A Novel Instrument for Measuring Drop Size and Liquid Water Content

Accurate measurement of the liquid water content and size distribution of water drops in clouds is a fundamental problem that crosses multiple disciplines in atmospheric science, including cloud physics, precipitation, satellite retrievals of cloud properties and climate prediction models. Current instruments used for measuring cloud liquid water content and drop size from aircraft rely on sizing individual cloud drops, which has inherent limitations due to the limited sample volume required by optical constraints. This results in a limitation of the size of drops that can be measured to about 50 microns diameter, and creates errors due to coincidence when more than one drop is in the optical viewing area. The novel instrument proposed here, called a cloud drop spectrometer, makes measurements of drop size and liquid water content from an ensemble of cloud drops, providing a sample volume that is orders of magnitude larger than single-drop measurements employed by current technology. In addition, the measurements are independent of airspeed and extend out to 200 µm, which overlaps far into the size range of optical imaging probes, which are typically used to measure larger diameter drops. Also, the cloud drop spectrometer independently measures liquid water content from an ensemble of drops. In contrast, liquid water content is computed from single-drop devices by cubing the size of individual drops, thereby also cubing the measurement error. The Phase I research will focus on fabricating a prototype cloud drop spectrometer that will be evaluated in the company’s calibration laboratory. In Phase II, SPEC will fabricate two versions of the cloud drop spectrometer: A robust version capable of being installed on a jet research aircraft, and a lightweight, low-power carbon composite version that can be flown on small uninhabited aerial vehicles and tethered balloon systems. The cloud drop spectrometer will take advantage of the company’s previous experience developing microphysics probes for research aircraft, and advances in electro-optics that facilitate miniaturization of computers and signal conditioning. Improvements to climate prediction models require better measurements of the properties of clouds, particularly stratus clouds that cover much of the Earth’s oceans and Polar regions. The cloud drop spectrometer will measure the complete drop size distribution from 2 to 200 microns and independently measure liquid water content, providing much needed improvement in measurements of cloud radiative properties that are a cornerstone of climate prediction models. The cloud drop spectrometer will also find applications in other disciplines, including aircraft icing certification in supercooled large drops, measurements of fogs, industrial, agricultural, and snow-making sprays.