Aerosol Effects on the Anvil Characteristics, Cold Pool Forcing & Stratiform-Convective Precipitation Partitioning & Latent Heating of Mesoscale Convective Systems

Principal Investigator(s):
Susan van den Heever, Colorado State University

Mixed-phase mid-latitude cloud systems, in particular mesoscale convective systems (MCSs), play a significant role in the hydrological and energy budgets of the mid-latitudes and the tropics, yet many aspects of their initiation and lifecycle remain poorly understood. We seek to build upon recent studies showing that the availability of cloud-active particles, both cloud condensation nuclei (CCN) and ice nuclei (IN), can affect the microphysical evolution of MCSs which in turn modulates the storm dynamic development and the production of precipitation. Several of the hypothesized aerosol-cloud-precipitation interactions are both counterintuitive and different from prevailing hypotheses, and depend strongly on how the hydrometeor population evolves for different aerosol types and abundances. To resolve these outstanding problems requires a combined observational and modeling strategy that can constrain key features of the input aerosol. Furthermore, the modeling component requires a cloud-resolving framework that also includes a detailed treatment of aerosol and cloud microphysics. For the observational component of our study, we focus on three MCSs that developed during the Midlatitude Continental Convective Clouds Experiment (MC3E), which took place near the Atmospheric Radiation Measurement (ARM)’s Southern Great Plains (SGP) facility in Oklahoma in April and May of 2011. MC3E deployed an unprecedented array of radars and other observing systems that are generating new insights into the initiation, development and organization of MCSs. The ARM Aerosol Observing System was deployed at SGP and provides characterization of surface aerosol including direct measurements of CCN concentrations. Initial investigations show that the aerosol environment at SGP during MC3E was influenced not only by local sources of particles, but smoke and dust transported to the region, creating aerosol layers above the surface.

Airborne observations of aerosol and cloud particle characteristics, and guidance from global aerosol models, will be used to form a picture of the vertical structure of aerosols available for ingestion into the storm. For the modeling component of our work, we will use the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS), with a newly revised microphysical scheme that includes multiple aerosol types and detailed aerosol-cloud interactions. RAMS will be run in a nested grid configuration with the resolution of the innermost grid at 1 km in order to explicitly resolve convection. Our proposed approach is to simulate the case studies in order to test and validate the aerosol and microphysical parameterization schemes of RAMS. We will then conduct additional sensitivity simulations in which we modify the aerosol initializations in order to explore the response of cloud evolution and anvil and precipitation development to variations in aerosols, especially CCN. Our specific aims are to investigate the impacts of aerosols on those features of MCSs that most directly impact the hydrological cycle and radiative budget of the earth. These include (1) the cloud anvils; (2) the total precipitation and the partitioning between convective and stratiform precipitation; (3) the vertical heating structure; (4) raindrop size distributions; and (5) the strength and intensity of the cold pools and updrafts. We are further proposing to examine the vertical and horizontal redistribution of aerosols by MCSs. Such relocation of storm-processed aerosols, particularly from the surface to the upper troposphere, has significant subsequent effects on radiative forcing. We expect this work to lead to new insights on aerosol-cloud-precipitation interactions in convective systems, and to provide guidance as to how these interactions can be included in regional and global climate models, where they will play key roles in projections of anthropogenically-induced changes to the hydrological cycle.