Simple Versus Complex Physical Representation of the Radiative Forcing From Linear Contrails: A Sensitivity Analysis

An off-line complex representation of the radiative forcing of linear contrails is applied for the first time to monthly mean 3-D distributions. This representation assumes the same temperature-dependent, spatially and time-varying functions of ice water content and particle size for contrails as for natural cirrus. This complex representation is contrasted with more commonly used simplified setups in which fixed contrail optical depth values [0.1 to 0.3] are prescribed and from which the results show differences covering a factor of 3 assuming fixed or variable contrail layer altitudes. This prescribed range of representative fixed altitudes resulted in differences covering a factor of 2 when the optical depth was also fixed. Prescribing fixed particle sizes also resulted in differences covering a factor of 2 if altitude and optical depth are also fixed. In contrast, the inclusion of the dependence of the contrail ice water content on temperature produced differences of around 20% or less when assuming the same ranges of altitudes and ice particle sizes, resulting in a much improved confidence in the radiative forcing estimates and more accurate spatial and temporal representations of the radiative interaction between contrails and the background meteorology. Assuming a contrail vertical extent of 500m, a 9mWm(-2) annual mean contrail radiative forcing is estimated, with an uncertainty range between 1 and 23mWm(-2) based on the ice water content's observed variability.