An aircraft-mounted instrument for high- resolution, in situ measurement of the abundances of liquid water and ice in clouds is undergoing development. This instrument is intended to overcome the dis- advantages of instruments developed previously for the same purpose. The disadvantages include various combinations of complexity, dependence on heaters and/or pumps, insensitivity to ice crystals, or dependence on droplet/crystal size. The present instrument is relatively simple, does not include a heater or a pump, and is expected (when fully developed) to be sensitive to both water droplets and ice crystals of any size.

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The Air-Sampling Probe feeds an aerodynamically heated sample stream to the surface-acoustic-wave hygrometer.

In principle, the fully developed version of the instrument would contain (1) a first air- sampling probe based a total-temperature probe, (2) a second air-sampling probe that would reject water droplets and ice crystals, and (3) two fast-reading electronic hygrometers — one for each probe. A simplified prototype of the instrument has been built from a modified commercial total-temperature probe and a surface-acoustic-wave hygrometer (see figure).

By virtue of its total-temperature feature, the first air-sampling probe would bring the incident airflow to rest (except for a slow internal sampling flow), thus raising the temperature of the sampled air. This aerodynamic heating would cause entrained water droplets and ice crystals to evaporate. The sample stream containing the water vapor generated by aerodynamic heating would be routed to the associated first hygrometer, which would thus be made to measure the total water content (vapor, liquid, and ice) of the ambient air.

The sample stream from the second probe would be routed to the associated second hygrometer, but, unlike the sample stream from the first probe, this stream would not contain the vapor from the water droplets and ice crystals; that is, its vapor content would consist solely of the ambient-air vapor. Then one could subtract the vapor reading of the second hygrometer from the vapor+liquid+ice reading of the first hygrometer to determine the liquid+ice content.

Of course, the cloud-water-content determinations made by use of the instrument depend largely on the assumption that all of the droplets evaporate and all of the ice crystals sublimate in the first probe as a result of aerodynamic heating (and, possibly, of impact upon the probe wall). It is planned to assess the validity of this assumption in tests in a wet wind tunnel.

This work was done by Flavio Noca and Michael Hoenk of Caltech for NASA’s Jet Propulsion Laboratory.