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Contenu archivé le 2024-05-14

A lagrangian experiment in the Arctic vortex


Although the ozone depletion mechanism in polar areas is generally understood, 3 D model simulations of cumulative loss in the winter/spring season in the Arctic, essential for forecasting what could happen in the future, fail to capture the loss rate and thus the amplitude of the depletion. The reason for this resides in an accumulation of uncertainties such as in difference between the real atmospheric temperature compared to that simulated in the Global Circulation Models (GCM); in the nature of PSCs formed at a given temperature to which the efficiency of heterogeneous reactions is extremely sensitive; and finally in photochemical reactions and photolysis rates.
The Lagrangian experiment is to perform a variety of measurements on-board 3 constant level (superpressure) and 2 vertical excursion (IR Montgolfier) long duration balloons together in the polar vortex in the winter of 1999 and to compare the results to ECMWF and UKMO analysis as well state of the art, transport, radiative transfer, microphysical and photochemical simulations.
Temperature, pressure, altitude, up and down welling infrared radiation, in situ PSC, ozone and water vapour, along the trajectory of the superpressure balloons and the same plus 03, N02, OC10, BrO, CH4 and H20 profiles from the Montgolfier, will allow to address a series of mechanisms extremely difficult to study by conventional means:
i)the possible temperature and wind bias in the GCM (ECMWF and UKMO) models,particularly away from radiosonde stations, on which all photochemicalsimulations are based;
ii) the amplitude of the temperature fluctuation under the influence oforographic and gravity waves compared to the mean, an extremely importantpoint since the PSC formation is controlled by the lowest temperature andnot by the mean at a grid point of the model;
iii) the average amplitude and the geographical fluctuation of the radiativecooling of the stratosphere during the polar night, the key of theozone-climate relationship, by the measurement of up and down-welling IRfluxes along the balloon trajectory and during their vertical excursions;
iv) the related diabatic sink in the vortex by the measurement of theconcentration of tracers (CH4 and H20);
v)the temperature / PSC relationship along the trajectory of the balloons(condensation, vaporisation, hysteresis ?) by the measurement of in-situbackscatter ratio, depolarisation factor (liquid or solid) and watervapour concentration;
vi) the concentration change of key constituents (ozone, N02, OC10 and BrO)along the balloon trajectory in relation with the above parameters andthe duration of sunlight exposure;
vii) the accuracy of GCM, radiative, microphysical and box and CTMphotochemical models in capturing the above mechanisms.

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