Assessing the Rosemount Icing Detector with In Situ Measurements

In situ measurements of microphysics conditions, obtained during 38 research flights into winter storms, have been used to characterize the performance of a Rosemount Icing Detector (RID). Characteristics of the RID were determined under a wide range of cloud environments, which included icing conditions within mixed phase, freezing rain, and freezing drizzle environments. Cloud conditions observed included temperatures between 0° and −29°C and liquid water contents (LWCs) up to 0.7 g m−3. The detection threshold for LWC was found to be 0.007 ± 0.010 g m−3 for the RID operated at an air speed of 97 ± 10 m s−1, which agrees well with theoretical predictions. A signal level of 0 ± 2 mV s−1 accounted for 99.6% of the measurements in clear air and 98.5% of the measurements in glaciated clouds, when the data were averaged over 30-s intervals. No significant response to glaciated clouds was found during any of the research flights, implying that the instrument can be used to segregate glaciated and mixed phase clouds. There was no change in the RID response between liquid and mixed phase conditions, suggesting that ice crystals neither eroded ice accumulation nor accreted to the RID surface under the range of conditions experienced. During sustained icing conditions, a linear relationship between the RID signal and LWC was observed after the RID signal exceeded 400 mV above the clear-air signal level. The LWC derived from the RID was found to agree with LWC measurements from Nevzorov probes within ±50% for 92% of the data. The relationship between the RID signal and LWC was unchanged for freezing precipitation environments with drop median volume diameters >100 μm. The Ludlam limit was estimated for low LWC values and was found to agree well with theoretical calculations. The analysis provides considerable insight into the strengths and weaknesses of the instrument for operations in natural icing conditions.