Low-Level Cloud Microphysical Properties under Different Air Masses at the ARM SGP site

Overcast low-level stratus cloud microphysical properties under different airmasses are examined at the DOE Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site during the period 2007-2016. The properties include cloud liquid water content (LWC), cloud-droplet effective radius (r_e), number concentration (N), and cloud optical depth (τ) retrieved from long-term ground-based measurements and Dong et al. (1998) parameterizations. For each selected single-layer stratus cloud case, the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back trajectory model is applied to track the airmass source regions. During the 10-yr period, most frequent trajectories came from the Gulf of Mexico (Southeast) and Canada (North) as well as trajectories that had originated from the North but hovered over the continental United States before reaching the ARM SGP site (North-South). Among these three sources of airmasses, one from the North is dominated by dry and clean air, which is represented by the low concentration of cloud condensation nuclei (CCN). The cases associated with air masses from the Southeast contained Gulf Coast pollution and biomass burning smoke from Mexico and Central America and had higher number concentrations of CCN. As for the North-South trajectories, despite of the relatively clean source region, the CCN concentration at the ARM SGP site was higher due to the entrainment of polluted air and moisture during airmass transport over the continental United States. The cloud microphysical properties have demonstrated a strong positive correlation with surface CCN, indicating that airmass type does have significant impact on the cloud microphysical properties. The aerosol-cloud interactions will be investigated based on the selected low-level stratus cases and tracked air masses associated with these time periods.