Extinction and optical depth of contrails

Abstract

[1] One factor limiting the understanding of the climate impact from contrails and aircraft induced cloud modifications is the accurate determination of their optical depth. To this end, 14 contrails were sampled for 2756 s with instruments onboard the research aircraft Falcon during the CONCERT (CONtrail and Cirrus ExpeRimenT) campaign in November 2008. The young (<10 min old) contrails were produced by 9 commercial aircraft with weights of 47 to 508 t, among them the largest operating passenger aircraft, the Airbus A380. The contrails were observed at temperatures between 214 and 224 K and altitudes between 8.8 and 11.1 km. The measured mean in-contrail relative humidity with respect to ice was 89 ± 12%. Six contrails were observed in cloud free air, the others were embedded in thin cirrus clouds. The observed contrails exhibited a mean ice water content of 2 mg m −3 and had a mean number concentration of 117 cm −3 and effective radius of 2.9 mm assuming asphericle particles with an aspect ratio of 0.5. Probability density functions of the extinction, with a mean (median) of 1.2 (0.7) km −1 , and of the optical depth t, with a mean (median) of 0.27 (0.13), are derived from the in situ measurements and are likely representative for young contrails from the present-day commercial aircraft fleet at observation conditions. Radiative transfer estimates using the in-situ measured contrail optical depth lead to a year-2005 estimate of line-shaped contrail radiative forcing of 15.9 mWm −2 with an uncertainty range of 11.1-47.7 mWm −2 . Citation:

A contrail cirrus prediction model

“…Figure 10 shows that the model computes optical depth values within the range 0.001-0.5 in most cases, with larger values for young contrails. This is fully consistent with observations and other models (Kärcher et al, 2009a;Atlas and Wang, 2010;Voigt et al, 2011). However, with respect to climate impact, it is the product of τ with the contrail width B which counts.…”

“…Contrails in subsaturated air masses reach small cover and, hence, contribute little to radiative forcing (Ponater et al, 1996). Nevertheless, such contrails may be important when comparing contrail properties to observations (Voigt et al, 2011). Also, soot properties may possibly get changed when processed in contrails (Lee et al, 2010).…”

“…3. This figure contains previously published results and, in addition, the result computed with the present approximation for one of the largest commercial aircraft (A380) as observed during the CONCERT campaign (Voigt et al, 2010(Voigt et al, , 2011 The initial sinking from state "0" to "1" (Fig. 1) is set to…”

Contrail life cycle and properties from 1 year of MSG/SEVIRI rapid-scan images

“…Minnis et al (2013) closely analysed 11 contrail outbreaks using Terra and Aqua MODIS data and derived τ values of contrail cirrus ranging from 0.32 to 0.43 and τ values of linear contrails between 0.13 and 0.17. The in situ measurements of τ taken by Voigt et al (2011) are also consistent with our findings. Further methods of deriving optical thickness show smaller average values: the model simulations by (Kaercher et al, 2010) yield mean values typically around 0.1 due to the amount of sub-visible contrails considered.…”

Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin

“…The CAS-DPOL measures particle size distributions between 0.5 and 50 µm at a 1 Hz time resolution (Baumgardner et al, 2011;Voigt et al, 2010Voigt et al, , 2011. Number concentrations are derived using the probe air speed measured at the instrument.…”