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Impact of Horizontal Temperature Gradient on CFC-11 Fields Retrieved from MIPAS LIMB Sounding: One-Dimensional Approach Versus Two-Dimensional Tomography

Enrico Arnone(1), Michael Kiefer(2), Massimo Carlotti(1), Elisa Castelli(3), Thomas von Clarmann(2), Bianca Maria Dinelli(3), Anu Dudhia(4), Michael Hoepfner(2), Sylvia Kellmann(2), Andrea Linden(2), Enzo Papandrea(1), Marco Ridolfi(1) and Gabriele Stiller(2)

(1) University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
(2) Institut fuer Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Karlsruhe, Germany
(3) CNR, Via Gobetti, 101, 40129 Bologna, Italy
(4) University of Oxford, Clarendon Laboratory Parks Road, OX1 3PU Oxford, United Kingdom

Abstract

Chlorofluorocarbons (CFCs) have been widely studied because of their action in depleting ozone and as greenhouse gases. While the CFC load in the troposphere is constantly declining since the mid-1990s, CFC concentrations in the stratosphere are close to their peak value, making temporally and spatially detailed observations essential both to discern their long term trends from short term variability, and to quantify their impact on ozone.

In this paper we have investigated the impact of correctly modelling the horizontal temperature variability in the retrieval of CFC-11 from limb sounding measurements of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the European Space Agency (ESA) ENVISAT satellite. As the satellite proceeds along its polar orbit, backward looking limb infrared measurements are taken by the MIPAS instrument guaranteeing continuous coverage during the dayside descending (North Pole to South Pole) and nightside ascending (South Pole to North Pole) parts of the orbit. Measurements of CFC-11 volume mixing ratio (VMR) were obtained with both a one-dimensional approach and a two-dimensional tomographic approach: the former assumes a horizontally homogeneous temperature structure within the atmospheric parcel spanned by each analyzed limb-scan while the latter models the horizontal structures of the atmosphere.

Evidence is found for a bias varying with latitude and season and reaching maximum values of 20% in the descending-ascending difference of the retrieved CFC-11 VMR in the one-dimensional retrieval which is not seen in the two-dimensional tomographic retrieval. This bias is largely explained by the impact of the temperature gradients encountered by the instrument during the two parts of the orbit. Although the atmospheric temperature structure is fairly similar along the day side and night side, during the ascending part of the orbit the observed horizontal temperature gradients along the instrument's line of sight reverse with respect to those seen in the descending part. As the temperature field of the atmosphere evolves with the season, the bias observed in the retrieved CFC-11 VMR maximises in coincidence with the regions of stronger temperature gradients and reverses with annual cycle. A bias is also observed in other molecules such as HNO3 and is under investigation in other targets. By simultanously retrieving MIPAS measurements from a complete orbit the tomographic approach can correctly account for the different horizontal temperature gradients observed by the instrument. A correction strategy to be applied to the one-dimensional retrieval is investigated.

 

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