Tropical Cyclone Supercells and Tornadoes: Gaps in the Knowledge Base Matthew Morin Dept. of Marine, Earth and Atmospheric Sciences North Carolina State University Collaborative Science, Technology & Applied Research Program (CSTAR)

Outline Background Gaps in the knowledge base TC tornado environmental conditions Sea to land transition Low-level baroclinicity Midlevel dry air intrusion Radar characteristics Gaps in the knowledge base Preliminary idealized TC simulation output Summary/discussion

Background The remnants of a dissipating TC can spawn tornadic outbreaks comparable to those of the Great Plains Clear evidence of embedded supercells: Tornadoes had multiple vortices beneath a wall cloud, persistent hook echoes, possible “right-movers”, observations of a “tail cloud” McCaul (1987)

McCaul (1991) TC tornado development is favored in the right-front quadrant of the tropical cyclone relative to the storm’s motion

McCaul (1991) SRH and low level vertical wind shear are enhanced by the interaction of the TC’s swirling flow with a steering current containing shear roughly parallel to the storm’s track

TC tornado environmental conditions McCaul and Weisman (1996) Parcel buoyancy is typically small in a TC environment However, strong updrafts are still possible when to the concentration of buoyancy is collocated with high shear allows for perturbation pressure gradient forces similar to those found in Great Plains supercells

TC tornado environmental conditions TC tornado composite environment TC tornado environmental conditions McCaul and Weisman (1996)

TC tornado environmental conditions McCaul and Weisman (1996) Parcel buoyancy is typically small in a TC environment However, strong updrafts are still possible when to the concentration of buoyancy is collocated with high shear allows for perturbation pressure gradient forces similar to those found in Great Plains supercells 8

Fort Sill, Oklahoma McCaul and Weisman (1996) Markowski et al. (2003)

Sea to land transition Wind profile is dramatically altered, especially in the near-surface layer Creation of an environment more conducive for spawning supercells and tornadoes increase in low level vertical wind shear as increased surface drag is experienced by the low level flow Baker et al. (2008) noted a maximum in midlevel rotational velocity in a tornadic cell associated with hurricane Ivan as it made landfall

Low-level baroclinicity The presence of low-level surface baroclinicity associated with thermal boundaries has been recognized as an important contributor to tornadogenesis (e.g. Rao et al. 2005; Schneider and Sharp 2007). The impact of such a boundary on the intensity of a tornado outbreak is realized in McCaul et al. (2004), where most of the tornadoes associated with Tropical Storm Beryl occurred along a thermal boundary as the remnants of Tropical Storm Beryl crossed South Carolina .

Dry air intrusions Midlevel dry intrusions play a significant role in modifying the thermodynamic environment such that development of more intense tornadic cells are favored In the dry air intrusion process, resultant evaporative cooling at mid levels increases the environmental lapse rate thus enhancing CAPE and surface-based instability (e.g. Curtis 2004; Baker et al. 2008) Lack of midlevel clouds due to the dry-air intrusion can increase diurnal heating thereby further promoting surface-based destabilization (Schneider and Sharp 2007) Curtis 2004

Gaps in the knowledge base What are the implications on the location and duration of the dry air intrusion? How does the vertical and horizontal location of the dry air intrusion impact local stability and ultimately tornado potential within the TC? How does the duration of the intrusion affect the thermodynamic profile over time? How does diurnal forcing influence the nature of TC tornado outbreaks? To what extent does it create/modify baroclinic boundaries?

Radar Characteristics Low-level mesocyclogenesis precursors include changes in the notch-lobe radar signatures and sudden tightening of the rotation signature (Spratt et al. 1997) From Schneider and Sharp (2007) Three precursors for TC-tornadogenesis a near gate-to-gate mesocyclone rotational velocity of > 20 kt a hook or appendage signature in the reflectivity data the presence of a velocity enhancement signature

Gaps in the knowledge base The physical mechanisms within TC-tornadic cells that cause a VES are not completely evident (Schneider and Sharp 2007). Schneider and Sharp (2007)

Preliminary idealized TC simulations Made possible thanks to: Renaissance Computing Institute (RENCI) supercomputers Kevin Hill & Dr. Gary Lackmann Aforementioned gaps in the TC knowledge base can be addressed using the WRF model with initial conditions for an idealized TC Nest down to storm scale to resolve TC supercells Sensitivity tests Add mean wind so that TC can eventually make “landfall” Vary parameters such as speed & strength of TC, angle of landfall trajectory, land characteristics, insolation, dry air intrusions, boundaries

Domain comparisons of TC outer rainband cells 6 km grid spacing

Domain comparisons of TC outer rainband cells 2 km grid spacing

Domain comparisons of TC outer rainband cells Domain 2 (new) 1.2 km grid spacing

Domain comparisons (different time & location) 240 meter grid spacing

Domain 3 zoomed in on RF quadrant of TC ~55.5 km Placement of the finest mesh for storm-scale analyses - Calculate max vorticity for each time step to quickly find where cells are potentially supercellular

Summary/discussion To what extent do supercells that form near the core of the TC differ from those forming within the TC’s outer rainbands? Gulf versus Atlantic landfalls Differences between the characteristics of TC supercells on day one of landfall and those still present and forming three days after landfall

Summary/discussion The physical mechanisms within TC-tornadic cells that cause a VES are not completely evident. What is the time and spatial scale of mesocyclogenesis and tornadogenesis precursors in TCs? What are the implications on the location and duration of the dry air intrusion? How does diurnal forcing influence the nature of TC tornado outbreaks? Meso- and storm-scale analyses of environmental changes within the TC as the highest reflectivity cells make landfall are needed to elucidate the small-scale gradients that are present and responsible for enhancing resultant supercells.

Your feedback is welcomed and appreciated Matthew Morin mjmorin@ncsu.edu