Homogeneous Ice Nucleation and Supercooled Liquid Water in Orographic Wave Clouds

This study investigates ice nucleation mechanisms in cold lenticular wave clouds, a cloud type characterized by quasi-steady-state air motions and microphysical properties. It is concluded that homogeneous ice nucleation is responsible for the ice production in these clouds at temperatures below about −33°C. The lack of ice nucleation observed above −33°C indicates a dearth of ice-forming nuclei, and hence heterogeneous ice nucleation, in these clouds. Aircraft measurements in the temperature range −31° to −41°C show the following complement of simultaneous and abrupt changes in cloud properties that indicate a transition from the liquid phase to ice: disappearance of liquid water; decrease in relative humidity from near water saturation to ice saturation; increase in mean particle size; change in particle concentration; and change in temperature due to the release of latent heat. A numerical model of cloud particle growth and homogeneous ice nucleation is used to aid in interpretation of our in situ measurements. The abrupt changes in observed cloud properties compare favorably, both qualitatively and quantitatively, with results from the homogeneous ice nucleation model. It is shown that the homogeneous ice nucleation rates from the measurements are consistent with the temperature-dependent rates employed by the model (within a factor of 10°, corresponding to about 1°C in temperature) in the temperature range −35° to −38°C. Given the theoretical basis of the modeled rates, it may be reasonable to apply them throughout the −30° to −50°C temperature range considered by the theory. The absence of convincing liquid water detection in past in situ cloud studies at temperatures below about −36°C is found to be the result of instrument detection thresholds. Evidence that droplets can exist, under liquid-subsaturated conditions, at temperatures at least as low as −40.7°C is presented. The findings are also used to discuss ice production via homogeneous nucleation in cirrus clouds. Relative humidity measurements from a newly developed airborne cryogenic frostpoint hygrometer were found to be reliable and crucial to studies of homogeneous ice nucleation.