On the regional climatic impact of contrails: microphysical and radiative properties of contrails and natural cirrus clouds

The impact of contrail-induced cirrus clouds on regional climate is estimated for mean atmospheric conditions of southern Germany in the months of July and October. This is done by use of a regionalized one-dimensional radiative convective model (RCM). The influence of an increased ice cloud cover is studied by comparing RCM results representing climatological values with a modified case. In order to study the sensitivity of this effect on the radiative characteristics of the ice cloud, two types of additional ice clouds were modelled: cirrus and contrails, the latter cloud type containing a higher number of smaller and less of the larger cloud particles. Ice cloud parameters are calculated on the basis of a particle size distribution which covers the range from 2 to 2000 mu m, taking into consideration recent measurements which show a remarkable amount of particles smaller than 20 mu m. It turns out that a 10 % increase in ice cloud cover leads to a surface temperature increase in the order of 1 K, ranging from 1.1 to 1.2 K in July and from 0.8 to 0.9 K in October depending on the radiative characteristics of the air-traffic-induced ice clouds. Modelling the current contrail cloud cover which is near 0.5 % over Europe yields a surface temperature increase in the order of 0.05 K.