Assessing the Winter Cloud Seeding Potential over the White Mountains of Arizona

Runoff from snowfall across the White Mountains of Arizona is the primary source of water and an important source of hydroelectricity for the city of Phoenix. The National Resources Conservation Services (NRCS) SNOTEL observations shows that the peak yearly median climatological snow water equivalent (SWE) observations across the higher terrain range from 178-mm (7.0”) to 243-mm (9.6”), and 294-cm (116”) of snow is annually observed at the local ski area. There is large variability in the year-to-year precipitation which seems to be tied to the ENSO phase. In addition, the majority of this precipitation is deposited during a few large storms.

To potentially help maximize this critically important snowpack, a winter cloud seeding feasibility study has been conducted. In this study four primary tasks are conducted and the results are presented.

Task 1 includes a 30-year assessment of climate data sets to determine the cloud seeding potential, storm frequency, snowfall and runoff. Next, in Task 2 a set of six winters covering a range of ENSO conditions is simulated using a 1-km horizontal resolution version of the WRF model centered over the high peaks of the White Mountains. The selected winter seasons represent wet and cold winters as well as winters with normal to above normal SWE. The hourly modeled temperatures, winds, stability, cloud microstructures are all characterized and used to create the hourly cloud seeding potential across each of the winters (December 1 – March 31).

In Task 3 a simulated cloud seeding generator network is sited across the mountains using the climatological and modeling analysis. Three multiday storms are selected for simulated seeding. Cloud seeding aerosols are released from the network and tracked as they interact with the model clouds over the White Mountains. The success of the seeding aerosols reaching the supercooled liquid water clouds are quantified. Finally in Task 4 the potential increased precipitation and runoff of a cloud seeding program over the White Mountains are estimated.