This award will allow the research team an opportunity to expand on their investigations into atmospheric convection, cloud electrification, and lightning. The three main areas of work are (1) the statistical analysis of regional differences in storm structure and lightning and their potential relationships to environmental parameters and aerosols, (2) the relationship between mesoscale precipitation systems and transient luminous events (TLEs), and (3) the meteorology surrounding large impulse charge moment changes (iCMCs).
In order to perform the statistical analyses required, the research group will use a recently developed algorithm that ingests and synthesizes radar, lightning, environmental, and aerosol data. The algorithm would be used with selected data sets to run basic statistical tests on different parameter populations in order to answer questions about the relationships of lightning with convective structure, the meteorological environment, and aerosols. For the second task, the group will analyze a number of case studies of TLE-producing events and compare them to prior case studies to address hypotheses relating to the production of TLEs in mesoscale convective systems (MCSs). They will focus on the stratiform charge layer, embedded convection in stratiform precipitation, and regional differences in MCS structure. For the third task, the researchers will study iCMCs, which are the products of the charge magnitude lowered to ground and the height above ground from which the charge is lowered, over the first 2 ms of a cloud-to-ground lightning strike. iCMCs can be measured by a recently developed instrument, and the researchers will use the observational data to place large iCMC observations in meteorological context to see what types of weather situations may produce these large charge moment changes.
The intellectual merit of the work includes the improved understanding of the relationships between storm structure and lightning behavior, and the influence of the environment on convection and lightning. Both topics will be enhanced by a greater amount of data, larger geographic reach, and additional instrumentation available for this work. The research will also help to improve knowledge about the connections between MCSs and TLEs and determine what information iCMC events can tell us.
The broader impacts of the research include the involvement of multiple graduate and undergraduate students and citizen scientists, widespread dissemination of results and data, and better understanding of high-impact convection and lightning.
This research grant focused on the study of convective (cumulus) clouds, namely the processes involved in how these storms electrify and produce lightning. In one objective, we used data from state of the art 3-D Lightning Mapping Arrays to estimate lightning flash rates in various regions around the U.S. and to understand differences in peak flash rates in terms of the environment within which the storms formed. Storms in Colorado had the highest peak flash rates and these storms also had very high cloud bases. We suggest that in storms with very high cloud bases, the storm updraft width is very broad, which reduces entrainment of dry air from the environment, promoting high supercooled water content, a necessary ingredient for storm electrification. The second phase of this research studied the phenomena of sprites, highly energetic lightning-like discharges that occur 50-90 kilometers above the Earth’s surface, well above the tops of even the deepest thunderstorms. Sprites are triggered by massively-energetic cloud-to-ground lightning discharges in the troposphere (the first 10-15 km of the Earth’s atmosphere). We documented the role of large Mesoscale Convective Systems in promoting massive positive cloud-to-ground lightning flashes, especially those that tap electrical charge in the large stratiform cloud regions of MCSs. We investigated the meteorological context of a new electrical measurement, the so-called Charge Moment Change (Coulomb-km). The CMC measures the charge lowered to ground in a CG lightning discharge, multiplied by the mean height the charge is lowered from. CMC’s in excess of 100-300 C-km have a very high probability for producing a sprite. Such large iCMC’s most often occur beneath regions of stratiform precipitation associated with MCSs. Finally, this grant supported preliminary research on the dataset obtained in the Deep Convective Clouds and Chemistry project (DC3). We have investigated relationships between storm variables and lightning flash rates which are used in convective-scale cloud-chemistry models that predict NOx production by lightning. NOx produced by lightning in the upper troposphere often leads to increased concentrations of ozone, an important greenhouse gas for climate change. One of the more interesting findings from this work is that total lightning channel length (that is,. the channel lengths of all flashes within a storm over a given time period) is uniquely related to flash rate. This suggests a new pathway for lightning-NOx parameterizations that depend on total channel length. Research on this topic continues under current NSF support.
