Links between the microphysics and macrophysics of low clouds over southeast Pacific Ocean (VOCALS Submission)

Satellite observations of clouds over the northeast and southeast Pacific reveal that dramatic structural changes occur frequently and spontaneously in overcast stratocumulus clouds. These clouds can rapidly transition from a closed cellular to open cellular regime with a corresponding decrease in cloud cover. The causes of this transition are at present unknown.

Recent research cruises to the southeast Pacific sampled both closed and open cellular convection using surface-based remote sensors. In this study we use the surface-based cruise data as well as satellite data to examine microphysical and macrophysical characteristics of marine boundary layer (MBL) clouds over the southeast Pacific (SEP). We also focus upon the MBL structure associated with different cloud regimes, their precipitation characteristics, cellular nature, and cloud cover.

Results suggest that precipitation is often of sufficient magnitude to exert a strong dynamical effect upon the MBL and is modulated by cloud microphysical variability. Moreover, heavier drizzle events tend to occur more frequently in the open cellular regions than in the closed cells. Satellite data suggest that the initial transition from closed to open cells tends to occur in regions of cloud with large cloud droplet effective radius. Drizzle may be more likely to form in these regions. Evidence is presented suggesting that the drizzle will exert a strong dynamical effect on the MBL through latent warming in cloud and evaporative cooling below. Additionally, drizzle scavenges cloud condensation nuclei, which could result in a runaway depletion process. MODIS data are used to estimate the effect of the open and closed cell regions upon the radiation balance. The climatological impacts of these findings are discussed.