Abstract
Abstract. Linear contrail coverage, optical property, and radiative forcing data over the Northern Hemisphere (NH) are derived from a year (2012) of Terra and Aqua Moderate-resolution Imaging Spectroradiometer (MODIS) imagery and compared with previously published 2006 results (Duda et al., 2013; Bedka et al., 2013; Spangenberg et al., 2013) using a consistent retrieval methodology. Differences in the observed Terra-minus-Aqua screened contrail coverage and patterns in the 2012 annual-mean air traffic estimated with respect to satellite overpass time suggest that most contrails detected by the contrail detection algorithm (CDA) form approximately 2 h before overpass time. The 2012 screened NH contrail coverage (Mask B) shows a relative 3 % increase compared to 2006 data for Terra and increases by almost 7 % for Aqua, although the differences are not expected to be statistically significant. A new post-processing algorithm added to the contrail mask processing estimated that the total contrail cirrus coverage visible in the MODIS imagery may be 3 to 4 times larger than the linear contrail coverage detected by the CDA. This estimate is similar in magnitude to the spreading factor estimated by Minnis et al. (2013). Contrail property retrievals of the 2012 data indicate that both contrail optical depth and contrail effective diameter decreased approximately 10 % between 2006 and 2012. The decreases may be attributed to better background cloudiness characterization, changes in the waypoint screening, or changes in contrail temperature. The total mean contrail radiative forcings (TCRFs) for all 2012 Terra observations were −6.3, 14.3, and 8.0 mW m−2 for the shortwave (SWCRF), longwave (LWCRF), and net forcings, respectively. These values are approximately 20 % less than the corresponding 2006 Terra estimates. The decline in TCRF results from the decrease in normalized CRF, partially offset by the 3 % increase in overall contrail coverage in 2012. The TCRFs for 2012 Aqua are similar, −6.4, 15.5, and 9.0 mW m−2 for shortwave, longwave, and net radiative forcing. The strong correlation between the relative changes in both total SWCRF and LWCRF between 2006 and 2012 and the corresponding relative changes in screened contrail coverage over each air traffic region suggests that regional changes in TCRF from year to year are dominated by year-to-year changes in contrail coverage over each area.
Highlights
Persistent linear contrails are aircraft-generated clouds that can form in ice-supersaturated zones of the upper troposphere and add to the naturally occurring cirrus coverage in air traffic regions
The results presented here demonstrate that interannual changes in air traffic density and potential persistent contrail frequency (PPCF) appear to have some influence on the change in satellite-detected contrail coverage (CC) between 2006 and 2012, some of the changes between the two years are more difficult to explain, especially the large increase in CC over the North Atlantic air corridor
The 2012 data show large changes in air traffic distribution across the Atlantic Ocean, including a 60 % decrease in air traffic between Europe and Latin America compared to 2006 that is not reflected in the unscreened contrail coverage
Summary
Persistent linear contrails are aircraft-generated clouds that can form in ice-supersaturated zones of the upper troposphere and add to the naturally occurring cirrus coverage in air traffic regions. As air traffic has increased, studies (e.g., Minnis et al, 2004; Eleftheratos et al, 2016) have observed increases in cirrus coverage in air traffic corridors, prompting research into the possible impacts of aviation on climate. Several studies have used satellite remote sensing to quantify linear contrail coverage (CC) and to determine the optical properties of these clouds. Meyer et al (2002) derived CC from AVHRR data over the same region and later over Southeast Asia (Meyer et al, 2007). Several studies have used satellite remote sensing to quantify linear contrail coverage (CC) and to determine the optical properties of these clouds. Mannstein et al (1999) developed an automated contrail detection algorithm (CDA) to detect linear contrails from Advanced Very High Resolution Radiometer (AVHRR) imagery and estimated CC over western Europe. Meyer et al (2002) derived CC from AVHRR data over the same region and later over Southeast Asia (Meyer et al, 2007). Minnis et al (2005) used the Published by Copernicus Publications on behalf of the European Geosciences Union
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