Composite Environments of Severe and Nonsevere High-Shear, Low-CAPE Convective Events

Abstract

Severe convection occurring in environments characterized by large amounts of vertical wind shear and limited instability (high-shear, low-CAPE, or “HSLC,” environments) represents a considerable forecasting and nowcasting challenge. Of particular concern, NWS products associated with HSLC convection have low probability of detection and high false alarm rates. Past studies of HSLC convection have examined features associated with single cases; the present work, through composites of numerous cases, illustrates the attributes of “typical” HSLC severe and nonsevere events and identifies features that discriminate between the two. HSLC severe events across the eastern United States typically occur in moist boundary layers within the warm sector or along the cold front of a strong surface cyclone, while those in the western United States have drier boundary layers and more typically occur in the vicinity of a surface triple point or in an upslope regime. The mean HSLC severe event is shown to exhibit stronger forcing for ascent at all levels than its nonsevere counterpart. The majority of EF1 or greater HSLC tornadoes are shown to occur in the southeastern United States, so this region is subjected to the most detailed statistical analysis. Beyond the documented forecasting skill of environmental lapse rates and low-level shear vector magnitude, it is shown that a proxy for the release of potential instability further enhances skill when attempting to identify potentially severe HSLC events. This enhancement is likely associated with the local, in situ CAPE generation provided by this mechanism. Modified forecast parameters including this proxy show considerably improved spatial focusing of the forecast severe threat when compared to existing metrics.

Structure and Environment of Tornado-Spawning Extratropical Cyclones around Japan

“…The environments shown in this study are somewhat similar to high-shear, low-CAPE (HSLC) environments that mainly appear for the cool season tornadoes in the southeastern US (Sherburn and Parker 2014;Sherburn et al 2016). Sherburn et al (2016) examined the composite HSLC environments and showed that the forcing of ascents for severe events were stronger than that for nonsevere events, which is similar to our results in MAM and DJF.…”

Distinct aerosol effects on cloud-to-ground lightning in the plateau and basin regions of Sichuan, Southwest China

“…The freezing process releases more latent heat to stimulate convection, allowing more ice particles to participate in the electrification process of collision and separation, thereby enhancing lightning activity (Khain et al, 2008;Mansell and Ziegler, 2013;P. Zhao et al, 2015;Shi et al, 2015). A similar enhancement in lightning activity due to aerosols was also found in oceanic regions, where aerosols and their precursors discharged by ships significantly enhanced lightning activity over shipping lanes (Thornton et al, 2017).…”

Using Overshooting Top Area to Discriminate Potential for Large, Intense Tornadoes

“…Although this might be counter to what one would expect, the cool season events considered here also have higher EF ratings than do the warm season events (Figure 5c). Note that "cool season" and "warm season" are defined simply by date, rather than by storm environments (e.g., as in Sherburn et al, 2016, who associated cool season tornado events with relatively low convective available potential energy and high vertical wind shear). In terms of storm morphology, we find somewhat expected results, with QLCS tornadoes (generally) having lower EF ratings ( Figure 5c) and thus smaller OTAs (Figure 5b) than supercellular tornadoes (see also Figure 3).…”