89th American Meteorological Society Annual Meeting

Tuesday, 13 January 2009
A Causational Argument for the Influence of Contrail Ice-particle Shape on Thermal Infrared Radiance Observations
Hall 5 (Phoenix Convention Center)
Robert P. d'Entremont, AER, Inc., Lexington, MA; and D. L. Mitchell
Poster PDF (515.2 kB)
Contrails and aviation-induced radiative forcing are receiving a renewed attention in the climate community by international government and private-industry concerns alike. Linear contrails behave like cirrus in that they both reflect and transmit incident solar energy and absorb upwelling Earth-emitted thermal energy. In the average, absorption of thermal energy wins out. Like cirrus, contrails are “greenhouse” clouds – but anthropogenic in nature – they trap more radiant energy than they shed, causing a net positive radiative forcing (net warming).

In order to asses the radiative impact of contrails on climate it is necessary to obtain reliable climatologies that represent accurately their full diurnal cycle over the globe. Since environmental meteorological satellites provide the only truly global observations of the Earth's atmosphere, there is motivation to assess the contrail state using visible and/or infrared sensor datasets. Many detection techniques exploit the linear-shaped nature of contrails, looking for spatial texture signals in multi-spectral infrared imager data. In an automated sense, linear contrail features can be confused with linear non-contrail signatures, thus confounding attempts to assess the global contrail state using large datasets of satellite imagery.

We will present a contrail-detetction algorithm that relies on distinct ice-particle shape and size dependences that are present in contrail infrared multi-spectral signatures. In our retrievals we use a particle-radiation interaction theory for ice-particle irregulars, called the Modified Anomalous Diffraction Approximation (MADA), that models explicitly a radiative process called tunneling. Mie theory (for spherical particles) predicts that tunneling (also known as optical resonance) is strongest when (1) the wavelength and effective particle size are comparable and (2) when the real part of the refractive index is high (relative to values for water and ice). But the overall tunneling efficiency depends on particle shape (Mitchell et al. 2006), and is at a maximum for spheres. As particle shape becomes more irregular, or as particle aspect ratio departs more from unity, tunneling becomes less influential. In MADA, tunneling strength is represented by the tunneling efficiency TE, which ranges between 0 (no tunneling) and 1 (spheres). Most of the particles in young contrails and in the small mode of cirrus are quasi-spherical in nature with high TE. This provides a quantitative, causational argument for the dependence of infrared brightness temperatures on ice-particle shape, and forms the basis of our contrail detection and retrieval technique. We will also discuss the transition of contrails into contrail cirrus, and how well these clouds can be distinguished from natural cirrus for purposes of assessing anthropogenic radiative forcing.

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