WMOD00393 - Laserfiche WebLink

Home Browse Search

610 WEATHER MODIFICATION aircraft, and the process has been simulated in numerical cloud models. The results show that hygroscopic seeding can promote the precipita- tion process in certain types of convective clouds. Numerical calculations suggest that the artificial embryos grow most rapidly in clouds with high liquid water concentrations and large droplets, that is, a vigorous cloud with a mari- time droplet distribution. However, these clouds readily produce precipitation by coales- cence without any artificial intervention, so con- tinental clouds may actually have more potential for change. One point of concern in warm cloud seeding experiments is whether or not the Langmuir chain reaction is activated. Some experimenters have decided that this chain reaction was initi- ated by their hygroscopic seeding, but the results are not universally accepted. In any case, numerical calculations show that the cost effectiveness of hygroscopic seeding is marginal if the Langmuir chain reaction is not operative. The chain reaction occurs only in clouds able to support raindrops of 2 to 3 mm in diameter, that is, updrafts exceeding 8 to 10 m/sec. A meeting was called by the World Meteoro- logical Organization in Kuala Lumpur, Malay- sia, in 1981 to consider the modification of warm clouds. While considering that hygroscopic seeding has considerable promise, the partici- pants were not able to conclude that it increases rainfall on a predictable basis. Despite the lack of conclusive proof of its effectiveness, hygro- scopic seeding continues to be applied in coun- tries of southeast Asia, including Indonesia, Ma- laysia, and Thailand. 2. Glaciogenic Seeding: Microphysical versus Dynamic Effects Most attempts to modify precipitation from convective clouds have involved the use of AgI or other glaciogenic seeding agents. Some inves- tigators have used glaciogenic agents to modify the dynamics of clouds, as opposed to merely stimulating the appearance of ice crystals to serve as precipitation embryos. Some calcula- tions have indicated that seeding for dynamic effects can be effective even in cases where ice crystal concentrations already exceed the opti- mum for using up the existing cloud water. The- oretical calculations show that freezing the su- percooled water in an updraft can raise the local temperature by as much as 1 to 2aC, depending on the liquid water concentration. Cloud models indicate that such a warming can intensify cloud circulations. In marginal situations, the stimu- lation could help a cumulus cloud grow dra- matically by penetrating a weak inversion into unstable air above the inversion. Because a cumulonimbus cloud may yield ten times as much rainfall as a cumulus congestus and merged cumulonimbus formations can increase the rainfall yield by another factor of ten, the motivation for experiments is obvious. The best known experiments to test these pos- sibilities were conducted over the Caribbean and in Florida in the 1960s. In the experiments over the Caribbean, clouds were selected as test cases and randomly treated or not treated. The treatment consisted of dropping of AgI pyro- technics into the clouds to produce rapid glacia- tion of their supercooled water. The clouds were observed by several aircraft to determine the rate of rise of their tops. Cloud growth was com- pared to that predicted by a one-dimensional cloud model, which used as input atmospheric sounding data obtained in the vicinity of the treated clouds. On the basis of 22 test cases, it was concluded that the cloud growth was related to the seeding treatment. The experiments over Florida provided fur- ther evidence of cloud growth and of increases in rainfall from individual clouds treated with AgI rather than a placebo. A number of hypothe- ses were put forth to explain the apparent in- creases in cloud height and rainfall. One held that the release of latent heat invigorated the updrafts in the clouds, leading to increased in- flow at cloud base and the processing of more cloud water. An alternative hypothesis, which was developed a few years later, held that the initiation of precipitation by the seeding stimu- lated the formation of downdrafts, which on reaching the ground lifted humid surface air to trigger new convection around the treated clouds. The observations currently available do not permit a determination of which (if either) of these hypotheses is correct. 3. Effects of Glaciogenic Seeding on Area Rainfall Operational projects on convective clouds do not provide convincing evidence regarding in- creases in precipitation. The lack of firm results is relatable to the variability of convective pre- cipitation and the low correlation coefficients (-0.4-0.6) obtained when convective precipita- tion in adjoining regions is compared. The Advi- sory Committee on Weather Control stated that