Monte Carlo Simulations in Aviation Contrail Study: A Review

This article provides a review of the role of stochastic approaches, in particular Monte Carlo calculations, in the study of aviation-induced contrails at different characteristic lengths, ranging from micrometers to the planetary scale. Pioneered in the 1960s by Bird, Direct Simulation Monte Carlo has for long time been considered unfeasible in extended dispersed-phase systems as clouds. Due to the impressive increase in computational power, Lagrangian Monte Carlo approaches are currently available, even for studying cloud formation and evolution. Some aspects of these new approaches are reviewed after a detailed introduction to the topic of aircraft-induced cloudiness. The role of Monte Carlo approaches in reducing the different source of uncertainty about the contribution of aviation contrails to climate change is introduced. Perspectives on their role in future experimental and theoretical studies are discussed throughout the paper.

Reducing Uncertainty in Contrail Radiative Forcing Resulting from Uncertainty in Ice Crystal Properties

The radiative forcing resulting from condensation clouds behind aircraft ("contrails") has been estimated to have an effect on the same order of magnitude as all accumulated aviation-attributable CO₂. However, contrail impacts are highly uncertain, with estimates of total contrail-driven forcing made in the past five years varying by a factor of 4. Two of the key driving uncertainties are the crystal shape and size, which describe the cloud optical properties. Here we combine data from high-fidelity scattering simulations of single crystals with in situ measurement of bulk contrail ice properties to bound the range of realistic optical properties for contrail ice. Accounting for the full range of measured contrail microphysical evolution pathways, and for a given estimate of contrail coverage, we find that the global net radiative forcing due to contrails in 2015 is between 8.6 and 10.7 mW/m². Relative to the midpoint, this uncertainty range is less than one-quarter of that recently reported in the literature. This reduction in uncertainty is primarily due to the elimination of spheres as a plausible long-term shape for contrail ice, leaving questions of contrail coverage and optical depth as the primary causes of contrail forcing uncertainty.

Formation and radiative forcing of contrail cirrus

Aircraft-produced contrail cirrus clouds contribute to anthropogenic climate change. Observational data sets and modelling approaches have become available that clarify formation pathways close to the source aircraft and lead to estimates of the global distribution of their microphysical and optical properties. While contrail cirrus enhance the impact of natural clouds on climate, uncertainties remain regarding their properties and lifecycle. Progress in representing aircraft emissions, contrail cirrus and natural cirrus in global climate models together with tighter constraints on the sensitivity of the climate system will help judge efficiencies of and trade-offs between mitigation options.

Aviation effects on already-existing cirrus clouds

Determining the effects of the formation of contrails within natural cirrus clouds has proven to be challenging. Quantifying any such effects is necessary if we are to properly account for the influence of aviation on climate. Here we quantify the effect of aircraft on the optical thickness of already-existing cirrus clouds by matching actual aircraft flight tracks to satellite lidar measurements. We show that there is a systematic, statistically significant increase in normalized cirrus cloud optical thickness inside mid-latitude flight tracks compared with adjacent areas immediately outside the tracks.