Effect of cloud‐scale vertical velocity on the contribution of homogeneous nucleation to cirrus formation and radiative forcing

Abstract

Ice nucleation is a critical process for the ice crystal formation in cirrus clouds. The relative contribution of homogeneous nucleation versus heterogeneous nucleation to cirrus formation differs between measurements and predictions from general circulation models. Here we perform large‐ensemble simulations of the ice nucleation process using a cloud parcel model driven by observed vertical motions and find that homogeneous nucleation occurs rather infrequently, in agreement with recent measurement findings. When the effect of observed vertical velocity fluctuations on ice nucleation is considered in the Community Atmosphere Model version 5, the relative contribution of homogeneous nucleation to cirrus cloud occurrences decreases to only a few percent. However, homogeneous nucleation still has strong impacts on the cloud radiative forcing. Hence, the importance of homogeneous nucleation for cirrus cloud formation should not be dismissed on the global scale.

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

“…The statistics contain relatively few nonpersistent cooling events in their flat leftmost tails ( n < 0.01 cm −3 ), which are therefore of minor importance for the mean nucleated ICNC. Otherwise, the overall shape of the simulated statistics is similar to the instantaneous distributions, reinforcing the current understanding that wave‐driven vertical wind speed variability and homogeneous freezing are important contributors to ice formation in cirrus (Hoyle et al, ; Shi & Liu, ). Instantaneous and stochastic results differ more in terms of total ICNC with greater forcing.…”

“…The random nature of wave perturbations motivates the use of probabilistic methods to describe them. Process models have studied aspects of homogeneous freezing using randomized small-scale dynamical forcing (Dinh et al, 2016;Hoyle et al, 2005;Murphy, 2014;Shi & Liu, 2016).…”

“…Field measurements leave little doubt that homogeneous freezing of supercooled aerosol particles is active in cirrus and responsible for at least the generation of the rightmost tail (>0.5 cm −3 ) of probability distributions of ICNC. (Gettelman et al, 2012;Kuebbeler et al, 2014;Penner et al, 2018;Shi & Liu, 2016;Zhou & Penner, 2014).…”

“…The main factor limiting INP effects is their low upper tropospheric number concentration (<0.1 cm −3 ; DeMott et al, ), which makes them inefficient in suppressing homogeneous freezing in the presence of sufficiently large vertical wind speeds (DeMott et al, ). Nonetheless, potent INPs are capable of modulating or, if sufficiently abundant, dominating cirrus properties according to global model simulations (Gettelman et al, ; Kuebbeler et al, ; Penner et al, ; Shi & Liu, ; Zhou & Penner, ).…”

Sensitivity of Homogeneous Ice Nucleation to Aerosol Perturbations and Its Implications for Aerosol Indirect Effects Through Cirrus Clouds

“…By including the aerosol size distribution in the homogeneous nucleation parameterizations, this will contribute to the reduction of aerosol longwave indirect forcing spread in models. We note that this spread in models can result from other sources of uncertainties in the representation of cirrus clouds, for example, the occurrence frequency of homogeneous nucleation, which depends critically on subgrid vertical velocity (Shi & Liu, ). Current models show the dominant role of homogeneous nucleation versus heterogeneous nucleation in the formation of ice crystals, especially in tropical upper tropospheric cirrus (Shi et al, ; Zhou et al, ), contrary to the observations (Jensen et al, ).…”