Using in situ microphysical observations in Direct Numerical Simulations to study the impact of hygroscopic seeding

The concept of hygroscopic seeding is that by introducing large hygroscopic particles, the activation of small natural aerosols is suppressed and the collision-coalescence process is promoted. Direct numerical simulation (DNS) of hygroscopic seeding in mono-disperse pristine environment show that seeding broadens the droplet size distribution (DSD), and drizzle-sized particles were observed at early cloud stage in seeded case compared to the non-seeded case. In this study, we extended the DNS to multi-disperse cases and the results were compared against the in-situ measurements.

During the Queensland Cloud Seeding Research Program in 2008-2009, a randomized hygroscopic seeding trial was conducted. For each potential seeding case, aerosol observations were collected just below the convective cloud base and in situ microphysical measurements were collected within 1000-2000 ft above cloud base. These measurements provide the background aerosol size distribution and initial cloud drop size distribution in clouds seeded with hygroscopic flares and those that were not seeded. These background aerosol observations, with and without the presumed impact of hygroscopic seeding flares, are used to initialize the DNS model to simulate the evolution of the cloud drop size distribution. The simulated cloud drop spectra are then compared to the observed cloud drop spectra.