The response of a simulated mesoscale convective system to increased aerosol pollution
Date
2016
Authors
Clavner, Michal, author
Cotton, William R., advisor
van den Heever, Susan C., advisor
Kreidenweis, Sonia M., committee member
Reising, Steven C., committee member
Birner, Thomas, committee member
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Abstract
Mesoscale Convective Systems (MCSs) are important contributors to rainfall in the High Plains of the United States and elsewhere in the world. MCSs are also producers of severe weather such as hail, tornados and wide-spread straight-line wind events known as derechos. It is therefore of interest to understand how different aerosols serving as cloud condensation nuclei (CCN) concentrations may impact these systems. This work focuses on the impacts of aerosols on the total precipitation amount, rates and spatial distribution of precipitation produced by an MCS, as well as the characteristics of a derecho event. Past studies have shown that the impacts on MCS-produced precipitation to changes in aerosol concentration are strongly dependent on environmental conditions, primarily humidity and environmental wind shear. Changes in aerosol concentrations were found to alter MCS-precipitation production directly by modifying precipitation processes and indirectly by affecting the efficiency of the storm’s self-propagation. Observational and numerical studies have been conducted that have examined the dynamics responsible for the generation of widespread convectively-induced windstorms, primarily focusing on environmental conditions and the MCS features that generate a derecho event. While the sensitivity of the formation of bow-echoes, the radar signature associated with derecho events, to changes in microphysics has been examined, a study on a derecho-producing MCS characteristics to aerosol concentrations has not. In this study different aerosol concentrations and their effects on precipitation and a derecho produced by an MCS are examined by simulating the 8 May 2009 “Super-Derecho” MCS. The MCS was simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated aerosol and microphysical parameterizations. Three simulations were conducted that varied in their initial aerosol concentration, distribution and hygroscopicity as determined by their emission sources. The first simulation contained aerosols from only natural sources and the second with aerosols sourced from both natural and anthropogenic emissions The third simulation contained the same aerosol distribution as in the second simulation, however multiplied by a factor of 5 in order to represent a highly polluted scenario. In all three of the simulations aerosol concentrations were derived from the output of GEOS-Chem, a 3D chemical transport model. In the simulated MCS, the formation and propagation of the storm was not fundamentally modified by changes in the aerosol concentration, and the total MCS-produced precipitation was not significantly affected. However, the precipitation distribution (convective vs stratiform) and derecho-strength surface wind characteristics did vary among the simulations. The more polluted simulations exhibited higher precipitation rates, higher bulk precipitation efficiency, a larger area with heavier convective precipitation and a smaller area with lighter stratiform precipitation. These differences arose because aerosol pollution enhanced precipitation in the convective region while suppressing precipitation from the stratiform-anvil. Higher aerosol concentrations led to the invigoration of convective updrafts which supported the formation of larger rain drops, and lofted more liquid cloud mass to higher levels, thereby increasing both collision-coalescence and riming processes. The presence of greater aerosol concentrations in the free troposphere, as well as in the boundary layer, reduced both collision-coalescence and riming within the stratiform-anvil region. As a consequence, the more polluted simulations produced the smallest precipitation from the MCS stratiform-anvil region. In order to understand the impact of changes in aerosol concentrations on the derecho characteristics, the dynamical processes which produced the strong surface wind were determined by performing back-trajectory analysis during different periods of the simulated storm. The analysis showed that two main air flows contributed to the formation of the derecho winds at the surface; a rear-inflow jet and an up-down downdraft associated with a mesovortex at the gust font. The changes in aerosol concentrations impacted the simulated derecho event by altering the main flow contributing to the formation of the derecho winds though the amount of melting and evaporation of hydrometeors. Earlier in the storm, changes in melting and evaporation altered the intensity of the storm-produced cold pool. This, in turn, modified the balance between the horizontal relative vertical vorticity generated by the cold pool and that of the low-level environmental shear. The smaller hail and rain hydrometeors in the cleaner simulation exhibited higher melting and evaporation rates due to the larger surface area, which contributed to the formation of a stronger cold pool and led to the tilting of the convective updraft upshear. This, in turn, shifted the flow associated with the derecho event to be predominantly from a Rear-Inflow Jet (RIJ). An increase in aerosol concentration led to a weaker cold pool and therefore an upright convective updraft which promoted the formation of a stronger mesovortex, and subsequently shifting the flow to be predominantly from strong downdrafts following an up-down downdraft (UDD) trajectory. The shift from a RIJ flow to a UDD led to stronger surface winds over a smaller area. As the storm matured, the derecho winds were found to be associated with the formation of a mesovortex at the gust front. At this time, a non-linear trend between aerosol concentrations to derecho intensity was apparent and was attributed to the non-linear trend in mesovortex strength.
Description
Rights Access
Subject
derecho
mesoscale convective system
RAMS
increased pollution
aerosol indirect effects
numerical analysis