Abstract
Glaciogenic cloud seeding has long been practiced as a way to increase water availability in arid regions, such as the interior western United States. Many seeding programs in this region target cold-season orographic clouds with ground-based silver iodide generators. Here, the “seedability” (defined as the fraction of time that conditions are suitable for ground-based seeding) is evaluated in this region from 10 years of hourly output from a regional climate model with a horizontal resolution of 4 km. Seedability criteria are based on temperature, presence of supercooled liquid water, and Froude number, which is computed here as a continuous field relative to the local terrain. The model’s supercooled liquid water compares reasonably well to microwave radiometer observations. Seedability peaks at 20%–30% for many mountain ranges in the cold season, with the best locations just upwind of crests, over the highest terrain in Colorado and Wyoming, as well as over ranges in the northwest interior. Mountains farther south are less frequently seedable, because of warmer conditions, but when they are, cloud supercooled liquid water content tends to be relatively high. This analysis is extended into a future climate, anticipated for later this century, with a mean temperature 2.0 K warmer than the historical climate. Seedability generally will be lower in this future warmer climate, especially in the most seedable areas, but, when seedable, clouds tend to contain slightly more supercooled liquid water.
Significance Statement
Cloud seeding has long been practiced commercially as a way to increase water availability in arid regions, such as the interior western United States. Many seeding programs in this region target cold-season clouds over mountains with ground-based silver iodide generators. Here, we use 10 years of hourly output from a well-calibrated regional climate model with a horizontal resolution of 4 km to determine how often suitable clouds are present over mountain ranges in the interior western United States. We find that suitable clouds are present 20%–30% of the time over many mountain ranges in the cold season, with the best locations just upwind of crests, especially over ranges in the northwest interior and greater Yellowstone area. Mountains farther south are less frequently seedable, but when they are, the seeding may be relatively more effective in producing extra precipitation—although this study does not attempt to quantify that extra precipitation. In a future climate, anticipated for later this century, seedable clouds become less common in the interior western United States, but cloud seeding may be slightly more effective.
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