMO) at 16:42 CET. The aircraft turned to the west and climbed to about 11 km altitude. It first reached Payerne at 17:18 CET, then Bilthoven at 18:15 CET, Lindenberg at 19:03 CET, and finally Garmisch-Partenkirchen (Zugspitze) at 19:52 CET The data are most accurate in the upper troposphere, i.e. close to the aircraft, but are remarkably reliable even in the lower troposphere, where the lidar signal is much weaker and, thus, noisier. The relative noise level within the dry layer additionally grew whenever the water-vapour density above the intrusion was enhanced to an extent that much of the radiation was absorbed. The lower-tropospheric performance was, thus, the best over Lindenberg (see Fig. 2) and becomes evident from the comparisons that are shown in the Appendix, with one exception.
In the lower panel of Fig. 2 also the backscatter ratio for 1064 nm is given, i.e. the ratio of the total backscatter coefficient and the Rayleigh backscatter coefficient. Any value exceeding 1.0 means the presence of aerosol, and high values around the upper end of the scale can be attributed to clouds. The data gaps (white areas) are mostly associated with the presence of clouds at the top of the boundary layer or cirrus clouds and the corresponding light loss.
Quite importantly, slightly enhanced aerosol was retrieved in the upper half of the intrusion layer along the entire flight path. The most reasonable explanation of this observation would be a downward transport of some of the enhanced stratospheric aerosol after the violent eruptions of Okmok and Kasatochi (to 15 and 13.7 km, respectively; Massie, 2015) starting on 12 July 2008 and on 7 August 2008, respectively, which was also registered with the stratospheric aerosol lidar at Garmisch-Partenkirchen (Trickl et al., 2013), up to about 19 km in October 2008. More information on this remarkable observation can be found in some of the following sections.
During the hours of the LUAMI campaign Payerne was located close to the western edge of the intrusion layer (Fig. 1). Nevertheless, the time series of the Raman lidar (Fig. 3a) verifies the presence of a very dry layer between 2 and 3 km during the entire period displayed, starting at 13:45 CET. The driest period with mixing ratios of 35 to 65 ppm started at about 17:30 CET (Fig. 3b); 50 ppm is a typical value as found in the tropopause region (Trickl et al., 2014). The relative uncertainties of the minimum mixing ratios specified for the period before 17:00 CET are 7 to 19 %, after 17:00 CET 5 to 9 %.
The presence of stratospheric air is confirmed by the 13:00 CET ozone profile (Fig. 4) that exhibits a 76.2 ppb maximum at 3.2 km, residing on a background of roughly 50 ppb. It is interesting to note that the corresponding RH minimum is downward shifted by about 0.3 km. The midnight (00:00 UTC or 01:00 CET) RH minimum was 1 %, presumably a truncation value (Trickl et al., 2014). This low value is in agreement with the drier situation revealed by RALMO for the night.
The time series of the CAELI system is depicted in Fig. 5. The noise at early times is due to clouds passing over the lidar. Two dry layers are visible. However, the minimum mixing ratios are of the order of 500 ppm (Fig. 6), which is beyond typical values in the lowermost stratosphere. By contrast, the noon sonde measurement at De Bilt (KNMI) (Fig. 6), as in the case of Payerne, shows the typical low-humidity cut-off at 1 % RH (about 70 ppm). Even 70 ppm are, again, within the range of values frequently found just above the tropopause. It seems that at the time of the lidar measurements in Fig. 5, the driest part of the intrusion was already over. Around midnight, the intrusion layer had almost disappeared as can be concluded from the 24:30 CET sonde measurement.
The two lidar systems agree well in a range up to 8 km (Fig. 6). There are just a few exceptions outside the specified uncertainties most likely due to insufficient spatial matching, or far-field detection of the DLR lidar. The agreement with the sonde data is not satisfactory due to the considerable time differences, except for the range between 3.2 and 7 km in the midnight profile.
In addition, a profile from the ECMWF analysis is shown. Outside the dry layers the agreement is reasonable, but just one of the two layers seen in the measurements is indicated. Another example can be found in the Appendix (Payerne, Fig. A1).
