ABSTRACT

The prediction of ice crystal nucleation in clouds has been a long-standing problem in atmospheric science. The difficulties in identifying ice nuclei that initiate ice crystals in a cloud may be due to 1) lack of information regarding chemical composition and activation spectrum of ice nuclei, 2) the multiple mechanisms by which the nucleation might occur, and 3) the limits of past instrumentation in estimating the number of the smallest ice particles. Past observations often exhibit a discrepancy: far fewer ice nuclei are observed than ice crystals, and especially so in maritime clouds at higher temperatures. Laboratory experiments and observations in clouds suggest that secondary ice production (where multiple ice crystals are produced by the activation of a single ice nucleus) may act under certain conditions, sometimes explaining the enhanced number of ice crystals, but not always. This deficiency of knowledge propagates into uncertainties in the prediction of precipitation in mixed phase clouds, where liquid and ice particles interact in a myriad of ways to form precipitation.

The objective of this research is to acquire new knowledge regarding primary and secondary ice crystal nucleation in maritime clouds, considering the influence of the warm rain process as a leading explanation for the first ice, and the high number concentrations of ice crystals sometimes observed in the past. Other candidate hypotheses to be explored include enhanced ice nucleation in evaporation zones
resulting from entrainment, additional ice nuclei supplied by intrusions of desert dust, and artificial enhancement of ice crystals by passage of a research aircraft through the clouds.

Intellectual merit. In this study, an unprecedented dataset will be collected that will have a detailed documentation of ice nuclei and the earliest appearance of the first small ice particles in maritime cumuli. Then the data will be analyzed considering both the dynamical and microphysical evolution of the clouds. High-resolution 3D numerical cloud simulations and Lagrangian microphysical calculations will also be conducted, critical for differentiating among the hypothesized ice nucleation mechanisms. The cloud dynamics control the temporal scales involved in ice crystal nucleation and growth, the transport of particles through the cloud, and the regions where different phases of hydrometeors can interact. The observations alone cannot capture the cloud motions and evolution in their entirety, making the numerical modeling essential to understand the evolution and transport of liquid and ice particles below and above the freezing level. Bulk (Eulerian) microphysics, including a 10-class ice scheme, will also be run in the simulations to test the ability of different microphysical processes to explain the observations. Finally, simulations initialized with and without desert dust will be compared to elucidate its effects on primary and secondary ice nucleation mechanisms.

Broader impacts. Numerous areas in atmospheric science will benefit from the understanding of ice crystal nucleation and its effects on convective precipitation, such as global and regional climate model predictions of clouds, numerical weather prediction of daily precipitation events, and predictions of tropical storms and hurricanes. Graduate students will benefit from participating in a field campaign, gaining experience in both observational analysis and numerical modeling, and presenting their research findings at scientific workshops and conferences. An outreach program conducted during ICE-T for university students at a local institution in the Caribbean will also benefit under-represented groups in atmospheric science.

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