Highly accurate measurements of cloud dropletsusing the Small-Angle Light Scattering Spectral Analyzer (SALSSA) probe

Accurate measurements of cloud microphysical properties are critical for improving numerical weather and Earth system models. Airborne measurements of properties critically important to improving model accuracy, such as drop size distributions and liquid water content, have historically suffered from significant uncertainties and large discrepancies between instruments. To address these weaknesses, DOE has requested development of an instrument for airborne platforms which greatly improves resolution and accuracy of drop size distribution and liquid water content measurements over current techniques. In response to this call, this project will develop and test a novel laser-based optical technique which takes advantage of the angular dependence of single particle scattering, rather than the integrated scattering intensity approach used by current instrumentation. This approach avoids the uncertainties inherent in integrated scattering approach caused by non-monotonic response to droplet size, instead relating relative intensities captured at high angular resolution to particle size, leading to a greater than 80% reduction in uncertainty over existing techniques. Multiple laser wavelengths will be used to further enhance the sizing accuracy. In Phase I, a breadboard optical system will be designed and built, then tested using NIST- traceable size standards and precisely sized water drops produced by a piezoelectric droplet generator. Test results will be analyzed using specialized software to validate instrument sizing response and determine overall accuracy. In future phases, the technique will be engineered and refined for deployment on airborne platforms (manned aircraft, UAV, balloon). Given the improvements this approach offers over existing instruments, which currently provide a ‘core’ measurement for airborne platforms, instruments using this approach will be adopted on research aircraft worldwide, in almost all mission profiles.