During the campaign the lidar data of RAMSES were prepared as 10 min and 30 min averages. The time series of the 30 min data are shown in the upper panel of Fig. 7, also using some 10 min data next to the data gaps. The measurements were continued until 06:00 CET on 18 October. During the period displayed the intrusion layer became continually thinner. The data do not exhibit a single minimum of the mixing. In the lower panel of Fig. 7, we, therefore, show the minimum values for the two driest zones in the upper panel separately. The minimum mixing ratios retrieved are 120 ppm, which is, still, in some agreement with conditions inside the “mixing layer” of the tropopause region (Trickl et al., 2014), but clearly higher than the minima observed at the other sites. The relative uncertainties of the RAMSES mixing ratios specified in the vertical range around the intrusion are just a few per cent.
The ozone profile measured by the balloon payload launched at 18:44 CET is shown in Fig. 8. Quite interestingly, the highest ozone peak (75 ppb) was observed at the upper end of the dry layer at an altitude of about 5.5 km, although just 0.1 km above the RH minimum (5 %). This is in agreement with the idea that the ozone rise in the lowermost stratosphere of the Arctic source region was transferred to Lindenberg without major change, assuming low interference by tropospheric air during the transport (Trickl et al., 2014). A similar behaviour is indicated for Payerne in Fig. 4.
The ozone structure above 6 km is not clear. There is an obvious anti-correlation of ozone and RH indicating stratospheric influence. However, the elevated RH values could indicate mixing with tropospheric air.
Sonde ozone and relative-humidity profiles above Payerne on 17 October 2008; the times are launch times.
Water-vapour time series of CAELI on 17 October 2008 (Bilthoven); the time of the aircraft overflight (18:16 CET) is marked by a red vertical line. In the graph, only the data from the far-field receiver are shown (above 1.7 km). The profiles are shown at the full native resolution of 10 s and 7.5 m.
In Fig. 9a the colour-coded plot of the water-vapour mixing ratio, derived from the radiosonde ascents at Lindenberg between 14 and 23 October, is shown. The plot benefits from the 6 h intervals between the launches at Lindenberg, shorter than the conventional 12 h. If one neglects uncertainties due to the graphical procedure applied, there is a strong hint on a direct connection of the dry layer to the stratosphere during the first half of 17 October (Julian day 291) that is also indicated in the upper panel of Fig. 7. The transverse drift of the fold away from Lindenberg is confirmed by the model calculations (Sect. 3.7).
The aerosol backscatter coefficients derived from the 354.84 nm RAMSES measurements are rather noisy due to the very strong contribution from Rayleigh backscattering at this short wavelength. Nevertheless, a small spike (backscatter ratio 1.05) is seen in the profile next to the DLR overflight at 5.08 km (not shown), residing on a broader pedestal between 3.8 and 5.2 km. This structure is in good agreement with the WALES results (Fig. 2). The result of a 3-h average is shown further below (Sect. 3.6).
Comparison of the water-vapour mixing ratio from the CAELI Raman lidar (18:23 CET, i.e. 15 min average between 18:15 and 18:30) and the airborne lidar (18:15 CET) at Bilthoven; the mixing ratios from the routine noon and midnight measurement at De Bilt (station code 6260) are given for comparison. The noon profile reveals a much more pronounced stage of the intrusion than the lidar data. In addition, the humidity result from a high-resolution ECMWF analysis for the time of the aircraft arrival is shown, again just indicating the intrusion layers (one of the two). The times for the lidar systems refer to the middle of a measurement, for the sonde the launch time (LT) was taken.
On 17 October 2008 a total of five measurements with the water-vapour DIAL
at UFS were made between 16:55 and 20:55 CET. Figure 10 gives an overview of
the profiles. The data are given as number densities, which is the primary
quantity measured by DIAL systems (not requiring the additional use of sonde
data). During that time period the intrusion layer descended by about 0.6 km. The minimum densities ranged between
The noon and midnight RH profiles of the Munich (Oberschleißheim, WMO
station 10868, 100 km roughly to the north) sonde type RS92 extend the
range of descent over southern Bavaria to 3.9 km (thick red arrow in Fig. 10)
It is interesting to note that, despite uncertainties of the sonde results, the value of 1 % RH has been found to be quite typical in the routine analyses of STT events at Garmisch-Partenkirchen since 2007. This value is clearly dominating for low to moderate travel times. For subsidence times beyond 10 days, the RH minima may grow to 2–6 %. In the current study 1 % RH was consistently observed in the sonde data in the vicinity of all sites involved but Lindenberg, where also the lidar minima are slightly higher.
