Satellite-based Investigation of Volcanic Ash-induced Ice Cloud Formation

Kurzfassung

There is evidence suggesting that volcanic ash can act as ice-nucleating particles (INP), supported by laboratory experiments and some atmospheric observations. However, largescale satellite-based validation of these findings is still limited. A key challenge in studying volcanic ash and ice clouds together lies in their contrary spectral signatures in the thermal infrared, which can introduce errors in satellite remote sensing retrievals when both cloud types are present. To address this, a nowcasting tool was developed to track volcanic ash and ice clouds detected by satellite retrievals (VACOS for volcanic ash and CiPS for ice clouds) and to finally relate temporally separated detections of both cloud types. Two optical flow methods, Dual TV-L1 and Farnebaeck, were tested and compared. These methods estimate the movement between consecutive frames, which is then used to predict future positions at various forecast times. The Farnebaeck method produced slightly better results for tracking volcanic ash, while both methods performed similarly for tracking ice clouds. The nowcasting tool, built using the Farnebaeck method, was applied to study the interaction between volcanic ash and ice clouds across five scenes from two eruptions (Eyjafjallajoekull 2010 and La Soufriere 2021). These scenes were chosen specifically for exhibiting behaviors relevant to this study. Results showed a decrease in volcanic ash pixels and an increase in ice cloud pixels over time, with the increase being more pronounced for ice clouds that originated from volcanic ash pixels in 4 of the 5 scenes. This suggests that volcanic ash may act as INP, facilitating ice formation. The temperature ranges observed in these scenes support conditions for heterogeneous nucleation through contact and immersion freezing. Additionally, the number concentration per area and effective radius of ice pixels increased over time in all scenes, with a higher increase for ice clouds that originated from volcanic ash pixels. There was a general trend of decreasing volcanic ash particle concentration with an increase in their effective radius, indicating that fewer but larger ash particles remained, further supporting the role of fine volcanic ash in ice nucleation. Statistical analysis of data from all five scenes also indicates an association between volcanic ash and ice clouds. However, these findings come with limitations due to potential errors in the nowcasting method and the retrievals used.