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

Abstract

Results are presented from measurements made with a counterflow virtualimpactor (CVI) in cirriform clouds containing crystals with dimensionstypically less than 30 mum. Independent measurements of crystal numberconcentration and cloud water content are presented, and for the firsttime consistent instrumentation is used to measure both the cloudcondensate and water vapor. The experiment was conducted over the Alpswhere the vertical wind field was very persistent in the stable air,which suggests the presence of standing gravity waves. For the twoflights presented in this paper, the relative humidity over ice withinthe cloud ranged from about 50% to 110%, and the cloud water contentranged from about 1 to 10 mg m-3. The number concentration was typicallya few hundred crystals per liter, reaching peak concentrations ofseveral thousand per liter. In general, the cloud water comprised lessthan 10% of the total water content. Often, the crystals were notdetected at all by concurrent PMS-230X optical array probe measurementsdue to the small size of the crystals. These results add to theunderstanding of the properties of cirrus, particularly for theconstituents of less than 30 mum in dimension.