Sensitivity study of global contrail radiative forcing due to particle shape

Global calculations of the radiative forcing (RF) that is due to contrails for various crystal shapes are performed. A radiative transfer model, which takes into account ice crystals' single‐scattering properties both calculated and acquired from the literature, is used to simulate solar and infrared fluxes. Contrail RF includes results obtained for 10 various, randomly oriented in space, crystal habits such as hexagonal plates and hexagonal columns with different aspect ratios, among others. The global and annual mean shortwave, longwave, and net contrail RF, averaged over all crystal models and assuming an optical depth of 0.3 at visible wavelengths, are −5.7, 16.8, and 11.1 mW/m2, respectively. Simulations performed for the clear‐sky (without background cloudiness) conditions show a similar net radiative forcing of 11.0 mW/m2, but stronger shortwave (−9.9 mW/m2) and longwave (20.9 mW/m2) RFs compared with those of the cloudy case. The influence of ambient clouds on the net contrail RF is small and depends only slightly on the contrail optical depth and particle shape. However, the particle shape has a strong impact on the contrail RF. A ratio of the RFs' standard deviation to the mean value, derived using 10 different ice particle models, is about 0.2 for the shortwave, 0.14 for the longwave, and 0.23 for the net radiation. An even larger uncertainty, besides the optical model, in global contrail RF estimations is the assumed optical depth. Model simulations show a 27% difference in RF related to a change of the contrail optical depth by ±0.1 from the reference value of 0.3 (at 550 nm). This value is almost invariant to the choice of crystal model, ranging between 25% and 28%. Key Points Large variability of contrails RF with ice crystal shapes Small influence of natural clouds on the net contrail radiative forcing Estimation of uncertainty of contrail radiative forcing