Simulations of dehydration effects from contrails in a global climate model

Abstract

We study the effects of humidity exchange between contrails and background atmosphere using the Contrail Cirrus Prediction Model CoCiP (Schumann, GMD, 2012) coupled with the climate model CAM3+/IMPACT (Penner et al., 2009; Zhou and Penner, JGR, 2014).

Contrail ice particles grow by uptake of humidity from ambient ice supersaturated air masses. By sedimentation, contrails contribute to dehydration of the upper troposphere and to humidity increases at lower levels. After advection in rising or subsiding air masses, contrail may release water at places higher or lower than where the contrail formed. The dehydration may reduce natural cloudiness and, hence, partly offset positive radiative forcing from contrail cirrus (Burkhardt and Kärcher, 2011). LES show thinner contrails when they overlap with others (Unterstraßer and Sölch, 2012).

The coupled CoCiP-CAM model simulates all individual contrails forming globally using a Lagrangian contrail model. The coupling accounts for conservation of humidity during contrail formation and sublimation. In the coarse global model, local changes in humidity from contrail formation become notable only when many contrails form.

The simulated contrails agree well with observations. The amount of water stored inside contrails is 1 million times larger than the amount of water emitted. Contrail ice particles sediment typically 700 m. Because of local dehydration, less contrails form, with less cover of optically thick contrails, but individual contrails have larger life time and width.