Abstract:
The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice
nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in
a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement
both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number
concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual
impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol
chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well
predicted in a 1Dparcel model framework. Two different empirical parameterizations were used in the parcel
model: a parameterization based on aerosol chemical type and surface area and a parameterization that links
ice nuclei number concentrations to the number concentrations of particles with diameters larger than
0.5 mm. This study shows that aerosol size distribution and composition measurements can be used to constrain
ice initiation by primary nucleation in models. The data and model results also suggest the likelihood
that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous
ice nuclei, at least for the lower temperatures examined in this case.
