A Lagrangian Study of Southeast Pacific Boundary Layer Clouds

Abstract

Low clouds lie at the heart of climate feedback uncertainties. The representation of clouds in global climate models relies on parameterization of many sub-grid scale processes that are crucial to understanding cloud responses to climate; low clouds in particular exist as a result of tightly coupled microphysical, mesoscale, and synoptic mechanisms. The influence of anthropogenic aerosols on cloud properties could have important ramifications for our understanding of how clouds respond to a changing climate. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS REx) sampled the persistent stratocumulus cloud deck located off the coast of Peru and Chile in the southeastern Pacific ocean. Several cloud features found in the stratocumulus deck during VOCALS exhibit signs of interesting aerosol-cloud interactions, including pockets of open cells (POCs). POCs are regions of open-cellular convection surrounded by closed cell stratocumulus, exhibiting not only a marked transition in mesoscale organization and cloud morphology, but also sharp microphysical gradients (especially in droplet concentration) across the boundary between open-cellular and closed cellular convection. In addition, precipitation is often higher at the POC boundaries, hinting at the importance of precipitation in driving their formation. In order to evaluate the microphysical characteristics of POCs prior cloud breakup, we use Lagrangian trajectories coupled with geostationary satellite imagery and cloud retrievals, as well as observational data from VOCALS REx and model data. In three of our case studies, we found regions of anomalously low droplet concentration 18-24 hours prior to POC formation (coupled with liquid water path similar to or higher than surrounding cloud), supporting a precipitation driven mechanism for POC formation. Another group of features with interesting aerosol-cloud interactions observed during VOCALS were mesoscale hook-like features of high droplet concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks", and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.