The Use of Trace Chemistry to Estimate Seeding Effects in the National Hail Research Experiment

Abstract

Four convective storms which occurred during the National Hail Research Experiment (NHRE) in northeast Colorado have been studied and confirm the hypothesis formed from an earlier study, that AgI aerosols released from aircraft into storms with defined cellular structure, are confined both temporally and spatially in the surface precipitation. This implies that portions of the storms receive substantial quantities of AgI aerosol using this seeding technique, whereas nearby precipitating portions of the same storms do not. Using radar, precipitation network data, upper air soundings and trace chemistry, estimates have been made of possible trajectories for the seeding aerosol and of times available for it to affect the cloud micro-structure. Compared with theoretical estimates, seeding rates used on the days investigated were occasionally large enough to achieve substantial phase change (water to ice) in the intended ice embryo competitive growth process. However, the evidence indicates that such levels were not achieved with sufficient regularity over the protected area to produce a hail suppression effect which could have been observed with the statistical methodology used in the NHRE analysis. The relationship between the quantities of precipitation sampled and their silver contents were investigated for each storm. The characteristics of the storms and the seeding of them varied considerably. Nevertheless, the results appear to fall into three categories; those where the AgI concentration was relatively constant over a wide range of precipitation amounts; those where the precipitation amounts were small and independent of silver contents; and those where there is a positive correlation between silver concentration and precipitation amount. For those cases where the Ag content is low and relatively constant, the results are consistent with the dominant capture processes being proportional to drop volume. For the two cases where precipitation amounts are small and independent of silver content, the rain fell from low radar reflectivity regions, at times long after seeding had occurred. The large variations of silver content are attributed to the non-uniform distribution of aerosol throughout the broad light precipitation regions. For the two cases where there is a positive correlation between silver concentration and precipitation amount, it is proposed that ice nucleation has played a dominant role, generating new ice embryos which in turn have resulted in increased rainfall. In both cases a silver concentration increase of 50 × 10−11 g mL−1 is associated with a precipitation increase of about 1700 g m−2, or 1.7 mm depth of water per square meter.