A Case Study of Cloud Seeding Impacts Using WRF-WxMod Simulations and SNOWIE Observations

The Seeded and Natural Orographic Wintertime clouds – the Idaho Experiment (SNOWIE) investigates the impact of glaciogenic cloud seeding from January through mid-March 2017 in the Payette Mountain region of Southwestern Idaho, resulting in 24 intensive observation periods (IOPs) with more than 200 hours of data collected. A recent modeling study by Xue et al (2022) on a well-observed case (IOP5) demonstrated that the cloud seeding model, WRF Weather Modification (WRF-WxMod), is capable of simulating and quantifying the glaciogenic seeding impacts on wintertime orographic clouds. Thus, this study aims to utilize WRF-WxMod to investigate another well-observed case and verify robustness of the module.

We focus on IOP19 (4 March 2017) and investigate cloud-seeding impacts through comparing WRF-WxMod simulations to observations from airborne and ground-based instruments. The instrument platforms include the University of Wyoming King Air aircraft (UWKA), two X-band dual-polarization radars, soundings, and surface instruments. From UWKA in-situ observation, there appeared two distinct cloud layers on the upwind side that merged over the furthest downwind portion of the line. Cloud-top generating cells in upper clouds are well detected through the Wyoming Cloud Radar (WCR) data. During the seeding missions, the seeding aircraft placed eight, northwest-southeast-oriented seeding lines solely using airborne ejectable flares with a total of 305 g of AgI released. Low-level clouds about 4 km deep were observed with cloud top temperatures of about -18℃ and liquid and ice water contents < 1 g m^-3 and < 0.6 L^-3, respectively.

In numerical models, several control (without seeding scenarios) experiments and seeding experiments (with seeding scenarios) have been conducted to investigate the seeding effects. The airborne AgI seeding of this case was simulated by the WRF-WxMod Model with different configurations including different driving datasets (ERA5, ERAI, NARR, CFS2), planetary boundary layer schemes (YSU and MYNN), and CCN values (climatology CCN and 30% climatology CCN). Preliminary results indicate that, through comparing WRF 900m simulations with sounding data, model has a good performance on simulating some variables including pressure, temperature, relative humidity and wind speed with Pearson Correlation values higher than 0.9. Compared with the UWKA in-situ measurements, the cloud droplet number concentrations and ice water content for all the flight legs agree well with simulation results driven by ERA5 with 30% climatology CCN values. In addition, precipitation enhancement has been observed near the Packer John radar site about one hour after the seeding materials ejecting. Therefore, applying the observation constrained approach on the 900 m sensitivity studies, a smaller member of the ensemble groups with better performance will be selected out to initialize the large eddy simulation (LES) to resolve finer-scale dynamic and microphysical characteristics such as the variations of different hydrometeors within the cloud-top generating cells in further studies.