Sensitivity Simulations of Ice Production in a Super-Cooled Orographic Cloud during the SNOWIE Field Campaign

To better understand how silver iodide (AgI) seeding impacts wintertime orographic clouds and to improve the model capability to simulate such impacts, the Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) field campaign was conducted between January–March 2017 based in Boise, Idaho. The ninth Intensive Observing Period (IOP9) from SNOWIE detected super-cooled drizzling orographic clouds using remote sensing and in-situ instruments. This IOP is thus simulated by the Thompson-Eidhammer (TE) microphysics scheme as part of the Weather Research and Forecasting (WRF) model.

It is found that the default natural ice nucleation in the TE scheme was too active resulting more ice-phase particles in the simulated orographic clouds than what were observed. In order to reproduce the almost pure super-cooled liquid cloud and the extremely low cloud droplet concentration, several sensitivity simulations have been conducted. A hypothesis of long-range transport of moisture and scavenging of natural aerosols associated with this event is proposed to explain the observed cloud properties. The natural cloud structures and precipitation in these sensitivity tests will be analyzed and presented.