Assessing the Collection Efficiency of Natural Cloud Particles Impacting the Nevzorov Total Water Content Probe

The Nevzorov Total Water Content Probe has been used extensively for characterizing the cloud water content (ice and liquid) in clouds where airframe icing and engine related problems might occur. Because of recent work done in the ice simulating wind tunnels, where a significant fraction of ice particles were observed to be ejected from the sensor instead of captured and evaporated, it is suspected that the Nevzorov Total Water probe and other similar hot-wire sensors are providing underestimates of ice water content and possibly even liquid water content in the presence of large drops. In-flight tests were performed in December 2004 where natural ice particles (dendritic ice crystals and aggregates) and water drops were photographed impacting a specially mounted Nevzorov probe installed on the NRC Convair-580. A high speed camera captured small fragments of ice particles and water drops being swept out of the collecting cone of the total water sensor after impact. It appears that the Nevzorov probe is detecting a decreasing fraction of the mass of individual particles (solid and liquid) with increasing particle size, although this effect could not be quantified with the available data. It should be emphasized that even when ice crystal shattering was occurring with subsequent loss of mass, the Nevzorov probe was still detecting a fraction of the ice. One of the interesting observations was the apparent difference in the visual impression of natural particle impacts during the flight tests versus simulated ice impacts in the wind tunnel tests. Particles did not “bounce” out of the sensor as they appeared to in the tunnel experiments, but appeared to shatter into small fragments, some of which would be ejected from the sensor in a complicated manner. This difference is postulated to be due to the higher density of simulated ice particles in the tunnel. The current tests documented impacts with natural cloud ice particles that would be classified as “fragile” and usually with a low density. The results suggest that both wind tunnel and insitu tests are needed to fully describe probe performance in ice particle environments.