Atmos. Chem. Phys. Discuss., 3, 3335–3359, 2003 Atmospheric ACPD www.atmos-chem-phys.org/acpd/3/3335/ Chemistry © European Geosciences Union 2003 and Physics 3, 3335–3359, 2003 Discussions Evidence of impact of aviation on cirrus cloud formation C. S. Zerefos et al. Evidence of impact of aviation on cirrus cloud formation Title Page Abstract Introduction C. S. Zerefos1, K. Eleftheratos1, D. S. Balis2, P. Zanis3, G. Tselioudis4, and C. Meleti2 Conclusions References 1Laboratory of Climatology & Atmospheric Environment, Faculty of Geology, National & Tables Figures Kapodistrian University of Athens, Greece 2 Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece J I 3Research Center for Atmospheric Physics & Climatology, Academy of Athens, Greece 4Goddard Institute for Space Studies, National Aeronautics and Space Administration, USA J I Received: 28 May 2003 – Accepted: 23 June 2003 – Published: 30 June 2003 Back Close Correspondence to: C. S. Zerefos ([email protected]) Full Screen / Esc Print Version Interactive Discussion © EGU 2003 3335 Abstract ACPD This work examines changes in cirrus cloud cover in possible association with avia- 3, 3335–3359, 2003 tion 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 Evidence of impact of 5 1984- 1998. Over areas with heavy air traffic, the effect of large-scale modes of natural aviation on cirrus climate variability such as ENSO, QBO and NAO as well as the possible influence of cloud formation the tropopause variability, were first removed from the cloud data set in order to cal- culate long-term changes of observed cirrus cloudiness. The results show increasing C. S. Zerefos et al. trends in cirrus cloud coverage, between 1984 and 1998, over the high air traffic corri- 10 dors of North America, North Atlantic and Europe, which in the summertime only over the North Atlantic are statistically significant at the 99.5% confidence level (2.6% per Title Page decade). In wintertime however, statistically significant changes at the 95% confidence Abstract Introduction level are found over North America, amounting to +2.1% per decade. Statistically sig- nificant increases at the 95% confidence level are also found for the annual mean cirrus Conclusions References 15 cloud coverage over the North Atlantic air corridor (1.2% per decade). Over adjacent locations with lower air traffic, the calculated trends are statistically insignificant and in Tables Figures most cases negative both during winter and summer in regions studied. Moreover, it is shown that the longitudinal distribution of decadal changes in cirrus cloudiness along J I the latitude belt centered at the North Atlantic air corridor, parallels the spatial distri- J I 20 bution of fuel consumption from highflying air traffic, providing an independent test of possible impact of aviation on contrail cirrus formation. Results from this study are com- Back Close pared with other studies and different periods of records and it appears as evidenced Full Screen / Esc in this and in earlier studies that there exists general agreement on the aviation effect on high cloud trends. Print Version Interactive Discussion © EGU 2003 3336 1. Introduction ACPD Cirrus clouds are the most common form of high-level clouds forming in the vicinity of 3, 3335–3359, 2003 the tropopause. In the tropics, their formation is mainly triggered by deep convection (Jensen et al., 1996); in the northern middle latitudes, they are associated to the ex- Evidence of impact of 5 istence of large ice supersaturated regions in the upper troposphere (Gierens et al., aviation on cirrus 2000). Cirrus cloud formation depends strongly on the existence of small particles cloud formation that provide the surface on which ice crystals nucleate (Jensen and Toon, 1997). Cir- rus clouds have alternately two opposite effects; an IR greenhouse effect and a solar C. S. Zerefos et al. albedo effect. Which effect dominates, depends strongly on optical properties. Thin 10 cirrus clouds usually produce a positive radiative forcing (RF) at the top of the atmo- sphere; thick cirrus clouds may cause cooling (Stephens and Webster, 1983; Fu and Title Page Liou, 1993; Fahey and Schumann, 1999). Abstract Introduction Contrails are visible line-shaped clouds that form in the wake of aircraft, when the relative humidity in the plume of exhaust gases mixing with ambient air temporarily Conclusions References 15 reaches liquid saturation, so that liquid droplets form on cloud condensation nuclei and soon freeze to ice particles (Schumann, 2002). Parameters determining contrail for- Tables Figures mation and persistence are analytically described in Schumann (1996). Contrails may evaporate quickly when the ambient air is dry, or persist for hours when the ambient J I air is substantially supersaturated with respect to ice (Jensen et al., 1998). Ice super- J I 20 saturated regions in the upper troposphere of the northern middle latitudes are very common (Gierens et al., 