Ozone and RH profiles during the sonde ascent launched at Lindenberg at 18:44 CET; for the RH of the RS92 sonde the final GRUAN data product was taken.
Time series of the water-vapour mixing ratio over Lindenberg during the period between 14 (Julian day 288) and 23 (Julian day 297) October 2008; this figure was derived from radiosonde ascents at intervals of 6 h. The mean flight times (in UTC) are marked by arrows. The thermal tropopause is indicated by a black line. The intrusion examined in this study is visible on Julian days 291 (17 October) and 292. The graphics indicate the passage of a major part of the tropopause fold over Lindenberg.
The Zugspitze in situ measurements showed a drop in relative humidity right after the end of the lidar comparison in agreement with further descent of the dry layer (Fig. 11). The minimum half-hour average, 7.2 %, was not reached before 01:00 CET, which indicates considerable slowing of the subsidence. A pronounced ozone rise to more than 73.3 ppb was found that started 4 h later than the beginning of the humidity drop. Both the peak ozone value and the delay are in agreement with the findings for Payerne and Lindenberg where, within the intrusion layer, an ozone rise towards higher altitudes was found. This observation must be considered in future data-filtering efforts of the half-hour averages for quantifying the stratospheric fraction of the Zugspitze ozone as described by Trickl et al. (2010). Carbon monoxide stayed above 110 ppb, which is rather typical and, again, indicates that the air mass originates just in the tropopause region (Trickl et al., 2014). Fully stratospheric CO values are substantially lower.
Water-vapour profiles from the measurements of Zugspitze DIAL on 17 October 2008; the red arrow marks the vertical position of the RH minimum (1 %) from the noon “Munich” radiosonde, observed during the ascent at 11:56 CET. The grey dashed line marks a mixing ratio of 100 ppm as determined from the same sonde ascent. In addition, a density profile for 75 % RH is given for a crude comparison (Munich, 13:00 CET). The corresponding profile for the following midnight shows significantly higher RH below 4.5 km since the intrusion had subsided to 2.78 km and, therefore, is not included here.
Zugspitze in situ measurements of ozone, carbon monoxide and
relative humidity on 17–19 October 2008; the stratospheric layer is clearly
visible in the H
As in the 1064 nm measurements of WALES aerosol the UFS measurements show
aerosol in the upper half of the intrusion layer, with a peak 817.2 nm
backscatter coefficient of about
In addition, the 3 h average for Lindenberg around the overflight time is
inserted into the upper panel in grey colour. The curve is rescaled by
multiplying the values with 354.84/817.2 according to a
Right panels: intrusion trajectories (black lines) intersecting a vertical control surface (green line) above northern Canada at three different times; left panels: intersection points of the trajectories on the control surface (see text); the blue contour lines are isentropes (in K).
The 5-day trajectories were released and preselected for deep subsidence from
the lowermost stratosphere as described in Sect. 2.2. In the next step, cross
sections transverse to the flow were prepared at a number of locations
between Canada and the Alps. Examples for four of the locations are shown
here. The PV contours (in colour), isentropes (as blue contour lines), the
interpolation points of the individual trajectories closest in time (within
Right panel: intrusion trajectories (black lines) intersecting a vertical control surface (green line) above the west coast of Greenland at three different times; left panel: intersection points of the trajectories on the control surface (see text); the blue contour lines are isentropes (in K).
The temporal development of the tropopause and positions of the trajectories
as they cross a first vertical surface (along 80
At the location of the transverse surface of Fig. 13, the tropopause is only
slightly distorted toward lower altitudes, during the entire period covered.
The beginning of the trajectories selected by the deep-STT criterion stays
east of 100
In the next cross section farther downstream (50
The best coincidence with the next transverse surface at 30
The next surface was selected from 50
The trajectories in Fig. 15 pass east of Payerne. Those covering Payerne reach the coastal area 6–18 h earlier (not shown).
