Parallel simulations of aerosol influence on clouds using cloud‐resolving and single‐column models

Abstract

The influence of the cloud condensation nucleus (CCN) concentration on the properties of low‐level clouds under the conditions observed over the north central Oklahoma on 24–25 September 1997 is examined in a series of 18‐hour simulations using a single‐column model (SCM) and a cloud‐resolving model (CM). Both models predict higher droplet concentration, smaller droplet size, and larger liquid water path in a “polluted” case (CCN concentration = 1000 cm−3) than in a clean case (CCN concentration = 250 cm−3), suggesting that the first and the second indirect effects act in unison under the considered conditions. A comparison of the simulations using the SCM and CM with the same two‐moment bulk microphysics parameterization highlights the dominant effect of the dynamical framework on both microphysical and macrophysical properties of modeled cloud. This effect is much stronger than the variations in each of the models resulting from changing CCN concentrations. However, the relative liquid water path sensitivity to CCN concentration is similar between the SCM and CM simulations. The CM simulations with the size‐resolved and the two‐moment bulk microphysical parameterization yield nearly identical structure of boundary layer. Even though these simulations are in much closer agreement with each other than they are with the SCM results, the variance from the microphysics treatment is still comparable to the effect of quadrupling CCN concentration.