Observations and Modeling of Aerosol-Cloud Interactions at ARM Sites in Alaska
Authors
Maximilian Maahn — Leipzig University
Gijs de Boer — University of Colorado
Jessie Creamean — Colorado State University
Graham Feingold — NOAA- Earth System Research Laboratory
Matthew Norgren — University of Colorado
Allison C. McComiskey — Brookhaven National Laboratory
Greg McFarquhar — University of Oklahoma
Matthew Shupe — University of Colorado
Amy Solomon — University of Colorado/NOAA- Earth System Research Laboratory
Christopher R Williams — University of Colorado, Boulder
Wei Wu — University of Oklahoma
Category
High-latitude clouds and aerosols
Description
Aerosols have a strong potential to influence cloud properties when acting as cloud condensation nuclei (CCN) or ice nucleating particles (INPs). In particular, they can impact the number, size, and phase of cloud particles and potentially cloud lifetime through aerosol indirect and semi-direct effects. These effects are of great importance for the radiation budget in polar regions due to the shortwave albedo and longwave emissivity of mixed-phase clouds.
Here, we summarize our efforts to enhance the understanding of aerosol-cloud interactions at the two Atmospheric Radiation Measurement (ARM) sites in Alaska: Oliktok Point (OLI) and North Slope of Alaska (NSA). While OLI is surrounded by petroleum production facilities located around Prudhoe Bay, NSA generally represents a more pristine Arctic environment lacking significant anthropogenic sources. Therefore, these two sites form together a natural laboratory to study the impact of anthropogenic pollution on Arctic cloud properties. In addition, both sites are subject to pollution originating from mid-latitudes and Alaskan forest fires.
To study aerosol-cloud interaction, we use airborne and ground based observations. The latter are obtained from the long-term aerosol data set at NSA, as well as from the recently installed aerosol samplers at OLI. These aerosol observations are combined with cloud properties retrieved using ground-based remote-sensing instruments such as radar, radiometer, and lidar. Initial results suggest an increase in ice water content during periods with less aerosol loading near the surface. In situ aircraft observations obtained in summer, 2015 during the ARM Airborne Carbon Measurements (ARM-ACME-V) campaign are compared for both Arctic ARM sites. This allows characterization of the impact of local anthropogenic pollution on cloud properties such as phase, liquid water content, and droplet size for similar synoptic and surface conditions. We found that local pollution leads to generally smaller cloud droplets at OLI.
In addition to these observational efforts, we are conducting numerical simulations using the Weather Research and Forecasting (WRF) model, run in Large-Eddy Simulation mode. Given the differing aerosol indirect effects of cloud droplet size distributions and cloud ice size distributions, and the uncertainty in ice formation mechanisms, we use these idealized simulations to quantify the impact of the ratio between CCN and INP on the properties of cloud-driven mixed layers.
