Climatology of Cold-Season Supercooled Liquid Water and Glaciogenic Cloud Seeding Potential in the Western United States, According to a 4-km Resolution Climate Reconstruction

The cold-season climatology of cloud supercooled liquid water (SLW) in the western United States is examined by means of a 4-km grid resolution, 43-yr dynamical climate reconstruction. This study is motivated mainly by the uncertainty of effectiveness of glaciogenic cloud seeding, intended to increase precipitation. A better understanding of the climatology of the presence and abundance of SLW is key to narrowing this uncertainty. It is found that SLW below 268C [cold enough for silver iodide (AgI)-based ice nucleation] is most common just upwind of the mountains in the Pacific Northwest and also rather common over the higher and drier mountains inland from there. SLW is often at levels very close to the ground over these mountains. SLW is far less common over the southwestern United States, with a downward trend over the 43-yr record. On the rare occasions that SLW is present over the Southern California mountains, the abundance of SLW tends to be exceptionally high. Much SLW is common just upwind of the near-coastal mountains in general. In these regions, SLW is distributed over a greater depth and is rarely found very close to the ground on account of a higher freezing level, compared to the inland mountains. The correlation of SLW with various environmental parameters is examined. Integrated vapor transport above the freezing level generally is a strong predictor for SLW, while the Froude number (a measure of flow blocking) is a poor predictor. Uncertainty related to model resolution and to the choice of cloud microphysics parameterization is acknowledged.