Radiative forcing by contrails

Abstract

A parametric study of the instantaneous radiative impact of contrails is presented using three dierent radiative transfer models for a series of model atmospheres and cloud parameters. Contrails are treated as geometrically and optically thin plane parallel homogeneous cirrus layers in a static atmosphere. The ice water content is varied as a function of ambient temperature. The model atmospheres include tropical, mid-latitude, and subarctic summer and winter atmospheres. Optically thin contrails cause a positive net forcing at top of the atmosphere. At the surface the radiative forcing is negative during daytime. The forcing increases with the optical depth and the amount of contrail cover. At the top of the atmosphere, a mean contrail cover of 0.1% with average optical depth of 0.2 to 0.5 causes about 0.01 to 0.03 Wm A2 daily mean instantaneous radiative forcing. Contrails cool the surface during the day and heat the surface during the night, and hence reduce the daily temperature amplitude. The net eect depends strongly on the daily variation of contrail cloud cover. The indirect radiative forcing due to particle changes in natural cirrus clouds may be of the same magnitude as the direct one due to additional cover.

Effects of optical depth variability on contrail radiative forcing

“…The behaviour of CRF(τ ) in this transition region depends on the ice crystal optical model (Markowicz and Witek, 2011, their Figure 9). Figure A.1 shows this transition region using g=0.8, A=0.2, θ =30 • , T b =280 K, and T c =223 K. Our results compare favourably with those discussed for spherical ice crystals in Meerkötter et al (1999, their Figure 3). A closer comparison with 1D RTM results is not straightforward since Meerkötter et al (1999) employ spectral models, provide diurnal averages of the flux changes, and consider a specific vertical distribution of absorbing gases and clouds (determining T b ), while we apply a broadband model with fixed values for g and θ and do not resolve the atmosphere below and above the contrail layer.…”

“…It causes CRF biases of 10−20% which are of similar magnitude to effects caused by other factors determining the optical response of contrails, e.g. optical depth of low-level clouds or ground albedo, altitude or ice water content of contrails, ice crystal habit, or 3D effects (Meerkötter et al, 1999;Gounou and Hogan, 2007;Yang et al, 2010;Markowicz and Witek, 2011;Schumann et al, 2011). We note that differences in the SW and LW CRF among various optical models-in which ice crystal radiative properties are treated by different methods-have been found to reach 44% and 23%, respectively, relative to the mean model value (Markowicz and Witek, 2011).…”

“…A closer comparison with 1D RTM results is not straightforward since Meerkötter et al (1999) employ spectral models, provide diurnal averages of the flux changes, and consider a specific vertical distribution of absorbing gases and clouds (determining T b ), while we apply a broadband model with fixed values for g and θ and do not resolve the atmosphere below and above the contrail layer. To estimate the accuracy of our parametrization, we have compared results for clearsky midlatitude summer continental conditions (Meerkötter et al, 1999, their Tables 4 and 5), using fixed values τ =0.52, θ = 30−60 • , and T b =288 K. Our individual flux changes in the SW and LW agree with the respective SW and LW estimates of Meerkötter et al (1999) within 30%, with F LW being more accurate than F SW . It is possible that the agreement could be improved by systematically fitting the values of g, A, and T b .…”

US jet contrail frequency changes: influences of jet aircraft flight activity and atmospheric conditions

“…The strong growth in commercial aviation during the past 30 years has led to speculation about its potential effects on regional-scale climate via increased jet contrail coverage (Changnon, 1981;Sassen, 1997;Meerkotter et al, 1999;Minnis et al, 1999;Duda et al, 2001;Travis et al, 2002Travis et al, , 2003, as well as changes in the aerosol load and gaseous composition of the upper troposphere and lower stratosphere (e.g. Schulte et al, 1997;Brasseur et al, 1998).…”

Optical properties of ice particles in young contrails

“…The microphysical and optical properties of contrails and contrail-induced cirrus clouds are quite different from those of natural cirrus clouds (Chylek and Hallett, 1992;Gayet et al, 1996;Petzold et al, 1997;Kuhn et al, 1998;Störm and Ohlsson, 1998;Schröder et al, 2000;Kärcher et al, 2007). Unlike a natural cirrus cloud within which ice crystals have a wide size spectrum, a contrail usually has a higher number concentration of small ice crystals (Gayet et al, 1996;Minnis et al, 1998;Ström and Ohlsson, 1998;Meerkötter et al, 1999;Schröder et al, 2000;Ponater et al, 2002). During the transition of contrails into cirrus clouds, the ice crystal size increases and the corresponding number concentration decreases, as articulated by Schröder et al (2000) who analyzed the microphysical properties of contrails and contrail-induced cirrus clouds observed during 15 airborne missions over central Europe.…”