No abstract

A new method of seeding convective clouds for the purpose of augmenting rainfall is being developed in South Africa. Flares that produce small salt particles (0.5-m mean diameter) are attached to the trailing edge of the wings of seeding aircraft and ignited in updrafts below the cloud base of convective storms. This method of delivery overcomes most of the difficulties encountered in the handling and the use of hygroscopic materials, difficulties that made seeding with ice nuclei (AgI) a more attractive option. The research that has led to the development of this new technique was prompted by an encounter with a storm with dramatically altered microphysics that was growing over a Kraft paper mill in the research area. Hygroscopic seeding flares were subsequently developed, and seeding trials began in October 1990. Successful seeding trials quickly led to the design and execution of a randomized convective cloud-seeding experiment, the results of which show convincing evidence of increases in the radar-measured rain mass from seeded storms when compared to the control or unseeded storms. Heightened reflectivities aloft seen by the real-time storm-tracking software and observed in the exploratory analysis raises the possibility of developing a radar-measured seeding algorithm that can recognize in almost real time a successful convective seeding event. The implications of such a development would have far-reaching effects on the conduct of future convective cloud-seeding experiments and operations. The authors' seeding hypothesis postulates that the hygroscopic seeding at cloud base accelerates the growth of large hydrometeors in the treated clouds, which harvest more of the available supercooled water before it is expelled into the anvils by the strong updrafts that are a characteristic of the local storms, thereby increasing the efficiency of the rainfall process. The validity of this hypothesis is supported by microphysical measurements made from an instrumented Learjet and the results of the randomized experiment, both of which are supported by numerical condensation-coalescence calculations. There are also indications that the hygroscopic seeding may have an impact upon the dynamics of the treated storms, lengthening their lifetimes by strengthening the coupling of the updraft-downdraft storm propagation mechanism. The apparent sensitivity of rainfall in convective clouds to the aerosol concentration, size, and chemical content may have climatic implications. Higher concentrations of small aerosols produced by pollution, biomass burning, etc., could adversely affect the efficiency of the rainfall process. The negative consequences of this effect would be magnified in regions that depend upon convective storms to provide the bulk of their annual rainfall.

No abstract

No abstract

SUMMARYOn 19 February 1970, a T-33 jet aircraft, equipped with fast-response meteorological instruments, obtained measurements of a wave structure and clear air turbulence. The clear air structure was simultaneously detected with a sensitive 10.7 cm wavelength radar at Wallops Island, Virginia. From a special network of radiosonde stations with separations of about 100 km, the detailed mesoscale synoptic conditions associated with the clear air structures were determined. The turbulence and wave structure occurred between heights of 2 and 3 km within an upper level frontal zone which had advanced well ahead of the surface cold front. The wave pattern consisted of two-dimensional Kelvin-Helmholtz billows. The billow velocity was close to the mean wind velocity of the sheared layer, and the Richardson number for the layer was about 0.15 which is consistent with the criterion for instability based on theoretical and wind tunnel results. The most intense turbulence occurred where the billows were rolling up or breaking.