Abstract
Abstract. 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 Geophysica, 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 Hector 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 Falcon, 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.
Highlights
Contrails are aircraft-induced cirrus clouds forming at low ambient temperature
The in situ data together with data from an upward-looking lidar on the German research aircraft “Falcon”, the CPOL radar near Darwin, and NOAAAVHRR satellites provide a sufficiently complete picture to distinguish between contrail and convective cirrus parts
The fresh convective clouds in the lower-right corner are the remains from Hector, which otherwise is to the right and in the back of the photo plane
Summary
Contrails are aircraft-induced cirrus clouds forming at low ambient temperature. Much has been learned about contrails from measurements behind commercial aircraft, typically at altitudes between 8 and 12 km and temperatures betweenPublished by Copernicus Publications on behalf of the European Geosciences Union.U. Contrails are aircraft-induced cirrus clouds forming at low ambient temperature. The water droplets freeze quickly and grow by uptake of ambient humidity in ice-supersaturated air (Kärcher et al, 1996). Contrails are generally expected to survive several minutes, essentially the wake vortex phase, only as long as ambient humidity exceeds ice saturation (Paoli and Shariff, 2016; Unterstrasser, 2016), though contrails have often been observed below ice saturation (Kübbeler et al, 2011; Jeßberger et al, 2013). If contrails persist long for icesubsaturated conditions, the contrail coverage could be larger than expected (Schumann et al, 2015; Bock and Burkhardt, 2016; Chen and Gettelman, 2016)
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