Atmospheric and climate scientists have long sought to understand the details of processes involved in how ice particles form in clouds. The number and size of ice particles in a cloud is important to the cloud’s lifetime, how it forms rain, and its impact on the Earth’s energy budget. However, scientists do not understand complicated ice formation as well as they understand less complex, water drop formation. Mixed-phase clouds, in which ice and water particles both exist at the same time, are particularly complex. This study used six years of radar data from Alaska to study ice formation events in mixed-phase clouds.
One type of ice formation is “secondary ice” production. This refers to ice particles that are created in clouds from already existing cloud particles. The data in this study showed that secondary ice events are more likely to occur in the presence of drizzle droplets. Drizzle droplets are smaller than raindrops but larger than cloud droplets. The secondary ice production is likely due to drizzle droplet freezing and then shattering. These surprising results show that drizzle shattering is more important to secondary ice formation than previously realized from laboratory data. The team observed that when secondary ice events occur, they can increase the number of ice particles in a cloud by a factor of 1,000. These results provide critical data for improving simulations of mixed phase clouds in climate and earth system models.
Researchers used the rich information content of multiyear, dual-polarization, radar Doppler spectra observations made at DOE’s Atmospheric Radiation Measurement user facility site in Utqiaġvik, Alaska to obtain a robust, statistical understanding of secondary ice production. The scientists used detailed radar spectra data, which provide information on particle shape, type, and fall velocity. They identified the presence of “rimer” particles (falling ice particles on which new ice may form and break off) and drizzle droplets (large liquid droplets).
In the presence of rimer particles alone, the data show fairly low ice multiplication within slightly supercooled clouds. This result suggests limited efficacy of the well-known rime-splintering or Hallet-Mossop process. However, the results show strong dependence of secondary ice production on increasingly large drizzle droplets, suggesting that the lesser-studied freezing fragmentation process is very efficient in these clouds. Cases with the presence of both fast rimer particles and drizzle drops had slightly greater secondary ice production over those with drizzle alone. The results indicate that ice number concentration can be enhanced by as much as 1,000 times during secondary ice production events. Although secondary ice production was observed to occur less than 10 percent of the time during the 6-year period in the study, it can have a significant effect on ice number concentration within a local region once it does occur.