Figure 2 : Map of the five scientific flights performed by the M-55 Geaophysica from the base of Ushuaia (Argentina) during the APE-GAIA mission
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4. The Antarctic flights
The scientific planning of the flights was first focused on the scientific objectives of the campaign listed in paragraph 2. Individual flight plans were then finalised in the field using detailed forecasts. These forecasts includes standard output from global numerical weather prediction models, as well as high resolution trajectory/contour advection models (to identify filaments), lee wave models (to identify mountain wave PSCs), 'both provided by the University of L'Aquila (Italy), and global chemistry transport models provided by the University of Leeds (UK). A map of the five scientific flights performed by the M55 aircraft over the Antarctic peninsula is shown in Fig. 2. Further details about the flight profiles are given in Table 2, where the specific objective of the flight is indicated along with the conditions or events that triggered the final choice of the route and of the timing.
Figure 2 : Map of the five scientific flights performed by the M-55 Geaophysica from the base of Ushuaia (Argentina) during the APE-GAIA mission
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990921 (daytime) |
SW ? Multi legs ? 2 altitude levels + dive (1st leg over 380 K isentropic surface) |
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Position of vortex edge: isentropic PV forecast maps |
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990923 (daytime) |
SE ? 2 legs ? 2 altitude levels + dive (1st leg over 420 K isentropic surface) |
Across vortex edge Chemistry of polar air |
Position of vortex edge: isentropic PV forecast maps |
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991002 (nighttime) |
S-NE-NW ? Triangle + dive (1st leg over 400 K isentropic surface) |
Lee-waves induced PSCs over the Antarctic peninsula |
Lee waves alert: MM5 mesoscale fields |
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991008 (nighttime) |
SE ? 4 legs ? 2 isentropic + dive (1st leg over 380 K isentropic surface (2nd leg over 420 K isentropic surface) |
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Forecast of filament: Hi. Res. DFT PV maps |
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991012 (daytime) |
S ? 2 legs ? 2 altitude levels + dive (1st leg over 410 K isentropic surface) |
Across vortex edge Chemistry of polar air |
Position of vortex edge: isentropic PV forecast maps |
Table 2 : Details about flight profiles and scientific objectives of the Antarctic flights
False colour images of the isentropic Potential Vorticity (PV) over the 400 K isentropic level (approximately 16 km) for the days of the APE-GAIA flights (at 12.00 GMT) are reported in Fig. 3, to locate the position of the vortex during the campaign period..
Figura 3 : Potential Vorticity maps over 400 K isentropic surface
4-1 Flight across the polar vortex boundary
The objective of the first flight (990921) was to obtain measurements throughout the tracer gradient which defines the boundary of the vortex, and to travel as deep as possible inside the vortex. This was also the task of the second flight (990923) which provided a further sampling of the polar vortex at slightly higher altitudes, as well as daytime observations of photo-chemistry activation inside the vortex, and of the last flight performed at a later time (991012): in this way, information on the chemistry of the polar lower stratosphere and the mixing between polar vortex and mid-latitude air was obtained both at the beginning of the campaign, during on-going ozone depletion, and at its end during the recovery phase.
An example of observations made by the in-situ instruments during the flight of 23rd September 1999 is shown in fig.4. Most species were observed by a single instrument, but in the case of ozone (bottom plot) two independent measurements are superimposed and show very good agreement. Ozone, water vapour and most tracers have very similar variations both in the general shape of the curves as well in some of the small features. The event of a rapid drop of concentrations synchronously observed for most species can be considered to identify the crossing of the vortex edge. Along with in-situ measurements, in the same flight, the limb sounding observations performed by MIPAS-STR and SAFIRE-A allowed to determine the 2-D distributions of different stratospheric compounds with a horizontal resolution of around 40 km (MIPAS-STR) and 100 km (SAFIRE-A) and with a vertical resolution of 2-3 km below the aircraft and column amounts above. As an example, colour contour plots of Ozone and ClO, as measured by SAFIRE-A during the 990923 flight is shown in Fig. 5, showing the enhancement of chlorine monoxide in daytime conditions inside the polar vortex and a clear anti-correlation with ozone mixing ratio values.
Figure 4: Altitude, latitude and in-situ measurements of minor constituents acquired during the flight of 23rd September 1999
Figura 5: 2D distributions of Ozone (upper panel) and ClO (lower panel) volume mixing ratio (in ppmv) measured by SAFIRE-A as a funtion of altitude and time (UTC) during the 990923 flight.
4.2 Detection of a large PSC event
On the 2nd of October the M55-Geophysica flew from Ushuaia toward the Palmer peninsula, following a three legs trajectory, with the second leg along the eastward branch of the peninsula, where MM5 PSU/NCAR mesoscale model forecast predicted lee-wave formation.
The temperature experienced on the second leg of the flight, with the aircraft cruising at altitude of 18-19 km for about 800 km, was below Tice. Both data recorded by the MAS laser backscattersonde and by the MAL microjoule lidar show clear evidence of a large PSC event in this region. The vertical structure of the cloud was recorded by the zenith channel of MAL. Two towering clouds can be discerned in fig.6, showing respectively the backscatter ratio and depolarisation ratio observed by the lidar, versus time. The min. and max. values in the temperature fields from the mesoscale MM5 run seems to be well anti-correlated with min. and max. of the backscatter values. The backscatter vs depolarisation scatterplot (fig. 7) is consistent with the hypothesis of a type II PSC.
Figure 6: Profiles of backscatter ratio observed by the microjoule lidar MAL above the flight level, superimposed to the temperature fields from global ECMWF analysis
Figure 7: Scatterplot of backscatter versus depolarisation ratio, showing both MAS (red triangles) and MAL (circles) datasets
4.3 Flight across a filament
The fourth scientific flight aimed at studying the physical and chemical conditions within and without a filament predicted from high resolution PV fields for the 8th of October 1999 between Tierra del Fuego and the Antarctic Peninsula at an altitude between 14 and 18 km.. Signature of the filament was already clearly present in the PV forecasted products since 96 hours in advance. To verify and study the occurrence of this structure a flight was planned, with a four legs track over two isentropic surfaces: 390K and 450K.
As forecasted, while flying quasi isentropically at 450 K, both in-situ and remote-sensing instruments detected an intense and localised variation of the measured tracers, that is supposedly related with the sampling of the vortex filament structures along the flight track. Also, high resolution PV products (not shown here) picture a three-dimensional filament with a strong horizontal gradient, a deep vertical shape and a high resolution internal structure that seems to fairly match with the preliminary analysis of the time series of experimental data. Further analyses, involving the complete set of tracers data available from onboard instrumentation, is needed to have a more complete understanding of this case.
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