Simulation of arctic diamond dust, ice fog, and thin stratus using an explicit aerosol-cloud-radiation model

In support to the development of the Northern Aerosol Regional Climate Mode l, a single column model with explicit aerosol and cloud microphysics is de scribed. It is designed specifically to investigate cloud-aerosol interacti ons in the Arctic. A total of 38 size bins discretize the aerosol and cloud spectra from 0.01- to 500-mum diameter. The model is based on three equati ons describing the time evolution of the aerosol, cloud droplet, and ice cr ystal spectra. The following physical processes are simulated: coagulation, sedimentation, nucleation, coalescence, aggregation, condensation, and dep osition. Further, the model accounts for the water-ice phase interaction th rough the homogeneous and heterogeneous freezing, ice nuclei, and the Berge ron effect. The model has been validated against observations and other mod els. In this paper, the model is used to simulate diamond dust and ice fog in th e Arctic during winter. It is shown that simulated cloud features such as c loud phase, cloud particle diameter, number concentration, and mass concent ration are in agreement with observations. The observed vertical structure of mixed-phase cloud is also reproduced with the maximum mass of liquid pha se in the upper part of the cloud. Based on simulations, a hypothesis is fo rmulated to explain the thermodynamical unstable mixed-phase state that can last several days in diamond dust events. The ice supersaturation time evo lution is assessed and is compared to its evolution in cirrus clouds. It is shown that the supersaturation relaxation time, defined as the time requir ed for supersaturation to decrease by a factor e, is more than 10 times the value found in cirrus clouds. Finally, the radiative contribution of arcti c diamond dust and ice fog to the downward longwave radiation flux at the s urface is evaluated and compared to observations.