Microphysical parameterization of arctic diamond dust, ice fog, and thin stratus for climate models

A parameterization is described for low-level clouds that are characteristi c of the Arctic during winter. This parameterization simulates the activati on of aerosols, the aggregation/coalescence, and the gravitational depositi on of ice crystals/water droplets and the deposition/condensation of water vapor onto ice crystals/water droplets. The microphysics scheme uses four p rognostic variables to characterize clouds: ice water content, liquid water content, and the mean diameter for ice crystals and for water droplets, an d includes prognostic supersaturation. The parameterization simulates stabl e clouds where turbulence and entrainment are weak, like ice fogs, thin str atus, and diamond dust. The parameterization is tested into the Local Clima te Model (LCM), which is the single column version of the Northern Aerosol Regional Climate Model (NARCM). NARCM is a regional model with an explicit representation of the aerosol physics and with the physics package of the C anadian Climate Center General Circulation Model version two. Since most cl imate models do not have prognostic size-segregated aerosol representation, an alternate method is proposed to implement the microphysical parameteriz ation into these models. The model results are compared to observations of diamond dust and ice fog at Alert (Canada) for the period 1991-94. Two aero sol scenarios are compared in the simulation: a natural background aerosol scenario and an acidic aerosol scenario. Results show that the LCM reproduc es approximately the time variation of the observed weekly frequency of the total ice crystal precipitation with a correlation coefficient of 0.4. Alt hough it overestimates diamond dust frequency and underestimates ice fog fr equency, the LCM predicts quite well the total precipitation frequency (ice fog and diamond dust added). The acidic aerosol scenario is in good agreem ent with the observations, showing a mean frequency of total precipitation over the 4 yr of 39% compared to the observed value of 37%. The natural aer osol scenario overestimates this frequency with a value of 47%. These resul ts were expected since recent aerosol observations have shown the predomina nce of sulfuric acid-coated aerosols in the Arctic during winter.