SBIR-STTR Award

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 s

Stratus and stratocumulus clouds are them most common cloud type, covering approximately 30% of the Earth’s surface. Precise measurements of the size distributions of water drops and liquid water are needed to improve satellite retrievals and climate prediction models. Current airborne instruments are limited to measurements of individual drops, which must be cubed to compute liquid water content, compounding the sizing error, and limits sampling statistics. A new airborne technology, adopted from instrumentation used in industry, measures drop size distribution and liquid water content from an ensemble of drops. The new instrument, a cloud drop spectrometer, has 100 times the sample volume, the measurement is independent of airspeed, and is not fraught with the errors associated with measurements based on individual drops. The company designed and performed feasibility laboratory tests on the state-of-the-art instrument cloud drop spectrometer. A custom annular detector with 64 photoelements was designed and fabricated that senses the circular diffraction rings from a field of cloud drops. The instrument was set up in the laboratory and reticles with known dot sizes were used as surrogates for various mono-dispersed and poly-dispersed drop size distributions that cover the size range in stratus clouds. The dots exhibit the same optical diffraction rings as drops in a cloud. The drop size distributions from 2 to 200 microns and liquid water contents from 0.01 to 5 grams per meter cubed were retrieved using a sophisticated numerical algorithm and compared with the known values of equivalent drop size and liquid water content. The agreement between the actual values of equivalent drop size distributions and liquid water contents with retrieved values were excellent. We propose to build a robust airborne version of the cloud drop spectrometer and flight test it on the company’s Learjet research aircraft. We will also fabricate a smaller, light-weight version of the cloud drop spectrometer that can be installed on mid-size uninhabited aerial vehicles and tethered balloons. Improved measurements of drop size distributions and liquid water content in stratus clouds will improve predictions and help prepare societies for global climate change. This will be especially important for Polar regions, which are warming at twice the rate of the global average. Improved measurements will also benefit potential future efforts to “brighten” stratus clouds and reflect more sunlight by increasing the total drop concentration via sprays and other cloud modification tech