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

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Abstract
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 second... Abstract 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 second...

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