Can Wintertime Orographic Clouds Be over Seeded by AgI?

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Tuesday, 6 January 2015: 4:00 PM
211B West Building (Phoenix Convention Center - West and North Buildings)
Lulin Xue, NCAR, Boulder, CO; and I. Geresdi, R. M. Rasmussen, S. A. Tessendorf, P. Holbrook, D. Blestrud, M. L. Kunkel, B. Glenn, and S. Parkinson

There are some observational and numerical evidences showing that clouds can be over seeded by introducing excessive amount of ice nuclei, such as in the case of volcanic irruption (Durant et al., 2008). So many tiny ice crystals were nucleated in the overseeded cloud that the available water vapor and liquid water cannot support any ice crystal to grow large enough to become precipitation. In this case, the overseeded cloud has lower precipitation efficiency than the natural cloud. Since AgI is used in wintertime orographic cloud seeding to enhance the snowpack, it is natural to ask the question: Can wintertime orographic clouds be over seeded by AgI?

A new bin microphysics scheme incorporating the AgI seeding mechanism has been developed recently based on Xue et al. (2013a,b) and Geresdi et al. (2014). The scheme is coupled into the Weather Research and Forecasting (WRF) model and calculates the evolution of the size distributions of water drops, pristine ice, snowflakes and graupel particles. Four modes of AgI nucleation, the immersion freezing, the deposition nucleation, the condensation freezing and the contact freezing, are also simulated in the scheme. Two-dimensional idealized WRF simulations will be performed to investigate how the increase of seeding material will impact the cloud microphysics and precipitation on the ground and how much of AgI is needed to over seed the cloud.

The detail analysis of the results will be presented on the conference.