Homogeneous ice nucleation and supercooled liquid water in orographic wave clouds

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. -from Authors