Resolving Aerosol Impacts on Drizzle
Climate models show a large range of potential changes in warm boundary layer cloud coverage, cloud microphysical properties, and corresponding feedbacks to surface temperature. One reason for this large intermodel spread is uncertainty in how aerosol particles modify the properties of warm clouds. A key question is the impact of aerosol particles on drizzle and precipitation formation. To date, metrics of these impacts have not shown consistent agreement among observations and simulations.
A team of U.S. Department of Energy-supported researchers used measurements from two Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) field campaigns to study aerosol impacts on drizzle formation in marine and continental conditions. The team extensively evaluated probability of precipitation and the response of cloud-base drizzle rate to liquid water path and the number concentration of cloud condensation nuclei (NCCN), a proxy for aerosol amounts. The study led to several key findings. First, both cloud-base drizzle rate and the probability of precipitation significantly increased with liquid water path and decreased with NCCN, supporting the concept of drizzle suppression by increasing aerosol amount. Second, the rate of change in probability of precipitation with increasing aerosol concentration (referred to as SPOP) agreed with results from a high-resolution aerosol-climate model, and both showed decreases at high liquid water paths. This decrease in the response to aerosols at high water paths indicates that autoconversion processes (which dominate rain formation at low liquid water paths) are more sensitive to aerosol perturbations than accretion processes (which dominate at high liquid water paths).
Finally, the study highlights differences in the SPOP value among observations from ground-based, aircraft, and satellite platforms and numerical simulations. SPOP values from the AMF data are similar to those from aircraft observations and higher than those from satellite data, indicating that the coarser-resolution satellite observations may not be the appropriate scale for evaluating these interactions. SPOP values from a high-resolution (4 km) climate model, which treats aerosol effects explicitly using an embedded cloud-resolving model, agree well with the AMF values, indicating that this model may be capable of representing aerosol-cloud-precipitation interactions well and can be used for further studies to examine the impact of these processes on climate.
Mann, J. A. L., J. C. Chiu, R. J. Hogan, E. J. O’Connor, T. S. L’Ecuyer, T. H. M. Stein, and A. Jefferson. 2014. “Aerosol Impacts on Drizzle Properties in Warm Clouds from ARM Mobile Facility Maritime and Continental Deployments,” Journal of Geophysical Research – Atmospheres 119(7), 4136-48. DOI:10.1002/2013JD021339.