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Abstract This study analyzes aboveground thermodynamic observations in three tornadic supercells obtained via swarms of small balloon-borne sondes acting aspseudo-Lagrangiandrifters; the storm-relative winds draw the sondes through the precipitation, outflow, and baroclinic zones, which are believed to play key roles in tornado formation. Three-dimensional thermodynamic analyses are produced from the in situ observations. The coldest air is found at the lowest analysis levels, where virtual potential temperature deficits of 2–5 K are observed. Air parcels within the forward-flank outflow are inferred from their equivalent potential temperatures to have descended only a few hundred meters or less, whereas parcels within the rear-flank outflow are inferred to have downward excursions of 1–2 km. Additionally, the parcels following paths toward the low-level mesocyclone pass through horizontal buoyancy gradients that are strongest in the lowest 750 m and estimated to be capable of baroclinically generating horizontal vorticity having a magnitude of 6–10 × 10−3s−1. A substantial component of the baroclinically generated vorticity is initially crosswise, though the vorticity subsequently could become streamwise given the leftward bending of the airstream in which the vorticity is generated. The baroclinically generated vorticity could contribute to tornado formation upon being tilted upward and stretched near the surface beneath a strong, dynamically forced updraft. Significance StatementSwarms of balloon-borne probes are used to produce the first-ever, three-dimensional mappings of temperature from in situ observations within supercell storms (rotating storms with high tornado potential). Temperature has a strong influence on the buoyancy of air, and horizontal variations of buoyancy generate spin about a horizontal axis. Buoyancy is one of the primary drivers of upward and downward motions in thunderstorms, and in supercell storms, horizontally oriented spin can be tipped into the vertical and amplified by certain arrangements of upward and downward motions. Unfortunately, the long-standing lack of temperature observations has hampered scientists’ ability to evaluate computer simulations and the tornadogenesis theories derived from them. We find that significant spin could be generated by the horizontal buoyancy variations sampled by the probes.
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Using Ensemble Sensitivity Analysis to Identify Storm Characteristics Associated with Tornadogenesis in High-Resolution Simulated Supercells
Abstract This study aims to objectively identify storm-scale characteristics associated with tornado-like vortex (TLV) formation in an ensemble of high-resolution supercell simulations. An ensemble of 51 supercells is created using Cloud Model version 1 (CM1). The first member is initialized using a base state populated by the Rapid Update Cycle (RUC) proximity sounding near El Reno, Oklahoma, on 24 May 2011. The other 50 ensemble members are created by randomly perturbing the base state after a supercell has formed. There is considerable spread between ensemble members, with some supercells producing strong, long-lived TLVs, while others do not produce a TLV at all. The ensemble is analyzed using the ensemble sensitivity analysis (ESA) technique, uncovering storm-scale characteristics that are dynamically relevant to TLV formation. In the rear flank, divergence at the surface southeast of the TLV helps converge and contract existing vertical vorticity, but there is no meaningful sensitivity to rear-flank outflow temperature. In the forward flank, warm temperatures within the cold pool are important to TLV production and magnitude. The longitudinal positioning of strong streamwise vorticity is also a clear indicator of TLV formation and strength, especially within 5 min of when the TLV is measured. Significance StatementTornadoes that form in supercell thunderstorms (long-lived storms with a rotating updraft) are heavily influenced by the features created by the storm itself, such as the temperature of a downdraft. In this study, many different iterations of a strong supercell thunderstorm are simulated, in which tornado-like features are formed at different times with widely different strengths. A statistical method is used to identify what the storms had in common when they produced a tornado-like feature, and what they had in common when one failed to form. This study is important because it highlights which storm features are most influential to tornado formation using an objective method, with results that can be used when observing supercells in the field.
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- Award ID(s):
- 1824713
- PAR ID:
- 10467591
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 151
- Issue:
- 10
- ISSN:
- 0027-0644
- Format(s):
- Medium: X Size: p. 2633-2658
- Size(s):
- p. 2633-2658
- Sponsoring Org:
- National Science Foundation
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