1999; Jensen et al., 2001). These regions and the number Back Close of aircraft flights in these regions control the area of the Earth covered by persistent Full Screen / Esc contrails (Fahey and Schumann, 1999). The influence of contrails on the radiation budget depends mainly on their coverage Print Version 25 and optical depth (Meyer et al., 2002; Schumann, 2002). Like thin layers of cirrus clouds, contrails usually produce a small heating at the top of the atmosphere. Re- Interactive Discussion gional and global estimates of contrail RF can be found in numerous studies (Meerkoet- ter et al., 1999; Minnis et al., 1999; Marquart and Mayer 2002; Meyer et al., 2002; © EGU 2003 3337 Ponater et al., 2002). IPCC follows Fahey and Schumann (1999) and retains a range ACPD of 5 to 60 mW/m2 for the present day contrail RF, around the best estimate of 20 mW/m2 (IPCC, 2001). 3, 3335–3359, 2003 Contrail occurrence and coverage have been observed using satellite and ground- 5 based observations. The mean coverage of line-shaped contrails is greatest over the Evidence of impact of USA, over Europe and over the North Atlantic (Fahey and Schumann, 1999). Sur- aviation on cirrus face observations of contrail occurrence from 17 bases across the continental USA cloud formation indicated that contrail occurrence varies substantially with location and season. Most contrails prevailed in winter and least during the summer with a pronounced minimum C. S. Zerefos et al. 10 in September (Minnis et al., 2003). Bakan et al. (1994), using NOAA/AVHRR infrared images, found a mean contrail cover of 0.5% over the eastern Atlantic/western Europe and showed that the highest contrail coverage (>2%) lies along the North Atlantic air Title Page routes during the summertime. Sausen et al. (1998) calculated a global contrail cover Abstract Introduction value of 0.09%, with maximum values exceeding 5% over certain regions in USA. Over 15 the USA and Europe, larger values occurred in winter than in summer. Calculations Conclusions References of the seasonal variability in contrail coverage over the US by Ponater et al. (2002) Tables Figures were found to be in agreement with previous studies (Minnis et al., 1997; Sausen et al., 1998). Over eastern Atlantic/western Europe they simulated a higher peak value in the summer compared to the winter, which was consistent to the results by Bakan et J I 20 al. (1994) but in contradiction to the findings of Meyer et al. (2002) and of Sausen et J I al. (1998). Few studies have examined trends in cirrus cloudiness. Wylie and Menzel (1999), Back Close showed evidence of a gradual increase in the occurrence of high clouds in the north- Full Screen / Esc ern middle latitudes (0.5% yr−1), while the change in the southern middle latitudes was 25 considered insignificant due to the very small cloud coverage. Boucher (1999) calcu- Print Version lated seasonal and annual mean global and regional trends in cirrus occurrence over land and ocean for the period 1982–1991. The highest increases in annual mean cir- Interactive Discussion rus occurrence were observed over the main air flight corridors (i.e. 13.3% per decade over the northeastern USA and 7.1% per decade over the North Atlantic air corridor). © EGU 2003 3338 These trends were attributed to condensation trail formation by aircraft exhaust gases, ACPD excluding the contribution of other possible causes such as the effects of the El Chi- chon and Mount Pinatubo volcanic aerosols, or long-term changes in relative humidity 3, 3335–3359, 2003 and climate variations related to the North Atlantic Oscillation. Minnis et al. (2001), cal- 5 culated trends in high cloud cover over air traffic and non-air traffic regions and found a Evidence of impact of statistically significant increase in high cloud coverage over areas where air traffic was aviation on cirrus heaviest relative to other regions, concluding that air traffic could cause an increase cloud formation in cloud cover. Trends in cirrus-high level clouds over regions with contrail coverage greater than or less than 0.5% were summarized in Fahey and Schumann (1999) (their C. S. Zerefos et al. 10 Table 3-5). Both satellite data and surface observations showed larger increases in cirrus-high level cloudiness in regions where contrails were expected to occur more frequently than in all other areas. Title Page Although satellite and surface-based observations of seasonal and decadal changes Abstract Introduction in cirrus frequency of occurrence over main air traffic regions suggested a possible rela- 15 tionship between air traffic and cirrus formation, Fahey and Schumann (1999) stressed Conclusions References the difficulty to attribute observed changes in cirrus coverage to aircraft emissions. The Tables Figures aim of the present work is to study changes in cirrus cloud cover over locations near and remote from air corridors and to compare them with earlier results with respect to evidence of effects from aviation.