A Radar-Based Climatology of Great Plains Thunderstorms

The thunderstorm is a high-impact weather phenomenon in the physical climate system that can produce a severe impact on humans and natural systems. As such, understanding its spatiotemporal variability, exposing patterns in its structure and behavior, and quantifying the statistical extremes in its occurrence, behavior, and spatiotemporal distribution are of great importance. Early attempts to develop thunderstorm climatologies have relied on data with poor spatiotemporal resolution and/or thunderstorm proxies with large errors and are, consequently, most accurately classified as thunderstorm event climatologies. These previous approaches were not able to represent thunderstorms in their true form as dynamic entities that can be identified, tracked, and counted. A robust thunderstorm climatology requires a method for identifying and tracking thunderstorms that uses data with a fidelity that is capable of resolving thunderstorm structure and lifecycle.

In this presentation we introduce preliminary results from a climatology of Great Plains thunderstorms developed using the ThOR algorithm (Thunderstorm Observation by Radar) which synthesizes Level II data from the NEXRAD network of surveillance Doppler radars, cloud-to-ground lightning recorded by the National Lightning Detection Network, and storm motion estimates from the NCEP North American Regional Reanalysis database. Key components of ThOR include 1) radar reflectivity quality control using a neural network to remove radar anomalies including ground clutter from anomalous propagation, 2) reflectivity filtering using a fuzzy logic technique to remove stratiform precipitation, 3) reflectivity cluster identification through image segmentation, 4) cluster tracking to develop “candidate” thunderstorm tracks, and 5) cloud-to-ground lightning attribution to classify “candidate” tracks as thunderstorms.

While the period of record for the data sets used to construct this climatology (2005-2007) is too short to reveal the manifestation of climate change in the number, distribution, or internal characteristics of thunderstorms, valuable insight into the distribution, structure, and behavior of thunderstorms can be identified. To this end, several diagnostic studies have been performed to interrogate the climatology. Results from an examination of the environments associated with thunderstorm initiation and a survey of the spatiotemporal patterns of the ratio of supercells to non-supercells will be highlighted in this presentation. Directions for future work will also be addressed.