Modelling the Precipitation Enhancement by Hygroscopic Cloud Seeding in Warm and Mixed-Phase Clouds Using UCLALES-SALSA

Weather modification and, in particular, rain enhancement via cloud seeding by aerosol particles are subjects of increasing interest because of the need to improve water security in many regions across the globe. In this work we employed the coupled large-eddy – sectional aerosol-cloud microphysics model UCLALES-SALSA to perform modelling studies of rain enhancement by hygroscopic cloud seeding. Although numerous field experiments to increase rainfall by deliberate emission of aerosol particles inside the clouds (i.e. cloud seeding) have been conducted, the effectiveness of the seeding is still subject to large uncertainties.

Since UCLALES-SALSA is designed for investigations of the aerosol-cloud interactions and cloud processing and scavenging of the aerosol, it is very well suited to study the cloud seeding efficiency. In the current work we focus on hygroscopic seeding. The model is evaluated in a simple marine stratocumulus setup and the results are compared observed estimates from field experiments. The simulated microphysical properties of the clouds show a response to the aerosol injection, which is consistent with observations. In addition, seeding increases the drizzle rates in our model, but the magnitude of the effect is very sensitive to the background aerosol characteristics and the seeding strategy. In a more recent work, the model is used to simulate the effects of hygroscopic seeding on convective mixed-phase clouds. The simulations are based on cloud events observed over the United Arab Emirates (UAE) and supported by observations of the aerosol and boundary layer properties from a field campaign deployed by the Finnish Meteorological Institute in the UAE during 2018. While the initial results are encouraging, the convective cloud systems comprise a much more complicated environment due to pronounced dynamical features and the mixed-phase cloud structure. Even though we focus on hygroscopic seeding, we must also account for the subsequent effects on the ice phase and the cold precipitation process.