Julien Savre

Intercomparison of large‐eddy simulations of Arctic mixed‐phase clouds: Importance of ice size distribution assumptions

Large-eddy simulations of mixed-phase Arctic clouds by 11 different models are analyzed with the goal of improving understanding and model representation of processes controlling the evolution of these clouds. In a case based on observations from the Indirect and Semi-Direct Aerosol Campaign (ISDAC), it is found that ice number concentration, Ni, exerts significant influence on the cloud structure. Increasing Ni leads to a substantial reduction in liquid water path (LWP), in agreement with earlier studies. In contrast to previous intercomparison studies, all models here use the same ice particle properties (i.e., mass-size, mass-fall speed, and mass-capacitance relationships) and a common radiation parameterization. The constrained setup exposes the importance of ice particle size distributions (PSDs) in influencing cloud evolution. A clear separation in LWP and IWP predicted by models with bin and bulk microphysical treatments is documented and attributed primarily to the assumed shape of ice PSD used in bulk schemes. Compared to the bin schemes that explicitly predict the PSD, schemes assuming exponential ice PSD underestimate ice growth by vapor deposition and overestimate mass-weighted fall speed leading to an underprediction of IWP by a factor of two in the considered case. Sensitivity tests indicate LWP and IWP are much closer to the bin model simulations when a modified shape factor which is similar to that predicted by bin model simulation is used in bulk scheme. These results demonstrate the importance of representation of ice PSD in determining the partitioning of liquid and ice and the longevity of mixed-phase clouds.

A theory-based parameterization for heterogeneous ice nucleation and implications for the simulation of ice processes in atmospheric models

A new parameterization for heterogeneous ice nucleation constrained by laboratory data and based on classical nucleation theory is introduced. Key features of the parameterization include the following: a consistent and modular modeling framework for treating condensation/immersion and deposition freezing, the possibility to consider various potential ice nucleating particle types (e.g., dust, black carbon, and bacteria), and the possibility to account for an aerosol size distribution. The ice nucleating ability of each aerosol type is described using a contact angle (θ) probability density function (PDF). A new modeling strategy is described to allow the θ PDF to evolve in time so that the most efficient ice nuclei (associated with the lowest θ values) are progressively removed as they nucleate ice. A computationally efficient quasi Monte Carlo method is used to integrate the computed ice nucleation rates over both size and contact angle distributions. The parameterization is employed in a parcel model, forced by an ensemble of Lagrangian trajectories extracted from a three-dimensional simulation of a springtime low-level Arctic mixed-phase cloud, in order to evaluate the accuracy and convergence of the method using different settings. The same model setup is then employed to examine the importance of various parameters for the simulated ice production. Modeling the time evolution of the θ PDF is found to be particularly crucial; assuming a time-independent θ PDF significantly overestimates the ice nucleation rates. It is stressed that the capacity of black carbon (BC) to form ice in the condensation/immersion freezing mode is highly uncertain, in particular at temperatures warmer than −20°C. In its current version, the parameterization most likely overestimates ice initiation by BC.

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

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.

The Free Troposphere as a Potential Source of Arctic Boundary Layer Aerosol Particles

This study investigates aerosol particle transport from the free troposphere to the boundary layer in the summertime high Arctic. Observations from the Arctic Summer Cloud Ocean Study field campaig ...