Analysis of Precipitation Augmentation Potential in Winter Orographic Clouds by Use of Aircraft Icing Reports

Aircraft icing reports from northern Utah and southeastern Idaho along with radiosonde and precipitation data for six winter seasons are utilized in an analysis of precipitation augmentation potential in winter orographic clouds. According to this analysis clouds with top temperatures warmer than a critical value are primarily composed of supercooled water and colder clouds are primarily composed of ice. This critical cloud-top temperature varies from ∼−20°C at zero to 10 m s−1 cross-barrier wind speeds (measured near the altitude of the barrier crest) to about −26°C at cross-barrier wind speeds between 10 and 20 m s−1. Deep convective clouds are excluded from the analysis. It is concluded that the critical cloud-top temperature is governed both by the temperature dependence of active ice nuclei, and the time available for glaciation, which is directly related to the cross-barrier wind speed. For cloud-top temperatures warmer than the critical value, the time available for glaciation is of secondary importance. The primary factor in this case is the cross-barrier wind speed, which determines 1) the critical cloud top temperature, 2) the amount of supercooled water, and 3) the cross-barrier flow of supercooled water. Thus, the precipitation augmentation potential depends approximately upon the square of the cross-barrier wind speed when the cloud-top temperature is warmer than a critical value which itself depends upon the cross-barrier wind speed. A cloud-seeding potential exists within the warm cloud-top temperature category at cross-barrier wind speeds ≳10 m s−1. Out of the total number of cases, 21% fall in this high-yield category.