| An increasing number of aerosol particles released by human activities cool the climate system by modifying the cloud radiative properties via the cloud formation process. Analysis of data from two Canadian field experiments shows that the enhancement of aerosol number concentration on a regional scale increases the cloud droplet number (N) and reduces the cloud droplet size, thus results in a higher cloud reflectivity (first indirect aerosol effect). The more numerous but smaller cloud droplets also decrease the precipitation efficiency, therefore prolonging cloud lifetime and increasing cloud reflectivity (second indirect aerosol effects). Additionally, anthropogenic aerosols influence the shape of the cloud droplet size distribution and modify the cloud reflectivity (spectral dispersion effect).; Not only aerosols, but also dynamical processes influence N. Data obtained from two Canadian field campaigns are applied into an adiabatic parcel model, which simulates the cloud formation process. Comparison of simulated N and observed N indicates that a single characteristic value proportional to the standard deviation of the in-cloud vertical velocity (w) distribution is a good representative for simulating N in marine stratus clouds.; A General Circulation Model (GCM) is a useful tool to understand the influence of aerosols on the climate. Different parameterisation schemes of the cloud nucleation process are tested using the single column model (SCM) and are compared against observational data from ACE-2. Based on Ghan's scheme and Lin and Leaitch's scheme, Peng's scheme modifies multiple parameters depending on the parcel model simulations. The predicted N using Peng's scheme shows a better agreement with the observed N and this scheme is expected to be further tested in the GCM. Also, the spectral dispersion effect is added in the parameterisation of cloud properties in the GCM, which reduces the predicted indirect aerosol effect by 0.2 W m-2 in the global mean. |
