ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-3-1633-2003 Evidence of impact of aviation on cirrus cloud formation Zerefos C. S. 1 Eleftheratos K. 1 Balis D. S. 2 Zanis P. 3 Tselioudis G. 4 Meleti C. 2 Laboratory of Climatology & Atmospheric Environment, Faculty of Geology, National & Kapodistrian University of Athens, Greece Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece Research Centre for Atmospheric Physics & Climatology, Academy of Athens, Greece Goddard Institute for Space Studies, National Aeronautics and Space Administration, USA 06 10 2003 3 5 1633 1644 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/3/1633/2003/acp-3-1633-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/3/1633/2003/acp-3-1633-2003.pdf

This work examines changes in cirrus cloud cover (CCC) in possible association with aviation activities at congested air corridors. The analysis is based on the latest version of the International Satellite Cloud Climatology Project D2 data set and covers the period 1984-1998. Over the studied areas, the effect of large-scale modes of natural climate variability such as ENSO, QBO and NAO as well as the possible influence of the tropopause variability, were first removed from the cloud data set in order to calculate long-term changes of observed cirrus cloudiness. The results show increasing trends in (CCC) between 1984 and 1998 over the high air traffic corridors of North America, North Atlantic and Europe. Of these upward trends, only in the summertime over the North Atlantic and only in the wintertime over North America are statistically significant (exceeding +2.0% per decade). Over adjacent locations with low air traffic, the calculated trends are statistically insignificant and in most cases negative both during winter and summer in the regions studied. These negative trends, over low air traffic regions, are consistent with the observed large scale negative trends seen in (CCC) over most of the northern middle latitudes and over the tropics. Moreover, further investigation of vertical velocities over high and low air traffic regions provide evidence that the trends of opposite signs in (CCC) over these regions, do not seem to be caused by different trends in dynamics. It is also shown that the longitudinal distribution of decadal changes in (CCC) along the latitude belt centered at the North Atlantic air corridor, parallels the spatial distribution of fuel consumption from highflying air traffic, providing an independent test of possible impact of aviation on contrail cirrus formation. The correlation between the fuel consumption and the longitudinal variability of (CCC) is significant (+0.7) over the middle latitudes but not over the tropics. This could be explained by the fact that over the tropics the variability of (CCC) is dominated by dynamics while at middle latitudes microphysics explain most of its variability. Results from this study are compared with other studies and for different periods of records and it appears that there exists general agreement as to the evidence of a possible aviation effect on high cloud positive trends over regions with congested air traffic.</p>