Large‐eddy simulation of three mixed‐phase cloud events during ISDAC: Conditions for persistent heterogeneous ice formation

Abstract

A Classical-Nucleation-Theory-based parameterization for heterogenous ice nucleation, including explicit dependencies of the nucleation rates on the number concentration, size, and composition of the ambient aerosol population, is implemented in a cloud-scale, large-eddy simulation model and evaluated against Arctic mixed-phase cloud events observed during Indirect and Semi-Direct Aerosol Campaign (ISDAC). An important feature of the parameterization is that the ice nucleation efficiency of each considered aerosol type is described using a contact angle distribution which evolves with time so that the model accounts for the inhibition of ice nucleation as the most efficient ice-forming particles are nucleated and scavenged. The model gives a reasonable representation of first-order (ice water paths) and second-order (ice crystal size distributions) ice microphysical properties. The production of new ice crystals in the upper part of the cloud, essential to guarantee sustained mixed-phase conditions, is found to be controlled mostly by the competition between radiative cooling (resulting in more aerosol particles becoming efficient ice nuclei as the temperature decreases), cloud-top entrainment (entraining fresh particles into the cloud), and nucleation scavenging of the ice+forming aerosol particles. The relative contribution of each process is mostly determined by the cloud-top temperature and the entrainment rates. Accounting for the evolution of the contact angle probability density function with time seems to be essential to capture the persistence of in-cloud ice production without having to, for example, increase the free tropospheric aerosol concentration. Although limited to only three cases and despite important limitations of the parameterization (e.g., the present version only considers dust and black carbon as potential ice nuclei), the results suggest that modeling the time evolution of the ice nuclei population ability to form ice is required to accurately model Arctic mixed-phase cloud processes.

Cloud response and feedback processes in stratiform mixed‐phase clouds perturbed by ship exhaust

“…Figures 3 and 4 show that additional INPs, which freeze via immersion freezing, could also stem from the background aerosol population by initiating heterogeneous nucleation at lower cloud-top temperatures. Therefore this study highlights the importance of the cloud-top radiative feedback not only for the cloud and boundary layer dynamics but also for the glaciation levels of the MPC, as had also been noted by Savre and Ekman [2015]. Christensen et al [2014] found that on average the ice water content detected in ship tracks was increased by 19%, the surface precipitation was increased by 30%, and the total water path decreased by 15% compared to the background.…”

“…Figures and show that additional INPs, which freeze via immersion freezing, could also stem from the background aerosol population by initiating heterogeneous nucleation at lower cloud‐top temperatures. Therefore this study highlights the importance of the cloud‐top radiative feedback not only for the cloud and boundary layer dynamics but also for the glaciation levels of the MPC, as had also been noted by Savre and Ekman [].…”

Vertical distribution of microphysical properties of Arctic springtime low-level mixed-phase clouds over the Greenland and Norwegian seas

“…The most trivial classification is the temperature regime. Savre and Ekman (2015), showed that it is one of the major factors (with cloud top entrainment) controlling the production of new ice crystals and the maintenance of MPCs. In the present study, two temperature regimes have been selected based on the mean cloud top temperature of each situation: the "cold" situations (−22 • C < T Top < −15 • C) and the "warm" situations (−15 • C < T Top < −8 • C).…”

“…A better assessment of the ice/liquid partitioning will improve our understanding of the life cycle and more precisely the persistence of MPCs since modeling studies show that this persistence is governed by a delicate balance between dynamical, radiative and microphysical processes occurring mainly in the boundary layer (Savre and Ekman, 2015). This understanding is still limited by the description of the microphysical processes related to the initiation and the maintenance of the ice phase.…”

“…This understanding is still limited by the description of the microphysical processes related to the initiation and the maintenance of the ice phase. The cloud processes responsible for the production of ice crystals in the upper part of the cloud seem to be mostly driven by the cloud top temperature and the entrainment rates (Savre and Ekman, 2015). In particular, the assessment of ice nuclei (IN) concentration and its time evolution is of primary importance but relies on a very limited set of in situ observations and needs to be improved (Ovchinnikov et al, 2014).…”