Northern Hemisphere contrail properties derived from Terra and Aqua MODIS data for 2006 and 2012
2018
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.
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