A Novel Instrument for Measuring Drop Size and Liquid Water Content

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 techniques.