Tornadoes in Environments with Small Helicity and/or High LCL Heights

Abstract

Recent studies have suggested that supercell tornado environments are usually associated with large 0-1-km storm-relative helicity (SRH) and relatively low lifting condensation levels (LCL heights). However, occasional tornadoes of significance occur in environments having characteristics that appear less supportive of supercell tornadoes, including small SRH values and/or relatively high LCL heights. Such tornadoes, whether associated with supercell or nonsupercell processes (more precisely termed mesocyclone and nonmesocyclone processes), present a challenge for forecasters. This empirical study uses a database of soundings derived from the Rapid Update Cycle model to examine thermodynamic characteristics of F1 and greater intensity tornado events associated with small SRH and/or high LCL heights. Results strongly suggest that many such tornado events are associated with steep lapse rates in the lowest few kilometers above ground. The low level of free convection heights, small convective inhibition, and sizable convective available potential energy below 3 km were also found to be of possible importance. These thermodynamic characteristics combined would likely reduce resistance to upward accelerations, potentially enhancing ascent for low-level parcels entering thunderstorm updrafts and, hence, low-level stretching. From prior research, if preexisting boundaries were available to provide surface vertical vorticity for stretching, such thermodynamic characteristics could be an important component of tornado events that involve nonmesocyclone processes. These same thermodynamic characteristics may also offer clues for the investigation of mesocyclone tornado events that do not fit well with accepted tornado forecasting parameters from prior studies.

The Carr Fire Vortex: A Case of Pyrotornadogenesis?

“…Specifically, the latent heating within the pyroCb and subsequent vertical stretching of antecedent near‐surface vorticity are key ingredients to the vortex formation. These factors differentiate this vortex from ordinary FGVs and are consistent with theories for nonmesocyclonic tornadogenesis in environments with high cloud bases and steep lapse rates (Agee & Jones, ; Davies, ; Wakimoto & Wilson, ).…”

Satellite detection of severe convective storms by their retrieved vertical profiles of cloud particle effective radius and thermodynamic phase

“…The combined physical considerations and preliminary statistical results suggest that clouds with extreme updrafts and small effective radii are highly likely to produce tornadoes and large hail, although the strength and direction of the wind shear probably would be major modulating factors. The generation of tornadoes often (but not always) requires strong wind shear in the lowest 6 km and low‐level helicity [ Davis , 2006]. According to the satellite inferences here this might be helping spin up the tornadoes in storms with very strong and deep updrafts that reach the anvil level.…”

“…As one would have expected those areas with tornadoes had warmer cloud base temperatures, greater CAPE and helicity values and slightly greater wind shear in the layer 0 to 6 km than the areas without tornadoes. Thus it comes as no surprise that the synoptic variables can be used to predict a general regional threat of tornadoes, as has been already done in previous studies [e.g., Hamill and Church , 2000; Dupilka and Reuter , 2006a, 2006b; Davis , 2006]. For a maximum similarity with the satellite analysis, the sounding analysis was done separately for satellite‐derived cloud base temperature Tb > 15°C and Tb < 15°C.…”

“…The existence of wind shear and low‐level storm relative helicity (rotation of the wind vector) were found to be with strong (at least F2) tornadoes [ Dupilka and Reuter , 2006a, 2006b]. However, even with small helicity, a steep low‐level lapse rate and large CAPE can induce strong tornadoes due to the large acceleration of the updrafts already at low levels [ Davis , 2006]. This underlines the importance of the updraft velocities in generating the severe convective storms, and the challenges involved in their forecasting based on sounding data alone.…”

Storm tracks and cyclone development using the theoretical concept of the Dynamic State Index (DSI)

“…McCoul and Cohen, 2002). The LCL describing the height at which an air parcel becomes saturated when it is lifted dry‐adiabatically is applied in the works of Davies (2006) and Craven et al (2002). Another variable quantifying the maximum energy of an air parcel which can be converted into kinetic energy during a possible ascent is called CAPE.…”