The Impacts of Mineral Dust on Organized Mesoscale Deep Convection
October 26, 2012
Robert Seigel
Committee: Sue van den Heever (advisor), Sonia Kreidenweis, Wayne Schubert, Jeffrey Niemann (Civil and Environmental Engineering)
Abstract
The overarching goal of this dissertation research is to investigate how mineral dust impacts various aspects of organized deep moist convection (DMC) using numerical modeling. From a bulk perspective, organized mesoscale DMC can be characterized by heating (i.e. latent heating precipitation production) above cooling (i.e. cold pools) that is long-lived. The balance between dynamics and thermodynamics for mesoscale systems dictates their level of organization and longevity, which can be modulated by mineral dust through direct and indirect effects.When a layer of dust exists in the atmosphere, it absorbs radiation such that a layer of increased stability results. Using experiments of idealized cold pools, it is shown that the strength and location of a vertically thin stable layer modulates the characteristics and propagation of cold pools. This direct aerosol forcing can then impact the organization of mesoscale DMC. Furthermore, observations have shown that dust can effectively serve as both cloud condensation nuclei (CCN) and ice nuclei (IN), which can indirectly impact the level of organization for mesoscale DMC. As increased dust aerosol within anvils can alter the ice crystal size distribution by serving as an additional source of IN, the impact of varying the mean hail diameter for squall lines is investigated. It is demonstrated that as hail size decreases, the intensity of the simulated squall line
increases. The increase in squall line intensity is attributed to a hydrometeor recirculation mechanism that is centered at the freezing level and more efficiently produces latent heating, driving enhanced buoyancy within the updrafts.
Finally, detailed analysis will be presented of a study that utilizes a nocturnal squall line to assess and isolate the individual responses in a squall line that arise (1) from radiation, (2) from dust altering the microphysics, as well as (3) from the synergistic effects between (1) and (2). To accomplish these tasks, we use RAMS set up as a cloudresolving model (CRM) and represent mineral dust as a background aerosol, which is shown to be the most effective source of dust ingestion. The CRM contains aerosol and microphysical schemes that allow mineral dust particles to nucleate as cloud drops and ice crystals, replenish upon evaporation and sublimation, be tracked throughout hydrometeor transition, and be scavenged by precipitation and dry sedimentation. Factor separation is used on four simulations of the squall line in order to isolate the individual roles of radiation (RADIATION), microphysically active dust (DUST MICRO), and the nonlinear interactions between these two factors (SYNERGY). Results indicate that RADIATION acts to increase precipitation, intensify the cold pool, and enhance the mesoscale organization of the squall line due to radiation-induced changes in the microphysics that appear to initiate from cloud top cooling. Conversely, DUST MICRO decreases precipitation, weakens the cold pool, and weakens the mesoscale organization of the squall line due to an enhancement of the warm rain process. SYNERGY shows little impact on the squall line, except near the freezing level, where an increase in mesoscale organization takes place.