Aerosol-cloud-drizzle variability in marine stratocumulus over the subtropical southeastern Pacific coastal region

The VAMOS Ocean-Cloud-Atmosphere-Land Study -Regional Experiment (VOCALS-REx) was undertaken from October to November 2008 over the subtropical southeastern Pacific to investigate physical and chemical processes associated with marine stratocumulus clouds that are central to the climate system of this region. The CIRPAS Twin Otter aircraft flew 19 flights in the coastal region (at 20oN, 72oW) during VOCALS-REx. Each flight included 3 to 4 soundings and near-surface, below-cloud, cloud base, in cloud, cloud top, and above inversion observations along fixed-height legs. This study used the aerosol, cloud, boundary-layer thermodynamics and turbulence observations from those 19 flights to investigate the boundary layer, and aerosol-cloud-drizzle variations in this region. The boundary layer structure and surface temperature and moisture fluxes showed relatively little variability and indicated little influence by meso-scale and large-scale systems during most of the observing period. Aerosol properties exhibited significant variability: accumulation mode aerosol concentration in the boundary layer varied from 200 to 700 cm-3. These typically represented about 80% of the total aerosol count. Aerosol number concentrations above the boundary layer were substantially smaller than those below (50 -250 cm-3) except for two cases where these values were elevated. Cloud droplet concentrations ranged from 50 to400 cm-3 over the 19 flights. The positive correlation between aerosol number concentration in the boundary layer and cloud droplet number concentration is consistent with the Twomey effect. Drizzle water content varied from 10-5 to 0.05 gm-3 and 6 flights out of 19 flights have mean drizzle water content larger than 0.0015 gm-3. The effects of liquid water path and mean droplet concentrations on the drizzle rates will be presented. Since the boundary layer conditions at this fixed point are so steady, the observations provide a unique data set for the evaluation of models operating at a variety of scales to study coastal marine stratocumulus during VOCALS REx.