Constraining the effect of aerosol on warm cloud formation

Accurate knowledge of cloud droplet number concentration is required to predict cloud albedo and precipitation efficiency. Calculation of cloud droplet number from aerosol properties is challenging even in the simplest available framework of an adiabatic parcel rising at constant updraft. Using classical Köhler theory, a minimum of 12 parameters are required to initialize a simulation. Several of these parameters can be condensed into a single hygroscopicity parameter which can represent aerosol composition. A second grouping termed the 'physicochemical activation parameter' can be constructed to combine the hygroscopicity parameter, surface tension, and the geometric mean diameter of the size distribution. The problem of cloud droplet formation can then be reduced to five unique parameters: aerosol number concentration, geometric standard deviation, updraft, condensation coefficient, and the 'physicochemical activation parameter'. We modified a parcel model to survey the parameter space in terms of those five inputs. Using this approach we systematically establish the sensitivity of simulated cloud droplet number concentrations to variations in hygroscopicity, surface tension, particle size distribution, updraft, and condensation coefficient. We find that droplet concentrations are always three times as sensitive to changes in mode diameter as to changes in aerosol hygroscopicity. Evaluating the various influences on overall sensitivity of cloud droplet number concentration reveals that at least 70% of the overall sensitivity stems from sensitivity to size distribution parameters, 20% stems from sensitivity to updraft and condensation coefficient, and 10% stems from sensitivity to aerosol hygroscopicity and solution surface tension.