Right panels: intrusion trajectories (black lines) intersecting a vertical control surface (green line) along the coast of the Netherlands, Germany, and Poland at three different times; left panels: intersection points of the trajectories on the control surface (see text); the blue contour lines are isentropes (in K).
Finally, a cross section slightly north of the Alps (44.5
In Fig. 16 we give three examples of model calculations again for 12:00 to 24:00 UTC on 17 October. During this time the best overlap of the trajectories with Payerne is found, in agreement with the growing dryness observed during this period. Due to the cut-off towards the north-east (mentioned above) the trajectory dots do not reach the high-PV contours, which is the case for a longer control surface.
Right panels: intrusion trajectories (black lines) intersecting a vertical control surface (green line) north of the Alps at three different times; left panels: intersection points of the trajectories on the control surface (see text); the blue contour lines are isentropes (in K).
It is obvious that the trajectory dots in the cross sections downstream the intrusion exhibit a higher spread. To some extent this is ascribed to the higher temporal jitter and to additional stratospheric contributions from outside the main descending air stream. There is not a perfect matching of the dots with the vertical contour of the fold for the earlier times. Later, during the driest phase observed over Payerne (lower two panels), there is a better agreement of the central axes. However, the trajectories for the beginning of 18 October no longer horizontally overlap with the Swiss station as can been judged by comparing the green bars in the right panels of Fig. 16.
There is growing evidence that ozone injection from the stratosphere is very
likely a much stronger source of tropospheric ozone than frequently thought
(e.g. Roelofs and Lelieveld, 1997; Trickl et al., 2010, 2011, 2014).
However, a quantification of STT remains a difficult task. The results
presented in this paper, together with the findings of the preceding studies
(Trickl et al., 2014, 2015), are an important prerequisite on the way to
quantifying STT based on observational data alone, at least at a few
suitable stations: the low concentrations of water vapour found in most deep
stratospheric intrusions examined suggest that the intrusion layers reach
high-lying atmospheric observatories with rather little modification during
the transport. Thus, the long-term observations of ozone, RH and
As pointed out in the earlier paper (Trickl et al., 2014) the detection of low free-tropospheric mixing means a considerable challenge for atmospheric modelling, particularly for narrow layers. As demonstrated by Roelofs et al. (2003), a very high spatial resolution is required for obtaining reasonable trace-gas distributions. Limitations in Eulerian models are imposed by numerical diffusion (Rastigejev et al., 2010).
The LUAMI measurements on 17 October 2008 have made a thorough
comparison of different high-quality instruments for water-vapour sounding possible,
in particular the CFH sonde, differential-absorption, and Raman lidar
systems. The airborne lidar served as a transfer standard. With respect to
the intercomparison of the instruments, the following main conclusions can
be drawn:
Apart from a generally excellent mutual agreement of the systems a high
capability of determining very low humidity levels was verified such as
those needed in the current study. The RS92 radiosonde (e.g. Miloshevich et
al., 2006; Vömel et al., 2007b; Steinbrecht et al., 2008; Dirksen et
al., 2014) was verified to reproduce RH values around 1 % indicating a
capability of resolving even lower values. The ground-based lidar systems
were found to resolve significantly lower humidity in the deep stratospheric
air intrusions since these layers are measured at relatively short
distances. The campaign was to a major extent based on lidar measurements. Lidars are
ideal due to the important (Vogelmann et al., 2011, 2015) advantage of volume
matching and of producing dense time series. Since the airborne DIAL provided
information of the spatial structure of water vapour also the quality of the
balloon-borne instruments could be judged. At Lindenberg the spatial matching
of WALES and the balloon was particularly good since the aircraft flew along
the wind direction. The signal of Raman lidar systems (Payerne, Bilthoven, and Lindenberg) is
proportional to the H The water–vapour data of DIAL systems in dry layers are noisy at all times
since they are based on absorption measurements in a noisy backscatter
signal. Under very dry conditions the noise can lead to pointwise negative
