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Abstract
Field investigations for the Precipitation Enhancement Project (PEP) were undertaken during the winter months of 1979–1981 in the upper Duero River Basin of Spain. The purpose of these studies was to examine what potential might exist for the enhancement of precipitation from different cloud types via ice-nucleus seeding of clouds. This paper describes procedures for estimating that potential. Specific regions within natural clouds were qualified as potentially seedable on the basis of observations by instrumented aircraft of persistent zones of supercooled water content. The observed “regions of potential” are described, and precipitation increases that might result from seeding the regions are estimated using two relatively simple models. Summed over all cloud types, and expressed as averages over the 100-km radius project area, increases of 10% and 23% are estimated with the two models for the days of seeding. For the February–May season as a whole, the increases are 0.75% and 1.8% of the normal precipitation (160 mm) for the season. The major contributions to these increases come from cumulus mediocris and cumulus congestus, and from shallow stratiform and clouds.
The low values obtained for the estimated increases indicate that a major augmentation of total seasonal precipitation in the Duero Basin is not likely to result from seeding the “regions of potential”. Consequently, within a 5-yr period envisaged for the project, it would be difficult to discern a seeding effect in terms of area-averaged precipitation. Useful increases in precipitation may possibly be produced by seeding the regions of potential, but demonstration of the seeding effects would have to be based on evaluations which are focused (in time and space) on the treated clouds or cloud regions.
Since no seeding was actually carried out during, or subsequent to these field studies, the validity of the criteria employed for defining the regions of potential, and the derived estimates, remain unverified. The criteria which define the regions of potential, and the methods of estimation developed in this study, can also be applied, in principle, to other situations, where precipitation enhancement is sought via seeding with ice nuclei.
Abstract
Field investigations for the Precipitation Enhancement Project (PEP) were undertaken during the winter months of 1979–1981 in the upper Duero River Basin of Spain. The purpose of these studies was to examine what potential might exist for the enhancement of precipitation from different cloud types via ice-nucleus seeding of clouds. This paper describes procedures for estimating that potential. Specific regions within natural clouds were qualified as potentially seedable on the basis of observations by instrumented aircraft of persistent zones of supercooled water content. The observed “regions of potential” are described, and precipitation increases that might result from seeding the regions are estimated using two relatively simple models. Summed over all cloud types, and expressed as averages over the 100-km radius project area, increases of 10% and 23% are estimated with the two models for the days of seeding. For the February–May season as a whole, the increases are 0.75% and 1.8% of the normal precipitation (160 mm) for the season. The major contributions to these increases come from cumulus mediocris and cumulus congestus, and from shallow stratiform and clouds.
The low values obtained for the estimated increases indicate that a major augmentation of total seasonal precipitation in the Duero Basin is not likely to result from seeding the “regions of potential”. Consequently, within a 5-yr period envisaged for the project, it would be difficult to discern a seeding effect in terms of area-averaged precipitation. Useful increases in precipitation may possibly be produced by seeding the regions of potential, but demonstration of the seeding effects would have to be based on evaluations which are focused (in time and space) on the treated clouds or cloud regions.
Since no seeding was actually carried out during, or subsequent to these field studies, the validity of the criteria employed for defining the regions of potential, and the derived estimates, remain unverified. The criteria which define the regions of potential, and the methods of estimation developed in this study, can also be applied, in principle, to other situations, where precipitation enhancement is sought via seeding with ice nuclei.
