A Major Artifact in Aerosol-cloud-interaction Studies Discovered from Azores Measurements

Li, Z., University of Maryland

Cloud-Aerosol-Precipitation Interactions

Convective Processes

Liu J, Z Li, and MC Cribb. 2016. "Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign." Journal of the Atmospheric Sciences, 73(11), 10.1175/jas-d-15-0364.1.


(a) Based upon ground measurements of aerosol number concentration and cloud properties, and (b) based upon Terra/MODIS Level 3 aerosol optical depth and cloud properties in the 1°×1° grid box closest to the site. The numbers above the bars in (b) are the number of samples that went into the calculation of the FIE in that bin. FIE: first indirect effect; LWP: liquid water path; LTS: lower-tropospheric stability.


(a) Based upon ground measurements of aerosol number concentration and cloud properties, and (b) based upon Terra/MODIS Level 3 aerosol optical depth and cloud properties in the 1°×1° grid box closest to the site. The numbers above the bars in (b) are the number of samples that went into the calculation of the FIE in that bin. FIE: first indirect effect; LWP: liquid water path; LTS: lower-tropospheric stability.

Science

Using 19 months of cloud and aerosol measurements made on Graciosa Island in the Azores, the response of marine boundary layer (MBL) non-precipitating cloud properties to changes in aerosol loading is examined. How meteorological parameters affect the diversity in the sensitivity of these MBL clouds to aerosol perturbations and the magnitude of the aerosol first indirect effect (FIE) is also investigated. These are important science issues in the study of cloud–aerosol–precipitation interactions.

Impact

A positive relationship between the FIE and atmospheric stability is derived from surface-retrieved cloud and aerosol information, contradicting the negative relationship found from satellite data. The contrast originates from differences in the observed cloud properties at different levels from satellite and surface sensors, which likely explains some major discrepancies in past studies concerning aerosol-cloud interactions.

Summary

By constraining the liquid water path (LWP) to a fixed range of values, the FIE is quantified by analyzing the relative susceptibility of the droplet effective radius to aerosol number concentration. FIE decreases as the LWP increases. Greater differences in the magnitude of the FIE are observed depending on atmospheric stability and vertical motion conditions. The magnitudes of the FIE calculated from surface-retrieved cloud properties are larger under more stable conditions, while the magnitudes of the FIE calculated from satellite-retrieved cloud properties show an opposite relationship. This happens because the satellite can only detect cloud droplet effective radii near cloud tops. The magnitude of the FIE changes more under ascending-motion conditions as LWP changes. It appears to be higher under ascending-motion conditions for clouds with low LWP and under descending-motion conditions for clouds with high LWP. The contrasting dependence of the FIE on atmospheric stability estimated from surface- and satellite-retrieved cloud properties implies that the dependence of the response of cloud properties to aerosol perturbations on thermodynamic conditions varies according to the approach used to retrieve cloud properties, an artifact that must be accounted for when studying aerosol-cloud interactions.