Clouds impact daily life over a range of scales from their impact on local weather and precipitation, to their influence on global climate as a result of their radiative properties. DOE through the Atmospheric Radiation Measurement program has been measuring cloud properties for over 25 years, primarily through suites of remote sensing instruments deployed at a number fixed and mobile sites. In situ aircraft observations, used for development and validation of remote sensing retrievals, are regularly carried out as part of this program. Measurement of ice water content in cloud, which is critical for understanding cloud radiative properties, is particularly challenging. Current approaches to measurement of ice water content suffer from a number of limitations and routinely disagree by 50% or more even under nearly ideal conditions. To address these limitations, the overall objective in Phase I is to develop an in situ probe for measuring ice water content, liquid water content, and water vapor mixing ratio via Raman spectroscopic techniques. Because the Raman scattering frequency shift is a distinct function of the scattering material, the scattering due to liquid water, ice, and water vapor are either completely independent or can be easily separated. Raman scattering will be measured on channels corresponding to the liquid, ice and vapor phases of water and a fourth for measuring scattering from nitrogen, which will be used to calibrate the system. Channels corresponding to scattering from liquid water will be chosen to minimize the contributions from the ice channel to the liquid channel and visa-versa while retaining sufficient signal strength. The prototype will consist of an annular sampling head containing an illuminated sample volume with light intensity enhanced through the use of a multiple-path cavity. The prototype design will give a sample cross section of ~300 mm2 corresponding to a sample rate of 30 L s-1 at a nominal airspeed of 100 m s-1. Phase I milestones include: the optical and mechanical design of the system; construction of the prototype; and demonstration that the signals from water vapor, liquid water, and ice can be detected, and that the signal from ice can be detected in the presence of liquid water. The design of the Phase I prototype, particularly that of the sampling head, will be informed by constraints on airborne instruments. In Phase II the instrument will be miniaturized and ruggedized so that it is suitable for operation onboard manned aircraft and Unmanned Aerial Systems. Current measures of cloud ice water content -- critical for understanding the impact of clouds on climate -- routinely disagree by 50% even under ideal conditions. A prototype instrument using Raman scattering to measure all phases of water in the atmosphere will avoid many limitations of current measurement approaches. Commercial Applications and
Benefits: It is anticipated that the prototype instrument, once developed through Phase II and beyond, will be widely adopted by throughout the airborne atmospheric research and commercial precipitation enhancement communities. Eventual deployment of this technology on commercial and military aircraft to warn of potentially hazardous icing and extreme ice water content conditions following extensive development, testing, algorithm development, and certification in Phase III and beyond is possible in collaboration with partners in the aerospace industry. Furthermore, the application of Raman spectroscopic techniques to in situ airborne measurements represents an innovative core technology that can be adapted to other measurements such as trace gasses and aerosol characterization.
