Determining the Importance of Initiation Mechanism and Background Environment on the Evolution of Deep Convection in Varied Regimes

Principal Investigator: Christopher Nowotarski, Texas A&M University

Co-Investigators: Anita Rapp, Sarah Brooks, and Milind Sharma (Texas A&M)

In addition to the direct hazards to humans and property (e.g., flooding, lightning, strong winds, hail, and tornadoes), moist convection plays a fundamental role in the global circulation and energy budget. Thus, accurate prediction of convection and its representation via parameterization in weather and climate models that cannot resolve convective clouds remains a critical challenge. 

Observations collected during the Tracking Aerosol Convection Interactions Experiment (TRACER) and Southeast U.S. (SE US) Atmospheric Radiation Measurement (ARM) mobile facility deployments provide a unique opportunity to answer critical questions about convection in subtropical and midlatitude regimes. The overarching goal of our project is to determine and quantify the influence of the mesoscale initiation mechanisms on the subsequent evolution of convective cells, in comparison to the impact of the background environment across different regimes. In  particular, we will determine how initial cell area and vertical velocity of nascent convection vary as a function of the initiation mechanism (sea-breeze front, outflow boundary, or boundary layer convective roll/cell).  We will explore the dynamical characteristics of nascent convection, such as initial cell width and cloud-top vertical velocity, that most significantly influence subsequent convective cell evolution. We will identify cases in which convection evolution is more sensitive to the dynamics of the  initiation mechanism than to the influence of the background meteorological and aerosol conditions. Finally, we will explore how this sensitivity varies between moist, weakly sheared subtropical, coastal environments (TRACER cases) and generally drier and more sheared midlatitude, inland environments (SE US cases).

To accomplish these goals, we will integrate ground-based profiling and radar  measurements from ARM deployments with data from operational radars and geostationary satellites. This will allow us to construct a comprehensive database of convective cell attributes, including their initiation mechanisms, meteorological conditions, and aerosol environments. We will investigate cloud-scale processes that result in observed differences in cell attributes based on initiation mechanisms. This will be accomplished using a series of high-resolution numerical modeling simulations to understand the role of initiation mechanism on cell evolution. Highly targeted analysis methods (e.g., passive tracer and trajectory analysis) will be employed to identify changes in the underlying cloud-scale processes.  Comparisons of bulk differences in the simulated convection between high-resolution and coarse domains will assess the necessity of representing mesoscale and microscale details for future cumulus parameterizations and climate-scale modeling efforts.   Finally, these analyses will be repeated using cells observed during the ongoing ARM SE US deployment in Northern Alabama where background conditions and initiation mechanisms are likely different from TRACER events. 

The primary impact of this project will be improved characterization of the frequency distribution of shallow and deep convective cells associated with different initiation mechanisms and background environments. Our work will help determine the extent to which mesoscale features that initiate convection need to be considered by cumulus parameterizations, and if so, inform ongoing and future efforts by others to advance parameterization of convection in global weather forecast and climate models.