Numerical Study of the Riming Growth of Graupel

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The growth of ice particles by riming is one of the central problems in cloud physics and has great impacts on many other atmospheric processes. This is the dominant process that forms graupel and hail. It occurs in deep convective clouds and it influences the further development of such clouds due to its thermodynamic and dynamic effects. It is also one of the most important processes leading to the electrification of thunderclouds. Quantitative knowledge of this process is necessary for accurate weather and climate predictions. Despite the critical importance of the riming growth process, there are only limited field and laboratory experimental data on riming growth available, and only very simplistic theoretical treatments of the process. In order to increase understanding of the process, a numerical study will be conducted on the growth of ice particles by accreting supercooled droplets, starting from a pristine ice crystal and following its growth to become a graupel. Two numerical models will be developed, one for simulating the fall behavior of the falling ice particles while accreting droplets (hence changing its shape); the other for determining the collision efficiency of the supercooled droplet hitting the ice particle. The first model involves solving numerically the relevant Navier-Stokes equations to obtain the flow field around the falling particle. Using these flow fields, solutions will be found to the equations of motion of supercooled droplets around the falling ice particle so as to determine their trajectories and hence their collision efficiency. Computations of the ventilation coefficients of the falling rimed particles will be performed as well. These numerical models will be used to perform computations to determine the collision efficiencies for a wide range of ice crystal/droplet size combinations and atmospheric conditions. This will yield an extensive data set of quantitative collisional growth rates of graupel in different conditions. The intellectual merit of the proposed work is that the results from this study will fill a large gap in available knowledge in this fundamental area of cloud physics and provide necessary quantitative information on how the riming growth process operates in clouds. Such information can be used to design better graupel riming parameterizations for use in storm-scale numerical models to understand better the convective cloud dynamics. It can also be used by larger scale weather/climate models to improve their large scale precipitation parameterizations. The experience gained and the numerical flow fields obtained from this study also will also be useful for the future study of ice-ice collisional growth. The broader impacts of the results of the study include their use to enhance our capability to predict weather and climate evolution through the use of numerical prediction models. The research activity will help train graduate students and young scientists who will contribute to atmospheric science research. The research results will be disseminated in scientific journals, meetings, and public lectures so that wider audience becomes aware of them. Recent research results are regularly incorporated into survey-level courses (aiming at college freshmen) by the principal investigator. They convey to the younger generation the excitement of scientific discoveries and their great contributions to society.