Aerosol Variability and Synoptic-Scale Processes over the Northeast Pacific
Subtropical marine boundary layer clouds play a significant role in cloud-climate feedbacks due to their warm temperatures and high reflectivity; small changes in cloud cover or droplet size resulting from changes in meteorology or interactions with aerosol particles can lead to large changes in the energy budgets of these clouds. During its first marine deployment, the second Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF-2) was installed on a container ship that made 36 transects between the port of Los Angeles, CA, and Honolulu, HI, to provide detailed observations of the meteorological variables and aerosol, cloud, and surface radiation properties in this region. The campaign provided one of the most comprehensive datasets available in this important marine boundary layer cloud region.
Scientists funded by the Department of Energy’s Atmospheric System Research (ASR) program combined the ARM in situ observations of aerosol concentrations with satellite retrievals and numerical model meteorological outputs to understand how the atmospheric circulation regulates the synoptic and monthly variations in aerosol concentration over a vast area of the northeast Pacific. Analysis of monthly observations of cloud condensation nuclei (CCN) reveal an annual cycle with peaks during spring and summer. The seasonal variation mostly occurs within 30° of the California coast, whereas monthly variations near Hawaii are modest. The analysis suggests that the annual cycle in CCN is consistent with a wind-transport mechanism associated with large-scale circulations and the California low-level jet, rather than with seasonal changes in precipitation. The study also examined the sensitivity of cloud properties to aerosol number concentration, and results support emerging new evidence from aircraft observations indicating that the aerosol-cloud interaction in low subtropical clouds is substantially stronger than that inferred from previous ground-based and satellite observations and from climate models.
Painemal, D., P. Minnis, and M. Nordeen. 2015. “Aerosol Variability, Synoptic-Scale Processes, and Their Link to the Cloud Microphysics over the Northeast Pacific During MAGIC,” Journal of Geophysical Research-Atmospheres, DOI: 10.1002/2015JD023175.