Atmospheric Tests of an Organic Nucleant in Supercooled Fog

Abstract

An organic artificial ice nucleant (Pseudomonas syringae) was dispersed as a fine powder from an aircraft within a supercooled fog over Mono Lake, California on 2 December 1989. The dispersal took place about 30 m below fog top at a temperature of -8°C. Two test runs about 3 to 4 km in length were made; 10 gm and 100 gm of nucleant were distributed over the test track during the first and second runs, respectively. Following each test run, a highly instrumented cloud physics aircraft flew tracks within the fog that were orthogonal to the original test track. Particle measuring probes (FSSP, 1D-C and 2D-C) were used to determine fog characteristics and quantify a seeding signature. The light seeding rate (10 gm over the test track) of P. syringae produced detectable seeding signature, probably because the monitoring aircraft missed the treated plume. The heavier rate (100 gm over the test track), however, produced an obvious seeding signature that was first detected about 5 minutes after release of the nucleant. The signature began with high concentrations of relatively small (50 to 100 um) particles, and the ice crystal concentration decreased with time as their size increased. The mean, median and modal particle sizes at a particular time are virtually equal in the plume, suggesting a common origin for the ice crystals. P. syringae clearly produced glaciation of a supercooled fog at temperatures of about -8°C in agreement with laboratory test results. Additional atmospheric tests are planned at warmer temperatures.