Ice Crystal Concentrations in Wave Clouds: Dependencies on Temperature, D>0.5 µm Aerosol Particle Concentration and Duration of Cloud Processing

Model formulas used to predict the concentration of heterogeneously nucleated ice crystals depend on combinations of cloud temperature, aerosol properties, and elapsed time of supersaturated-vapor or supercooled-liquid conditions. The validity of these formulas is questioned for many reasons. For example, there is concern that constraining measurements from ice nucleus counters may not be applicable to conditions within atmospheric clouds. In response to this concern, this work analyzes airborne measurements conducted both upwind of, and within, middle-tropospheric wave clouds. Level-flight measurements made near cloud base were used to derive 82 cloud parcel streamlines; four parameters were derived for each streamline: 1) minimum cloud temperature along the streamline, 2) aerosol particle concentration (diameter, D>0.5 µm) measured within ascending air, upwind of the cloud, 3) ice crystal concentration measured in descending air downwind of the water-saturated cloud region, and 4) the duration of water-saturated conditions along the streamline. The latter are between 55 to 425 s and the minimum temperatures are between -33 to -14 °C. Values of minimum temperature, D>0.5 µm aerosol concentration and crystal concentration were fitted to the formula developed by DeMott et al. (2010, D10); overall, there is reasonable agreement between fitted concentrations, derived here, and concentrations predicted using D10. However, clouds colder than average (<-25 oC), and which also ingested D>0.5 µm aerosol concentrations that were larger than average (>8 per standard cubic centimeter), contained ice at concentrations that were systematically larger than the prediction of the D10 formula.