Lightning Channel Locations, LNOx Production, and Advection in Anomalous and Normal Polarity Thunderstorms
November 30, 2017
Trenton Davis
Hosted by Steven Rutledge (Advisor), Mary Barth (NCAR), Emily Fischer, Steven Reising (Electrical and Computer Engineering)
Abstract
While stratospheric ozone is essential in blocking ultraviolet radiation from reaching the Earth’s surface, this process warms the environment rendering tropospheric ozone undesirable. Relatedly, tropospheric ozone production is widely studied in atmospheric science today as global climate modelers attempt to estimate future warming within the troposphere. Nitrogen oxides (NO + NO2 = NOx), known collectively as NOx, serve as a precursor to ozone production in the presence of low concentrations of hydrocarbons. Due to lower concentrations of these hydrocarbon species in the upper troposphere, NOx tends to experience a longer lifetime (on the order of days) and greater ozone production at these heights, especially above the boundary layer. Lightning produces an appreciable amount of NOx (a.k.a. LNOx) but exactly how much and where this source of NOx ends up is poorly understood. Therefore, it is important that this source of NOx be further investigated.Numerical modeling methods are attempting to fill this void through parameterizing the nature of lightning within thunderstorms. Often, the vertical distribution of flash channels (and LNOx) is produced according to a parameterized flash rate within a defined vertical profile and reflectivity volume threshold. The structure and intensity of thunderstorms is highly variable though, causing the location of lightning to also change from one thunderstorm to the next. Furthermore, one remaining goal of the Deep Convective Clouds and Chemistry (DC3) field campaign (May – June 2012) was to investigate the contributions to upper tropospheric LNOx between storms of normal and anomalous charge polarity.
To address this remaining goal, five cases with over 5600 total flashes are analyzed in detail from DC3, three in northern Colorado and two in northern Alabama. Lightning sources are combined into 3-dimensional (3-D) flash channels and flash channel parcels, with each parcel containing the LNOx produced from its parent flash channel. Parcels are then advected forward throughout the lifetime of each storm using 3-D wind fields produced from dual-Doppler analyses. Results reveal significantly more flashes and flash channels within anomalous polarity thunderstorms at a mean initiation height around 5 km and roughly half of flash channel parcels advecting to final heights above 8 km. Contrary to previous assumptions, an appreciable fraction of these parcels and NOx contributions remain in the boundary layer of these storms though. In the two normal polarity cases, flash channels initiate around 8 km with roughly half of the flash channel parcels remaining near or above 8 km. While both storm types appear to transport roughly 50% of their flash length parcels to the upper troposphere (z > 8 km), significantly higher flash counts and total flash length in the anomalous polarity storms lead to much higher mixing ratios of LNOx in the upper troposphere. These results may help chemistry modelers in parameterizing LNOx formation in varying thunderstorm polarity structures, which will also benefit global climate models in terms of tropospheric ozone production.