Aviation-Climate Change Research Initiative (ACCRI) Subject Specific White Paper (SSWP) on Contrails/cirrus optics and radiation In this report, the current state of science on the climate impacts of contrails and contrail-induced cirrus is reviewed, including an analysis of the uncertainties. In so doing, we identify ten key problems: • The uncertainties in contrail and contrail-induced cirrus coverage estimated from different detection algorithms and measurements. • The uncertainties in optical thicknesses of contrail and contrail-induced cirrus clouds from different analyses. These differences result in distinct differences in the radiative forcing of contrails and contrail-induced cirrus clouds. • Ice particle sizes vary from contrails to contrail-induced cirrus clouds. But the measurements for ice particle sizes are strongly dependent on the detection approaches, particularly for small ice particles. • Different ice habits have been found in contrails and contrail-induced cirrus clouds. In turn, different mixtures of ice habits must be used in the study of the radiative forcing of contrails and contrail-induced cirrus clouds. A better understanding of the single-scattering properties of ice crystals in contrails and contrail-induced cirrus clouds is necessary. • Shortwave radiative forcing varies substantially with solar zenith angle. The diurnal variation of air traffic is an important factor for the radiative forcing calculations, but is often neglected in the calculation of the impact of contrails and contrail-induced cirrus. • New techniques should be developed to accurately detect contrails and contrail induced cirrus clouds. This is critical for understanding the microphysical and optical properties of these clouds. • Our current understanding of the effects of aerosols emitted by aircraft on cirrus cloud formation and the interaction between aerosols and cirrus clouds needs to be improved. Furthermore, the chemical effects of black carbon and sulphate particles on the microphysical properties (e.g., the refractive indexes) of ice particles in contrails and contrail-induced cirrus clouds are poorly known and need to be improved. • There are no studies on the contrails embedded within natural cirrus clouds. This is quite important since the aircraft flight heights are often at the heights where ice clouds frequently occur.5 • Representation of contrails and contrail-induced cirrus clouds in global atmospheric models needs to be improved. There are shortages of observations for validation of GCM results, and the representation of the aerosol-cirrus interaction in GCMs needs substantial improvements. Furthermore, there is an urgent need to develop radiation schemes that are suitable for use in GCMs. • More and better measurements of supersaturation in the atmosphere. 2.2 Radiative forcing contrails and contrail-induced cirrus clouds a. Current state of science As one of the most visible anthropogenic effects in the atmosphere, contrails and contrail-cirrus are a common sight in the skies over regions with heavy air traffic. Fig. 12 shows the effect of persisting contrails on the diurnal temperature range (Travis et al., 2002). This range tends to be reduced by the occurrence of contrails, which affect the transfer of solar and infrared radiation, as demonstrated by measurements taken around 09/11/01, when all civil and commercial aircraft were temporarily grounded. faa.gov/about/office_org/headquarters_offices/apl/research/science_integrated_modeling/accri/media/ACCRI_SSWP_VI_PING.pdf
Posted on: Fri, 19 Jul 2013 20:53:46 +0000
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