humidity values. However, as concluded previously (Trickl et al., 2014), the
comparisons confirmed that also DIAL systems can rather reliably determine
low values: As found by Trickl et al. (2014), the minimum uncertainty of the
ground-based Zugspitze DIAL in dry layers in the lower free troposphere
under optimum conditions is of the order of 25 ppm (roughly The minimum water-vapour mixing ratios observed above most sites
participating were clearly below 100 ppm during the driest periods, the
lowest values having been about 35 ppm (Payerne) or less (Zugspitze). The
dryness above Payerne is a remarkable fact since this station was close to
westernmost edge of the intrusion. The low values harden the conclusions of
Trickl et al. (2014) that significant mixing of the stratospheric air during
the downward transport to 3–4 km takes only place if there is external
interference from nearby frontal systems or convection. Stratospheric air
layers can travel over very long distances without losing much of their
characteristics (Trickl et al., 2014, 2015). Sometimes they survive with
minor mixing even when travelling once around the globe (Trickl et al.,
2011). The formation of the tropopause fold has started significantly earlier and at
slightly higher altitudes than anticipated from the daily forecasts received
since autumn 2000 (Zanis et al., 2003). However, the success of the forecasts
(Trickl et al., 2010) could be due to the fact that a minimum start pressure
of 250 mbar (about 10.5 km) stays within the range of lowered tropopause
positions in the start region of the folding, even in summer. The trajectory bundles transversely propagating in the folds initially stay
rather narrow, narrower than the fold structure marked by the PV Downward motion occurs all along the path, initially faster on the west
side. The changes in trajectory density and position shifts of the trajectory
bundle qualitatively confirm the time periods of the driest parts of the
layer in the observations at the different sites.
With respect to the dynamics of the intrusion some interesting findings
could be found based on the measurements and the modelling study. In
particular, the observations, carried out in a rather wide region, together
with the model calculations have led to a thorough characterization of the
intrusion. The cross sections prepared with LAGRANTO trajectories nicely
show the development of the main intrusion layer from the source region in
arctic Canada on its way to the Alps. The main conclusions are
The LUAMI measurements of water vapour, ozone, and aerosol have indicated
another behaviour of descending stratospheric layers. As hypothesized by
Trickl et al. (2014) the ozone and aerosol distributions in the intrusion
layer is in agreement with the idea of a rather unperturbed transfer of the
vertical distribution of these species in the source region to Europe: An
increase of ozone from tropospheric values at the bottom of the layer to
elevated values at near the top of the intrusion was documented at three
stations, the location of the aerosol peak in the upper part for the entire
DLR flight. The straight air flow out of the lowermost stratosphere revealed
by the model calculations (transverse to the fold) confirms this idea. More
cases must be analysed to harden these findings.
As in the vast majority of the ozone observations with the lidar at
Garmisch-Partenkirchen the peak O
The aerosol seen in Figs. 2 and 12 in the upper half of the dry layer seems to reflect the behaviour of ozone, which increased backscatter coefficients towards the layer top. It is reasonable to assume that the lower volcanic peak was located just above the tropopause in a major part of the Northern Hemisphere. Stratospheric aerosol in intrusion layers has been rarely reported (e.g. Browell et al., 1987; Langford and Reid, 1998). We have seen indications in the ozone plus aerosol soundings at Garmisch-Partenkirchen in 2009 following the Sarychev eruption, or after a 1991 pyro-cumulonimbus in the Québec province of Canada (Carnuth et al., 2002; Fromm et al., 2010). STT has been seen as the most important removal mechanism in the mid-latitudes, limiting the stratospheric dwell time of aerosol in the mid-latitude stratosphere to 1 year and less (Trickl et al., 2013). As a consequence, also the stratospheric impact of boreal smoke plumes (e.g. Fromm et al., 2008, and Fig. 1 of Trickl et al., 2013) or particle formation from aircraft emissions at high cruising altitudes strongly diminishes within less than half a year.
In Figs. 6 and A1 (see below) high-resolution ECMWF profiles are presented.
These profiles were calculated for the entire flight track (Wirth et al.,
2009b). The ECMWF analysis shows roughly the same H
The data can be obtained from the authors of this paper.
