Studies on the competition between homogeneous and heterogeneous ice nucleation in cirrus formation

We study the reduction of ice crystal numbers produced by homogeneous freezing due to INPs in both, individual simulations assuming constant updraft speeds and in ensemble simulations based on a stochastic representation of vertical wind speed fluctuations. We simulate and interpret probability distributions of total nucleated ice crystal number concentrations, showing signatures of homogeneous and heterogeneous nucleation.

At typically observed, mean updraft speeds (approximate to 15 cm s(-1)) competing nucleation should occur frequently, even at rather low INP number concentrations (<10 L-1). INPs increase cirrus occurrence and may alter cirrus microphysical properties without entirely suppressing homogeneous freezing events. We suggest to improve ice growth models, especially for low cirrus temperatures (<220 K) and low ice supersaturation (<0.3).