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rainband seeds

Rainband seeds

Willoughby et al.(1985)

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Subject: C5a) Why don’t we try to destroy tropical cyclones by seeding them with silver iodide:

Concentric eyewall cycles naturally occur in intense tropical cyclones (wind > 50 m/s [100 kt, 115 mph]). As tropical cyclones reach this threshold of intensity, they usually – but not always – have an eyewall and radius of maximum winds that contracts to a very small size, around 10 to 25 km [5 to 15 mi]. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. A concentric eyewall cycle occurred in Hurricane Andrew (1992) before landfall near Miami: a strong intensity was reached, an outer eyewall formed, this contracted in concert with a pronounced weakening of the storm, and as the outer eyewall completely replaced the original one the hurricane reintensified.

Actually for a couple decades NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide – a substance that serves as a effective ice nuclei – into the rainbands of the storms. The STORMFURY project , as it was called, proposed that the silver iodide would enhance the thunderstorms of the rainband by causing the supercooled water to freeze, thus liberating the latent heat of fusion and helping the rainband to grow at the expense of the eyewall. With a weakened convergence to the eyewall, the strong inner core winds would also weaken quite a bit. Neat idea, but it, in the end, had a fatal flaw: there just isn’t much supercooled water available in hurricane convection – the buoyancy is fairly small and the updrafts correspondingly small compared to the type one would observe in mid-latitude continental super or multicells. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called “concentric eyewall cycles”.

Contributed by Stan Goldenberg and Hugh Willoughby

Thus nature accomplishes what NOAA had hoped to do artificially. No wonder that the first few experiments were thought to be successes. To learn about the STORMFURY project read Willoughby et al. (1985). To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990).

—Leah Crane, Freelance Writer

When tropical cyclones pass over the land, they can be disastrous to coastal areas. Scientists can paint a fairly accurate picture of cyclones’ trajectories and intensities, but their ability to quantitatively predict the heavy rainfall that accompanies these storms is less advanced. The quantitative forecasts require a more precise understanding of the physics of tropical cyclones, down to the size of a single raindrop.


Crane, L. (2016), Reading raindrops: Microphysics in Typhoon Matmo, Eos, 97, Published on 09 December 2016.

The raindrop distributions that the authors observed in Typhoon Matmo were notably different from those of other storms, including seemingly similar typhoons in Taiwan: Matmo had a higher concentration of smaller raindrops than usual. Some degree of the difference can be accounted for by the environment, but further study will be required to fully probe the microphysical processes involved. A diverse set of data about tropical cyclones will help researchers build and validate numerical models of storms so that they can predict precipitation more precisely and mitigate damage. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2016JD025307, 2016)

The team found that, generally, the rainband of Typhoon Matmo contained lots of small drops (with radii under 1 millimeter) and only a few larger ones. As the storm progressed, the drops increased in size, and the rainfall intensified; both updrafts and downdrafts of air in the storm built in intensity along with the rain. These updrafts carried water vapor to higher, colder parts of the storm, allowing ice crystals to grow. However, ice accounted for only a small percentage of the storm’s water, suggesting that Matmo’s rainband was dominated by warm rain processes. Eventually, the number of raindrops dwindled as the storm dissipated.

Typhoon Matmo made landfall in Fuqing, Fujian Province, China, in July 2014. As it moved inland, the storm passed over an intensive observation area of the Observation, Prediction, and Analysis of Severe Convection of China (OPACC) project, where the microphysical and kinematic structures of its rainband were captured by OPACC’s instruments. The data provided the researchers with insights into the size distribution of raindrops and allowed them to make inferences about the movement of air and water through the storm.

In a new study, Wang et al. used relatively new observational methods to examine microphysical processes in a typhoon’s rainband, the line of heavy showers—generated by temperature differences—that spirals in toward the storm’s center and gives a hurricane its distinctive whorl shape.

Rainband seeds

Farmers near Mt Arapiles in Victoria will look to bank some of the 50mm that fell over the weekend by controlling summer weeds.

“You don’t usually see the heat and the moisture we saw with this event in southern Australia.”

There was then gusty dry storms characterised by high winds, with 98 kilometres an hour gusts recorded at Horsham airport.