WEATHER MODIFICATION BY AIRCRAFT CLOUD SEEDING

BERYULEV G.P.

Head,
Department of Cloud Physics and
Weather Modification
Central Aerological Observatory
Rosgidromet, Russian Federation

:: Excerpt ::

Improving weather conditions and flush flood control

Throughout the halfcentury history of the artificial modification of hydrometeorological processes, which is an important direction in experimental meteorology, the focus of attention has been on designing techniques and technical aids to dissipate clouds and fogs, as well as to prevent or reduce precipitation.

In particular, it is practicable to carry out artificial weather modification operations aimed at:

A. Dissipation of stratiform clouds.
B. Destruction of cumulonimbus clouds by a dynamic technique in order to prevent shower rains and thunderstorms.
C. Inducing an early fall out of precipitation from cloud systems on the windward side of a target area through artificial seeding of these cloud systems, which leads to the formation of a ‘shadow‘ of precipitation, i.e. to precipitation reduction over the given site.

To crystallize one cubic km of supercooled cloud, it is usually enough to seed it by several hundred grams of dry ice or several grams of silver iodide. After seeding a cloud or fog by ice particles in favorable conditions an intensive cloud crystallization process begins, and in 510
minutes ice crystals are observed to fall out of the cloud.
In this case, one passage of a seeding airplane results in producing a dissipation zone with an average 35 km width. The full clearing of the target site from cloud drops and precipitation particles occurs in 35-50 minutes after seeding.

Aircraft liquid nitrogen generator of ice particles

The methods to destroy developing convective clouds differing in intensity, from cumulus congestus to cumulonimbus, using a dynamic technique, i.e. artificially generated downdrafts, were theoretically justified by scientists from the Russian Institute of Applied Geophysics and thoroughly tested under laboratory and field conditions by specialists from the CAO. It has been found out that downdrafts in the upper cloud part can be produced by an artificial air jet directed downward, through seeding powders or dispersing water mass in it.

The seeding of 30 kg or more of coarsedispersion powders (per cloud top) resulted in the destruction of singlecell isolated clouds within 10-20 minutes and frontal ones within 30-35 minutes.

Results of improving weather conditions during festivities to mark Moscow Day ( 1 September, 2001 )

The other two methods out of the four mentioned at the beginning of this section use weather modification techniques similar to that employed in the first method aimed at the dissipation of clouds and fogs. The zone of reduced precipitation is commonly referred to as “precipitation shadow”. In both cases it is possible to estimate the distance of advance seeding relative to the protected territory so as to prevent undesirable clouds and precipitation from reaching it.

In some synoptic situations, overseeding may prove to be the most appropriate procedure. This is due to its capability to reduce precipitation significantly and to its faster action facilitating the
production of an artificial crystallization zone (with reduced or no precipitation) over a protected territory, which is especially important in conditions of a complex and variable wind field. In cloud overseeding operations, the distance of seeding paths from the borders of a protected territory is chosen so as to be approximately the same as the distance of a halfhour or one hour wind transport of clouds.

The methods described can be applied with the help of instrumented aircrafts. Instrumentation includes systems to release pyrotechnic flares, devices to seed granulated dry ice, generators of ice particles, using liquefied nitrogen, and systems to introduce 25-30kg powdery material packages which open automatically after their release. All the procedures and technical aids described above were employed successfully in the activities associated with eliminating the consequences of Chernobyl disaster and improving weather condition in Moscow (November 7, 1986; May 89, 1995; September 5-7, 1997; July 13 and 19, 1998; May 9, 2000; September 23, 2000; September 12, 2001; June 12 and 15, 2002; August 31 and September 1, 2002), Tashkent (1994-2002), and Astana (June 9-10, 1998).

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