Hurricane Mitigation with Surfactants

Jonathan I. Katz [my last name]@wuphys.wustl.edu

Hurricanes are powered by the latent heat of condensation of water evaporated from the surface of warm tropical oceans. They are heat engines driven by the temperature difference between the warm ocean surface and cool air at high altitudes. When a hurricane travels over land or over cooler water it rapidly loses strength. The reason for this is that the evaporation rate from land or cool water is much less than that from warm water. The power available to a hurricane depends on the temperature difference but is also proportional to the rate of evaporation.

If the ocean surface in the path of a hurricane could be cooled it would reduce the strength of the hurricane. This is probably true, but it would be very difficult to cool the ocean surface. The upper (approximately) 50 meters of ocean are rapidly mixed and its heat capacity is enormous. If a thin film at the very surface were cooled (as it is naturally by evaporation) it would immediately mix (it would be convectively unstable) with the much deeper mixed layer, and the surface temperature would return to its warm value; evaporation also increases the salinity of the surface film which also drives mixing.

It has been suggested that deep cold water could be mixed into the warm mixed layer by introducing air bubbles that would make the cold water buoyant. Unfortunately, if the bubbles are small they will dissolve rapidly (the water is saturated with respect to air at one atmosphere, and undersaturated with respect to the air in the bubbles, which is at several atmospheres' pressure at depths of tens of meters). If they are large enough not to dissolve rapidly, they will rise through the water column and be lost rapidly. This may be readily demonstrated quantitatively.

It is also readily estimated that the energy required to mix the deep cold water with the warm mixed layer over the width and length of a hurricane track, is prohibitive. This energy must be supplied by pumping air into the bubbles. Alteratively, mechanical stirring might be considered, but the energy problem remains, in addition to the difficulty of ensuring that stirring actually mixes warm with cold water. Simple experiments with a household aquarium containing dense and less dense water (most easily accomplished by dissolving sugar to make a dense solution; the interface is then readily visible) show that once a stable structure is set up it is remarkably hard to mix.

Cooling the ocean surface is therefore probably not feasible. Instead, I therefore suggest that the evaporation from the warm ocean surface be reduced by applying surfactants. This is a very old idea. Even a monolayer (typically 0.0000001 cm, or 0.0000001 gm/square cm) of surfactant has been demonstrated to reduce evaporation from reservoirs by a large factor. Its chief drawback is that the surfactant is rapidly dispersed by the wind. However, surfactants may be effective when only temporary coverage (perhaps one day) is required while the hurricane passes.

To cover a swath 100 km wide and 1000 km long with a monolayer of surfactant would require about 100 tons; ten monolayers (to allow for replenishment as the initial monolayer is dispersed by wind and waves) would require about 1000 tons. These are not large quantities; the surfactants are common industrial chemicals that cost a few dollars per pound (or less than $10,000,000 for the quantity required, a tiny fraction of the damage done by a hurricane striking populated land).

Surfactant could be distributed from dispensers on the ocean bottom, where they would not be affected by wind or waves. These dispensers would contain a ``controlled leak'', an aperture designed to release the contents over the duration (perhaps a day) of a hurricane's passage. The containers might be dropped from aircraft along a hurricane's predicted track immediately before its passage. Alternatively, they might be distributed by ships across the continental shelf well in advance of hurricane season, with a valve that opens when commanded by an acoustic signal.

Would this method significantly reduce the strength of a hurricane, and how much surfactant would be required? Would wind and waves disperse the surfactant before it could have a significant effect? Only experiment can answer these questions.



Jonathan Katz
Thu May 13 12:39:11 CDT 1999