Long-lived contrails and convective cirrus above the tropical tropopause
This study has two objectives: (1) it characterizes contrails at very low temperatures and (2) it discusses convective cirrus in which the contrails occurred. (1) Long-lived contrails and cirrus from overshooting convection are investigated above the tropical tropopause at low temperatures down to −...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/2311/2017/acp-17-2311-2017.pdf |
| Summary: | This study has two objectives: (1) it characterizes contrails at very low
temperatures and (2) it discusses convective cirrus in which the contrails
occurred. (1) Long-lived contrails and cirrus from overshooting
convection are investigated above the tropical tropopause at low
temperatures down to −88 °C from measurements with the Russian
high-altitude research aircraft M-55 <q>Geophysica</q>, as well as related observations
during the SCOUT-O3 field experiment near Darwin, Australia, in 2005. A
contrail was observed to persist below ice saturation at low temperatures
and low turbulence in the stratosphere for nearly 1 h. The contrail
occurred downwind of the decaying convective system <q>Hector</q> of 16
November 2005. The upper part of the contrail formed at 19 km altitude in
the tropical lower stratosphere at ∼ 60 % relative humidity over ice
at −82 °C. The ∼ 1 h lifetime is explained by engine water
emissions, slightly enhanced humidity from Hector, low temperature, low
turbulence, and possibly nitric acid hydrate formation. The long persistence
suggests large contrail coverage in case of a potential future increase of
air traffic in the lower stratosphere. (2) Cirrus observed above the strongly
convective Hector cloud on 30 November 2005 was previously interpreted as
cirrus from overshooting convection. Here we show that parts of the cirrus
were caused by contrails or are mixtures of convective and contrail cirrus.
The in situ data together with data from an upward-looking lidar on the
German research aircraft <q>Falcon</q>, the CPOL radar near Darwin, and
NOAA-AVHRR satellites provide a sufficiently complete picture to distinguish
between contrail and convective cirrus parts. Plume positions are estimated
based on measured or analyzed wind and parameterized wake vortex descent.
Most of the non-volatile aerosol measured over Hector is traceable to
aircraft emissions. Exhaust emission indices are derived from a self-match
experiment of the Geophysica in the polar stratosphere in 2010. The number
of ice particles in the contrails is less than 1 % of the number of
non-volatile aerosol particles, possibly because of sublimation losses and
undetected very small ice particles. The radar data show that the ice water
content in convective overshoots is far higher than measured along the
flight path. These findings add insight into overshooting convection and are
of relevance with respect to hydration of the lower stratosphere. |
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| ISSN: | 1680-7316 1680-7324 |