EPCAPE

Eastern Pacific Cloud Aerosol Precipitation Experiment

15 February 2023 - 14 February 2024

Lead Scientist: Lynn Russell

Observatory: AMF, EPC

Coastal cities provide the opportunity to characterize marine clouds and the substantial effects of manmade particles on cloud properties and processes. La Jolla lies to the north of San Diego, California, but it is often about a day downwind of the major pollution sources located in the ports of Los Angeles and Long Beach. The large dynamic range of aerosol particle concentrations combined with the multi-hour to multi-day persistence of stratocumulus cloud layers makes the site ideal for investigating the seasonal changes in cloud and aerosol properties as well as the quantitative relationships between cloud and aerosol properties.

The focus of this project is to characterize the extent, radiative properties, aerosol interactions, and precipitation characteristics of stratocumulus clouds in the Eastern Pacific across all four seasons at a coastal location, such as the Scripps Pier and the Scripps Mt. Soledad sites in La Jolla.  An important enhancement to this study will be the collection of simultaneous in-cloud aerosol and droplet measurements to investigate the differences in these cloud properties during regional polluted and clean marine conditions. The combined observations will provide an unprecedented set of constraints for the following questions:

1. Cloud and aerosol climatology: What are the seasonal and diurnal cycles of marine stratocumulus cloud and aerosol properties on the northeastern Pacific coast? 
2. Cloud radiative fluxes: How do cloud properties, including the ratio of direct-to-diffuse radiation, change as coastal clouds are advected inland?
3. Aerosol-cloud interactions: Will retrieved cloud properties reflect the regional signatures of aerosol?

Each of these questions reflects a topic of current controversy in the literature that cannot be addressed without the type of comprehensive data set proposed here.

The relevance of this campaign to the ARM mission is its strategic location in an accessible and economically important region of the world that lacks long-term observations of its frequent, persistent, and climatically important coastal stratocumulus cloud cover.

The clouds lie in one of the largest regions of upwelling-driven stratocumulus layers that are most likely impacted by aerosol indirect effects, but climate models do not accurately simulate the processes that control their radiative effects. Furthermore, the coastal orography incites significant additional uncertainties related to cloud turbulence, air motion spectrum, and drop size distributions.

Finally, the aerosol in the region ranges from a clean marine background to frequent intrusions from a large and regionally homogeneous, well-characterized, surface‑based pollution source (the Los Angeles-Long Beach urban port megacity), providing a large dynamic range of aerosol conditions for investigation.

To complement and leverage EPCAPE, the Office of Naval Research (ONR) will support a 90-flight-hour deployment of the Naval Postgraduate School’s Twin Otter research aircraft during the May/June 2023 Southern California Interactions of Low cloud and Land Aerosol (SCILLA) experiment. Those research flights will provide a regional context for the point measurements in La Jolla and will add extensive below-, in-, and above-cloud measurements spanning an area encompassing a wide range of meteorological and, especially, aerosol conditions. The SCILLA team from the University of California Riverside and Clemson University will be supported by the DOE Atmospheric System Research (ASR) program to significantly augment the aerosol measurement capabilities on the Twin Otter in order to provide a much more comprehensive description of the sources, processing, chemistry, radiative and cloud-altering properties, and spatial distribution of the aerosols upwind of the San Diego area in general, and the EPCAPE study sites in particular.