Eulerian and Lagrangian Analyses of Aerosol Transport in Three Deep Convective Storm Morphologies

Biological aerosol particles (bioaerosols) are known from laboratory and observational studies to be highly effective cloud condensation nuclei and ice-nucleating particles. However, the actual impact of these particles on the microphysics of convective storms is not well-understood. A key reason for this knowledge gap is a lack of understanding of the spatial extent, timing, and magnitude with which these particles are entrained into and transported by these storms. We address this question of bio- and other aerosol ingestion and transport using a combination of Eulerian and Lagrangian approaches to characterize the possible pathways of these particles within three different deep convective storm morphologies.

We conduct high-resolution (250m horizontal grid spacing) idealized simulations of isolated deep convection, a squall line, and a supercell using RAMS, a non-hydrostatic cloud-resolving model. We use an idealized, massless, non-interactive tracer quantity to emulate the transport of aerosol particles in isolation from their microphysical feedbacks. We implement two tracer emission mechanisms intended to represent two different aerosol production mechanisms. Fixed-source tracer is emitted at a constant rate in fixed areas of the simulation domain and is representative of features like urban aerosols. Rain-sourced tracer is produced at each timestep in surface gridpoints where the rainfall rate exceeds a threshold value, a mechanism which is derived from observations in the literature that rainfall is correlated with the aerosolization of biological particles. We conduct multiple experiments for each emission mechanism and storm morphology, varying the locations of the fixed-source tracer emission regions and the threshold rainfall rates of the rain-sourced tracer. We analyze the timing and magnitude of in-updraft tracer concentrations across storm morphologies and experimental setups to determine the potential importance of bioaerosol entrainment and transport from the earth’s surface. Preliminary results from this Eulerian approach indicate that the greatest overall in-updraft tracer concentrations arise from the fixed-source tracer in isolated convection, but that this maximum value is short-lived. By contrast, the highest values of mean in-updraft rain-sourced tracer are attained in the other (organized) storm morphologies and are maintained for the duration of the simulation. These results indicate significantly different entrainment and transport pathways between morphologies and tracer emission mechanisms.

We also conduct offline backward and forward parcel trajectory calculations using the same simulations to investigate the evolution of the environments experienced by individual parcels. With this Lagrangian perspective, we are able to characterize the path of surface parcels from regions of aerosol production into storms, assessing not only their ultimate trajectories but their potential for microphysical activity as the parcels are entrained and transported. We present analysis of the parcels' residence time in environments where condensation and/or heterogeneous ice nucleation are possible in order to estimate the potential for microphysical impacts across storm morphologies and aerosol emission mechanisms. By combining Eulerian and Lagrangian approaches to investigate the pathways by which aerosols are entrained and transported within deep convective storms, we present a clearer picture of the potential availability of bio- and other aerosol particles to microphysical processes in different convective and aerosol environments.