Water Vapor, Condensed Water, and Crystal Concentration in Orographically Influenced Cirrus Clouds

Results are presented from measurements made with a counterflow virtual impactor (CVI) in cirriform clouds containing crystals with dimensions typically less than 30 μm. Independent measurements of crystal number concentration and cloud water content are presented, and for the first time consistent instrumentation is used to measure both the cloud condensate and water vapor. The experiment was conducted over the Alps where the vertical wind field was very persistent in the stable air, which suggests the presence of standing gravity waves. For the two flights presented in this paper, the relative humidity over ice within the cloud ranged from about 50% to 110%, and the cloud water content ranged from about 1 to 10 mg m−3. The number concentration was typically a few hundred crystals per liter, reaching peak concentrations of several thousand per liter. In general, the cloud water comprised less than 10% of the total water content. Often, the crystals were not detected at all by concurrent PMS-230X optical array probe measurements due to the small size of the crystals. These results add to the understanding of the properties of cirrus, particularly for the constituents of less than 30 μm in dimension.