Phase-Doppler Interferometry for High Efficiency Characterization of Cloud Droplets

Radiometric and cloud microphysical properties in boundary layer clouds is of importance in advancing our knowledge on how these properties regulate Earth's moisture and energy budgets and, hence, climate. The representation of clouds in even the most sophisticated climate models remains a challenge and, as a result, cloud properties represent the largest source of error in our estimates of climate sensitivity. Detailed, precise, and accurate measurements of cloud microphysical characteristics remain in short supply and high spatial resolution measurements of cloud properties from flight-based platforms would improve our understanding of cloud microphysics, ultimately resulting in a better representation of clouds in climate models. An advanced instrument based on phase-Doppler interferometry capable of measuring droplet size and velocity with very high accuracy is being developed to provide high spatial resolution measurements of cloud properties. This instrument has the unique advantage over competing instruments in that the issue of coincidence, when more than one particle resides in the sample volume, may be resolved. The overall objective is to increase the sample volume by multiple orders of magnitude without compromising measurement accuracy while providing 1-meter spatial resolution of cloud properties during in-flight acquisition. During Phase I, it was demonstrated that the described instrument is capable of simultaneously measuring droplet size and velocity for a wide range of sizes and velocities with very high accuracy. Increasing the sample volume by a factor of 30 to 100 times was investigated and found to be feasible. To avoid or minimize errors due to coincidence, an advanced signal detection approach along with signal parsing was proposed and evaluated. This approach allows the recovery of individual particle signals for coincident events. Algorithms to control the probability of coincidence and adapt the instrument to prevailing conditions were developed. Methods to characterize the effective probe volume and validate flux and liquid water content measurements were developed. During Phase II, methods to increase the probe volume to allow for high data rate acquisition will continue to be developed and refined and a breadboard optical system will be constructed for further testing. Probe volume will be characterized over a broad range of configurations and flux and liquid water content measurements will be validated for a complete range of conditions. A fully autonomous instrument that can be easily operated in a wide range of applications by users without prior knowledge of operating principles will be developed. The described instrument will allow characterization of microscale turbulence from tethered balloons. Other industries that rely on spray characterization will also benefit from this technology, including the development of anti-viral sprays, pharmaceutical inhalers, agricultural spray technologies, and inkjet printing technologies.