Radiative forcing by contrails

Abstract

Abstract. A parametric study of the instantaneous radiative impact of contrails is presented using three di erent 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 e ect 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.

Parameterization of contrails in the UK Met Office Climate Model

“…Using the off-line Edwards-Slingo code with the 144 (longitude) × 72 (latitude) × 23 (altitude) resolution, the LW, SW and net RFs at the top of the atmosphere can be estimated. Table 1 shows the results obtained by the model for the global mean RFs, compared with similar cases presented by Meerkötter et al [1999], Myhre and Stordal [2001], and Stuber and Forster [2007]. As already shown by these earlier studies, we note that the shape of the ice particles plays an important role, with aggregates and hexagonal cylinders exerting significantly less net RFs than spherical ice particles.…”

“…As in work by Meerkötter et al [1999], a 100% homogeneous contrail coverage is assumed at 250 hPa, with a constant optical depth t = 0.52 at 0.55 mm, an ice water content IWC = 21 mg m −3 , and a generalized mean effective size D ge = 23mm. Following Fu [1996], the generalized mean effective size is given by…”

“…When they form in dry unsaturated air, contrails are usually short-lived, but when the ambient relative humidity exceeds ice saturation, they persist and can develop into extended cirrus cloud layers [see, e.g., Schumann, 1996]. Existing studies have shown that for these persistent contrails, their daily average LW radiative forcing (RF) effect is larger than their SW effect, meaning that contrails cause a positive net RF, and therefore a warming [see, e.g., Meerkötter et al, 1999]. The magnitude of this net positive RF estimated by various studies for the air traffic of the year 1985, varies from a value of 2.0 mW m −2 [Stuber and Forster, 2007] to a value of 17 mW m −2 .…”

Cirrus cloud retrieval with MSG/SEVIRI using artificial neural networks

“…Fu and Liou, 1993;Zhang et al, 1999;Wendisch et al, 2007). Because of the tenuous nature of cirrus clouds, the reflection of solar radiation can be outweighted by the thermal effect (Meerkötter et al, 1999), leading to a positive net radiative forcing, as is the case for thin cirrus (Jensen et al, 1994;Chen et al, 2000). Despite the constant progress in cirrus research and the continuous development of more advanced instruments and retrieval algorithms, the understanding of the physical processes that govern the cirrus life cycle…”

Contrails: Causal Inference Using Propensity Scores

“…Geologists asserted that contrails (1) decrease the daily high temperature by blocking incoming sunlight, (2) increase nightly low temperatures by preventing escape of greenhouse gases, and, therefore (3) decrease the daily temperature range on the Earth's surface below (e.g., Meerkotter, et al, 1999). This contrails effect was estimated to be 1.98°F or 3.24°F (Travis, et al, 2002); the greater of these is hereafter referred to as the Travis estimate.…”