We present in the following the results of the instrument comparisons that are not primarily relevant to the scientific discussion of this paper.
The DLR Falcon passed over Payerne at 17:18 CET, i.e. during the transition period towards the lowest water-vapour mixing ratios (Fig. 4). The comparison of the two lidar systems and the profile obtained from an extra RS92 ascent is shown in Fig. A1. The minimum mixing ratios from the DLR DIAL and from the sonde agree well, whereas the minimum for the Raman lidar is slightly higher. This deviation is outside the uncertainty specified for RALMO (12 ppm), but inside that of the WALES data (as high as 200 ppm due to the strong radiation loss in the moist layer above the intrusion). The RALMO profiles are not shown beyond 5 km due to a deteriorating performance caused by the background noise from residual daylight. For comparison, the uncertainty at the humidity minimum during the dark phase was 3 ppm.
In addition, a profile of the water-vapour mixing ratio from the ECWMF T7699L91 analyses is given (see Sect. 2.1.1.). The agreement outside the intrusion is rather good, but the intrusion is not only strongly underestimated but also vertically shifted in the model output.
In Fig. A2 the Lindenberg measurements around the time of the Falcon overflight are shown together with a profile from WALES. Two separate panels are given since the DLR profiles used for the comparison with RAMSES and the sondes slightly differ, the balloon horizontally propagating along the flight path. The comparisons are highly satisfactory. No systematic bias is found, and deviation clearly exceeding 5 % exist just in a few altitude ranges. In the intrusion layer the uncertainty of the WALES mixing ratio is 40 ppm (see lower panel of Fig. 7), i.e. much smaller than over Payerne due to less absorption. The RAMSES data are displayed for measurement times of 10 and 30 min. An improvement by the longer averaging is seen only above 8 km where the noise of the 10 min data is high.
Comparison of the water-vapour mixing ratio from the Raman and the airborne lidar, and from a sonde ascent at Payerne; in addition, the corresponding humidity result from an ECMWF analysis is given, which barely shows the intrusion layer. The times for the lidar systems (DLR: top of panel) refer to the middle of a measurement, for the sonde the launch time (LT) was taken.
Figure A3 shows comparisons between the UFS 817 nm DIAL and the DLR 935 nm DIAL WALES. For this comparison we kept the smoothing interval of the UFS DIAL rather low, dynamically (nonlinearly) growing from about 25 m at 3 km to about 125 m at 10 km (definition: VDI, 1999). Three WALES profiles are given for time intervals before, around and after the overflight of the mountain.
As one would expect from the co-ordinates the best agreement is found for
the second WALES profile. In the altitudes ranges up to 4.5 km (e.g. moving
spike at the concentration maximum) and between 5.3 and 7.3 km there is a
considerable change in density along the flight path. At 6.6 km the H
The uncertainty of the Zugspitze DIAL in the dry layer is of the order of
Comparison of Zugspitze (UFS) DIAL and WALES; three WALES density profiles are shown around the time of the overflight. The best agreement was found for the best matching of the co-ordinates. The UFS data were smoothed less than in Fig. 10. The density profiles for 100 % RH is given in dashed lines (Munich, 01:00 and 25:00 CET).
The authors thank H. P. Schmid for his interest and support. The Zugspitze in situ data were generated by H. E. Scheel, who passed away in June 2013 after unfortunate surgery. W. Steinbrecht provided high-resolution radiosonde data of the German Weather Service for Stuttgart and Munich. The great support by the UFS team is acknowledged. The development of the Zugspitze water-vapour DIAL has been funded by the Bavarian Ministry of Economics and German Bundesministerium für Bildung und Forschung within the programme Atmosphärenforschung 2000 (ATMOFAST project: Atmospheric Long-range Transport and its Impact on the Trace-gas Composition in the Free Troposphere over central Europe; ATMOFAST, 2005). The observations of volcanic aerosol at Garmisch-Partenkirchen (UFS) contribute to NDACC (Network of the Detection of Atmospheric Composition Change) and EARLINET (European Aerosol Research Lidar Network, currently partly founded by ACTRIS 2).The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: E. Gerasopoulos Reviewed by: two anonymous referees