A Remote Microphysical Study of Severe Wind-producing Convective Storms

Convective storms that generate hail, lightning, and damaging winds have been identified as a formidable hazard to life and property. Even more impactful are stronger storms that generate and loft liquid-phase hydrometeors to high altitudes where freezing occurs and collisions between drops, graupel, and ice crystals lead to electrification. Condensate loading, sometimes combined with the lateral entrainment of subsaturated air in the storm middle level, initiates the convective downdraft. The subsequent melting of frozen hydrometeors and subcloud evaporation of liquid precipitation, in conjunction with precipitation loading, result in the cooling and negative buoyancy that accelerate the downdraft in the unsaturated layer. A downburst, in general, is defined as a strong downdraft that induces an outburst of damaging winds at or near the ground, and a microburst as a very small downburst with an outflow diameter of less than 4 km and a lifetime of less than 5 minutes. Previous studies of the microphysical structure of downburst-producing convective storms have entailed analysis of polar and geostationary satellite imagery and derived products, meteorological Doppler radar, and in-situ surface wind observations. The current study expands upon previous analysis by incorporating lower tropospheric vertical wind and temperature profile data generated by the Cooperative Agency Profilers (CAP) system that consists of Boundary Layer Profiler (BLP) instruments, operating at a frequency of 915 MHz, and Radio Acoustic Sounding System (RASS) instruments. In addition, Geostationary Lightning Mapper (GLM) data from Geostationary Operational Environmental Satellite (GOES)-16 will also be displayed and analyzed to better explain the role of lightning in a downburst-producing convective storm.

Selected thunderstorm events that demonstrate the physical process of downburst generation as observed simultaneously by the GOES-16 Advanced Baseline Imager (ABI) and GLM, Doppler radar (NEXRAD), and boundary layer profilers will be analyzed in this paper. Vertical sounding profile data from the CAP system has been applied as a supplement to Sounding/Hodograph Analysis and Research Program in Python (SHARPpy)-generated thermodynamic profiles to further study the favorable environment for severe convective storm winds. On the afternoon of 1 August 2017, severe downburst-producing thunderstorms occurred in the United States Mid-Atlantic region and over southern California that resulted in tree damage, downed power lines, and traffic disruptions. As shown in Figure 1, for both of these events, GOES-16 ABI water vapor (WV) – thermal infrared (IR) channel brightness temperature difference (BTD) imagery at 2-km resolution displayed a high level of detail in storm structure and was effective in identifying storm-scale features, including cold cloud tops (red shading) and dry-air intrusions (white arrows). Corresponding GLM imagery displayed lightning events in close proximity to downburst events at the time of downburst occurrence.