Computational Modeling Studies of Fundamental Aerosol-Cloud Interactions

Citation

Lebo, Zachary John (2012) Computational Modeling Studies of Fundamental Aerosol-Cloud Interactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HH11-8N22. https://resolver.caltech.edu/CaltechTHESIS:03292012-204204319

Abstract

Basic questions regarding the interaction between changes in human activity and the atmosphere remain unanswered. Among these, the link between aerosol particles and cloud formation and development, especially in an altered climate, is a large point of uncertainty in recent climate projections. This should come as no surprise given the uncertainty in model parameters required to predict droplet activation, even in the most detailed models used for climate predictions. Here, a detailed spectral mixed-phase microphysics scheme and a state-of-the-art continuous two-dimensional (2-D) aerosol-droplet microphysics scheme have been developed and coupled to the Weather Research and Forecasting (WRF) model to more closely analyze the effects of aerosol perturbations on single clouds or cloud systems with the hope of ultimately improving numerical parameterizations used by microphysics schemes in general circulation models (GCMs).

The continuous 2-D aerosol-droplet model is developed to explicitly treat the entire spectrum of aerosol, haze droplets, cloud droplets, and drizzle drops while allowing the solute mass spectrum to evolve within the droplets. In other words, the aerosol mass is conserved and regeneration of aerosol particles upon droplet evaporation is physically accurate without any a priori assumptions. The model is tested by performing simulations of marine stratocumulus and the results are compared with those from the aforementioned bin and bulk models. It is shown that microphysical processing of aerosols alone results in a large shift in the aerosol spectrum toward larger particles (via collision-coalescence of droplets). This could have potentially large effects on the activation of regenerated particles. Future studies with the model will address the need for better parameterizations of the aerosol regeneration process.

The spectral mixed-phase microphysics scheme is used in conjunction with a two-moment bulk microphysics model to study the effect of aerosol perturbations on deep convective clouds. The bin model shows that with an increase in aerosol number concentration comes an invigoration and a decrease in precipitation. On the other hand, the bulk model suggests that the storm ought to weaken and precipitation will increase in a more polluted environment. The invigoration predicted by the bin model is a result of the suppression of the collision-coalescence process that permits more droplets to be lofted above the freezing level, hence increasing the bulk freezing rate aloft. The additional freezing and subsequent deposition acts to increase the latent heating and thus increase buoyancy. However, the cloud particles in the polluted cases are now smaller and more numerous and consequently have a shorter evaporation/sublimation timescale and a longer sedimentation time-scale. The ultimate result is for precipitation to decrease in conjunction with a moistening of the mid- to upper-troposphere. The difference in the sign of the aerosol effect between the two models is thought to be related to the saturation adjustment scheme used in the bulk model and is addressed by including an explicit treatment of condensation and activation within the bulk model, similar to the algorithm utilized in the bin model. The results of the inter-model comparison demonstrate the significance of the saturation adjustment assumption on the sign and magnitude of the aerosol effect on deep convective clouds.

Thesis Files