The Alpenglow Instruments All Phase Water Probe - Instrument Development and Preliminary Results

Description

Alpenglow Instruments LLC, supported by Phase-I and II funding from the DOE SBIR program, is developing a Raman scattering-based in situ probe for simultaneous measurement of ice water content (IWC), liquid water content (LWC), and specific humidity. Raman scattering is measured at three wavelengths chosen to optimize retrieval of LWC, IWC, and specific humidity (a fourth channel, corresponding to Raman scattering from N2 is used to maintain internal calibration). This probe is intended to provide in situ measurements in cloud from an airborne platform with a high sample volume and free from the difficulty of capturing and evaporating ice particles in order to measure IWC. In contrast to lidar applications, the APWP measures in close proximity (~ 3 cm) of the instrument itself as a result of a sample volume defined by the intersection of the transmitted laser beam and receiver field of view which intersect at an angle of ~ 60 starting approximately 5 cm from the face of the instrument. The receiver field of view combined with the diameter of the transmitted laser beam results in an instantaneous sample volume on the order of 250 cm3. The proximity of the sample volume to the probe itself allows a compact, low-power laser source to be used. An Explorer One 355 nm laser (Spectra-Physics) is used. This is an air-cooled, diode pumped laser with a pulse energy of 80 J at a repetition rate of up to 10 kHz. As a result of the small size of the laser, which consists of a single head unit 165 x 95 x 76 mm in size, the APWP has been designed to fit in a 125 mm diameter PMS cannister – the de facto standard for mounting in situ probes on manned research aircraft. The laser itself, along with the optical detectors for the four channels, fits within the PMS cannister. The probe optical head which houses the beam expanders and mirrors for the transmitted beam as well as the receiver optics and light distribution, extends ~30 cm forward from the canister with the sample volume located directly beneath. The APWP is currently undergoing bench and environmental testing in preparation for deployment on the University of Wyoming King Air in late-spring 2019. Probe optical and mechanical design along with technical challenges encountered in the development of the APWP will be presented. Results from bench and environmental testing along with preliminary results from the King Air deployment will be presented.