Assessing Secondary Ice Production in Continental Clouds Based on ARM Mobile Facility Synergistic Remote Sensing Observations

Abstract

Ice crystals play an important role in radiation and precipitation formation. However, the prediction of ice number concentration has proven to be problematic, because the predicted primary ice number concentration is often smaller than the observed by several orders of magnitude. The difference between the observed and the predicted number concentration has been explained by so-called secondary ice production (SIP), which includes hypothesized mechanisms like rime splintering, shattering of large frozen drop, and ice-ice collision breakup.
In the past, these mechanisms were assessed largely by airborne in-situ measurements from field campaigns. While these intensive field observations have generally supported the hypothesized mechanisms, it remains unclear which mechanism dominates; what the trigger requirements are; what their typical time scale is; how their production rate depends on ambient atmospheric conditions; and importantly, how they influence the subsequent precipitation and radiation. This calls for the need for long-term, frequent, robust observations for characterizing secondary ice production occurrences and processes.
To address the need, we will provide the most essential observable in studying secondary ice production, the number concentration of pristine ice, using advanced ARM radar measurements. The capability of multiple-wavelength, dual-polarization, and Doppler information makes it possible to reveal detailed ice particle properties. The various radar scan strategies further empower us to follow secondary ice production evolution and to quantify spatial variability of ice number concentration. We will use these observations to quantify the typical time scale and trigger requirements for each secondary ice production mechanism, and understand how well the three hypothesized mechanisms can explain observed secondary ice production events. The dependence of observed secondary ice production rates on atmospheric conditions will be formulated and used to assess the impact of secondary ice production processes on cloud-precipitation-radiation-dynamics interactions.
Results from this project will be of considerable interest to the observational and modeling communities, in particular those interested in mixed-phase cloud microphysics and dynamics. This project will also have a wider impact on our understanding and modeling of the radiative impact of secondary ice formation.