Assessing Cloud-Phase Conditions

Abstract

In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 ?m, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L?1. In contrast, the FSSP measurements of the droplet spectra less than 30 ?m appeared free of ice-crystal contamination, independent of the ice-crystal concentrations observed. Glaciated cloud conditions always had FSSP-measured median volume diameters greater than 30 ?m and particle concentrations less than 15 cm?3, whereas similar measurements in entirely liquid-phase clouds were observed less than 4% of the time. Images of drops greater than or equal to 125 ?m in diameter, which were collected in warm clouds greater than 0°C, were used to calibrate geometric criteria, which were, in turn, used to segregate 2D images into circular and noncircular categories. It is shown that, on average, between 5% and 40% of ice crystals greater than or equal to 125 ?m in diameter will be classified as circular, depending on the particle size, with the percentage decreasing with increasing particle size. In liquid-phase clouds, between 85% and 95% of the 2D images will be correctly classified as circular for all particle sizes. At temperatures less than ?4°C, a Rosemount icing-detector threshold of 2 mV s?1, corresponding to a maximum LWC of 0.002 g m?3, was used to help to identify glaciated and nonglaciated conditions. A methodology for segregating liquid, mixed, and glaciated cloud regions, based on instrument responses to liquid and ice hydrometeors, was developed and applied to the CFDE dataset. The results were used to determine the relative frequency of liquid, mixed, and glaciated cloud conditions for the data collected during the two field projects. Approximately 40% of the in-cloud observations at temperatures less than 0°C were assessed as liquid phase. The fractions of mixed-phase and glaciated-phase conditions were 26% and 34% for CFDE I and 46% and 14% for CFDE III, respectively. Because the ice-crystal responses for each instrument depend on the aircraft sampling speed and the ice-crystal sizes and concentrations, the results may be limited to conditions similar to those in clouds in midlatitude winter storms. Regardless, the results may have application to several fields, including development of parameterizations for numerical modeling, precipitation formation, remote sensing, ice multiplication, radiative transfer, and aircraft icing investigations. Important implications for aircraft icing investigations are discussed.