Figure 1. Response of the concentrations within the two aerosol modes to extreme cloud base altitudes (cba) and cloud fractions (cf).
Bimodal spectra observed by DRI CCN spectrometers in two aircraft field campaigns showed opposite microphysics relationships. Coastal California MASE stratus clouds associated with bimodal CCN had higher droplet concentrations with smaller sizes, narrower spectra, and less drizzle than MASE stratus clouds associated with unimodal CCN. But in Caribbean ICE-T cumuli associated with bimodal CCN, there were lower droplet concentrations of larger droplets and greater drizzle than ICE-T clouds associated with unimodal CCN. Therefore, the main cause of bimodal CCN in stratus clouds, chemical transformations, augment both first and second indirect aerosol effects on these most climatically important clouds; i.e., higher droplet concentrations and precipitation suppression.
Surface aerosol modality measurements at SGP were compared with remote cloud fraction and cloud-base altitudes. When these correlations were time-adjusted to account for aerosol movement from clouds to surface, clouds were consistently implicated as the source of aerosol bimodality. Fig. 1 shows the response of concentrations within the two aerosol modes to clouds. In each panel the detected cloud events within the noted ranges are considered at zero time. Then the concentrations within the accumulation and Aitken modes at various times after each of these cloud events is determined. Then mean concentrations are computed within each cloud range at each hour and then plotted against the number of hours after each cloud event. Panel A shows that after low clouds, accumulation mode concentrations increase for 7 hours while panel C shows that Aitken mode concentrations decrease for at least 5 hours after such low clouds are observed. Panel A also shows that when there are no cloud bases below 8 km the accumulation mode concentrations decrease for 5 hours while panel C shows that Aitken particle concentrations increase for 5 hours after these no-low-cloud events. Panel B shows that accumulation mode concentrations increase following high cloud fractions while panel D shows that Aitken mode concentrations decrease after such events. Panel B also shows that accumulation mode concentrations decrease following instances of no clouds while panel D shows that Aitken mode concentrations increase following no-cloud situations. Since the larger-sized accumulation mode is mostly a result of cloud processing, these results indicate that clouds make most of the accumulation mode.
Because of around-the-clock, year-round coverage, surface measurements provide more data more economically than aircraft campaigns. Thus, aerosol and remote-sensing measurements at ARM sites can be used to advance understanding of cloud processing.
