Polarimetric Retrievals of Cloud Droplet Number Concentration: Towards a Better Understanding of Aerosol-Cloud Interactions

A longstanding source of uncertainty within the climate system is our understanding of clouds and their response to aerosols. The resulting cloud optical property changes constitute the largest uncertainty in our understanding of 20th century climate change. Central to being able to monitor and better understand the effects aerosols composition, size and concentration have on cloud reflectivity are accurate observations of the cloud droplet number concentration. Cloud droplet number concentrations couple aerosol properties to changes in cloud brightness. In the first portion of this dissertation, I present the development and evaluation of two techniques for observing cloud properties. The first is a new method of observing cloud droplet number concentration that uses polarimetric measurements and requires relatively few assumptions. The theoretical derivation is first presented followed by a method of implementation using NASA’s airborne Research Scanning Polarimeter (RSP). I use data obtained during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). Comparing cloud droplet number concentration retrievals with in situ measurements made by a cloud droplet probe during NAAMES shows strong agreement between measurements over a range of meteorological conditions and cloud types. Multilayered clouds are ubiquitous within Earth’s atmosphere, yet detecting their presence and height has been a longstanding challenge for passive remote sensing instruments. Retrieving the cloud top height is also an important part of the droplet concentration retrieval, and detecting the presence of multilayered clouds supports interpreting results. For this second technique, I present an assessment of RSP cloud top height retrievals, which are based on the concept of parallax. By comparing RSP cloud top height retrievals to the Cloud Physics Lidar (CPL), the technique is found to be capable of determining the presence and heights of up to three cloud layers, which is innovative for a passive remote sensing instrument. A second element essential to addressing the uncertainty in cloud’s response to aerosols is to better understand processes and drivers of cloud properties. Air-campaign studies offer opportunities to study high temporal and spatial resolution measurements that are needed to better understand the complex processes between aerosols, clouds and meteorological properties. My final investigation uses the two developed cloud property retrievals, in conjunction with other in situ and remotely sensed data, to undertake a broad investigation quantifying connections observed between aerosols, clouds and meteorology. I find a well- defined link between cloud microphysical property changes and marine biogenic aerosol concentrations. Changes in cloud properties are consistent with the Twomey effect, whereby an increase in cloud condensation nuclei is associated with increases in droplet concentrations and decreased droplet sizes. I also observe complex, non-linear secondary effects of aerosols on clouds such as cloud thinning and decreased droplet distribution width. I conclude this study by integrating my findings and discussing plausible linkages between aerosol, cloud and meteorological properties within the context of existing concepts