Ground-based vs. Aircraft Seeding (2)

Aircraft Seeding - Aircraft seeding with silver iodide (AgI) can be done via droppable flares or through combustion in place (wing-tip generators or burn-in-place flares).  Because of aircraft movement, the result is a “line source” of AgI across the wind, or a vertical curtain in the case of droppable flares.  In either case, atmospheric motions combined with ice particle fall speeds will produce a “curtain” of seeding effects whose eventual shape is determined by the transport and diffusion (T&D). Past observations have shown serious difficulties achieving adequate SLW cloud volume filling with aircraft-released line sources because of limited T&D upwind of mountain targets, but more research is needed in this area.

Furthermore, since the bulk of SLW is frequently confined to the lowest 3000 feet above the surface (page 1), it presents a problem for aircraft seeding.  Safety restrictions frequently prohibit aircraft operation that close to mountainous terrain. The FAA mandates a minimum flight level over mountains of 2,000 feet above the highest terrain within a horizontal distance of four nautical miles.  Icing conditions, especially during darkness, add to the concern.  There may be some mountain barriers where aircraft seeding might be worth consideration, particularly where ground seeding is not feasible (e.g., ground generators are prohibited in wilderness areas).  Such mountains should be relatively isolated with sufficiently warm upwind valleys, e.g., California's Central Valley.  This terrain permits aircraft to safely descend below the freezing level when airframe icing becomes excessive, or where they could remain well upwind where exposure to icing would be limited.  Aircraft seeding is substantially more expensive than ground seeding, so the value of water augmentation would need to be great  enough to justify such an option.

Gravity waves (page 1) frequently appear over mountains in winter, producing pockets of SLW at relatively high altitudes.  These phenomena may sometimes present opportunities for aircraft seeding, especially since they occur at altitudes too high for effective ground seeding.  This possibility was recently suggested by a feasibility study for a weather modification pilot project in Wyoming.  As related on page 1, however, these gravity waves and associated SLW are very transient and it is unclear if they can be located or predicted so as to be effectively seeded.   Moreover, any snowfall from seeding such high-altitude SLW will be unlikely to reach the ground in significant amounts, unless a downwind mountain range is located in a favorable position relative to the gravity waves.

Ground-based seeding - The seeding agent released from an AgI generator* represents a continuous point source that produces potential ice nuclei downwind.  If the AgI particles reach sufficiently cold SLW cloud, a plume of seeded ice crystals will result.  The AgI and/or seeded crystal plume spreads horizontally and vertically downwind, and the plume meanders, according to T&D conditions.  Physical evaluations that directly track AgI plumes using aircraft and ground-vehicle instrumentation, or that use co-released tracer gases and snowpack trace chemistry, have shown that plume spread is usually between 15-30° in the horizontal and less than 2,000-3,000 feet in the vertical.  The latter dimension matches well the typical depth of SLW in the mountains, suggesting that ground seeding can routinely reach the SLW if generators are otherwise properly sited.  What constitutes proper siting?  Given the relatively narrow horizontal plume spread and meander and local topographic influences on plume transport, it is quite possible that seeding plumes may frequently miss the target area (poor siting).  Proper siting should ensure that the target area is affected by the plume often enough with sufficient sizes and quantities of ice crystals, so as to produce significant precipitation.  To achieve these effects in the target, we recommend siting that produces routine overlap of  plumes from multiple generators.

Proper siting should also form seeded plumes that allow for sufficient ice crystal growth while crystals are transported through the SLW cloud zone, so as to drop precipitation over the target area before those crystals reach the mountain crest.  Otherwise the seeded crystals will descend with the airflow on the downwind (lee) side of the crest, with tiny SLW cloud droplets evaporating and ice crystals sublimating (to vapor) before snowflakes become large enough to fall as precipitation.  Larger seeded crystals may be transported downwind of the crest before sublimating, however, and these can continue growing by "chaining together" among themselves (aggregation) and with natural crystals and snowflakes.  Consequently, the area receiving additional snowfall from seeding may extend well downwind of the initial crest.  Indeed, studies have shown such seeding increases as far as 100 miles downwind of the expected target.  And, in the common case of secondary, parallel mountain barriers, seeded crystals may experience a second "growth spurt" in the SLW zone created by the downwind barrier.  In any event, siting is a complex process, which must be informed by current research and tailored to T&D conditions within a specific area.

Finally, research has shown that temperature inversions frequently form in mountain valleys during winter, even during the cloudy conditions of winter storms.  These inversions can inhibit T&D of seeding materials in the vertical, so that valley-based generators do not seed at altitudes of SLW cloud and thereby fail to affect the target area.  The frequency of such episodes of seeding material "trapping" is dependent on local climatology and terrain, but should be investigated in the process of generator siting.  If trapping is a persistent problem, higher-altitude generators, above the typical inversion top, are recommended.  

* Liquid propane (LP) dispensers can also produce significant quantities of ice crystals and seeding plumes.

Summary
The decision to use aircraft or ground seeding is complex and varies with many factors, including terrain, climatology, meteorology, and cost.  We attempt to summarize the salient factors with a simple table that compares the pros and cons of aircraft and ground seeding.  It is important to understand that, even within ground seeding approaches, "one size does not fit all."  For example, one may seed with lower-elevation, valley AgI generators, higher-altitude AgI generators, or even higher-altitude propane dispensers.  Within these options, one may choose between manual or remote-controlled generators/dispensers.

The primary sources for this information are:

1.  Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane by Super and Heimbach, September 2005

2.  Optimizing Cloud Seeding for Water and Energy in California by Hunter, March 2006

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