Lemon and Doswell (1979) Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm evolution and mesoscyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107,

Lemon and Doswell attempt to synthesize the observations and limited modeling studies available through 1979 into a conceptual model of the supercell and mesocyclone structure. Their paper establishes the importance of the rear flank downdraft in tornadogenesis in supercells. Their conceptual models are still valid today with some modifications They also evaluate the likely source of the strong vorticity associated with tornadoes.

SW NE Observations suggest that the following sequence occurs in some supercells: Initially, a rotating updraft is present (not shown…would be panel before (a)) Dry (low  e ) impinges on the rotating updraft aloft (7-10 km) creating a downdraft. The downdraft/updraft interface becomes a second center of rotation: the incipient TVS The new rotation center begins to dominate the flow until a single rotation center (coincident with the TVS) is present along the updraft/downdraft interface within the rotating updraft. The TVS migrates under the rotating updraft Rotating updraft Tornado vortex signature

Stippled = updraft Shaded = downdraft Dual-Doppler analysis of the 8 June 1974 Storm over Oklahoma City that produced 3 tornadoes Storm relative winds (1 grid length = 10 m/s) Initial mesocyclone corresponds to central updraft region where echoes are weak (called the Bounded Weak Echo Region BWER) Mesocyclone centered on strongest vertical velocity gradient later in storm when tornadoes occurred

8 June 7424 May 73 Union City, OKManhattan, KS “Clear slot” actually wrap around curtain of precip in rear flank downdraft Wall cloud and updraft Evidence of the rear flank downdraft is the clear slot or curtain of precipitation that appears SW of the wall cloud/tornado

Radar data from tornadic supercell (from Barnes 1978) Shaded = 1.5 km reflectivity 32 dBZ or greater Contoured = 7.5 km reflectivity Heavy line = 15 dBZ Light lines = 32, 45, 52 dBZ Black dot = storm top Winds = storm relative at 6.75 km 1 full barb = 10 m/s Hook echo Flanking line of clouds along rear flank gust front Weak echo along rear flank of storm marking location of rear flank downdraft Heavy precipitation and forward flank downdraft Radar data used to develop conceptual model of supercell structure

Schematic plan view of a tornadic supercell at the surface. T Red line = radar echo. Gust fronts and “occluded” region depicted with blue frontal symbols. UD = Updraft FFD = Forward flank downdraft RFD = Rear flank downdraft (RFD) T = Tornado Streamlines are relative to the ground

Three dimensional view of the development of the mesocyclone and tornado within a supercell

Storm relative flow at mid-levels between the time of the top left panel and the top right panel on the 3-D depiction Strong temperature gradient Original mesocyclone Transformed mesocyclone (i.e. TVS location) Cyclonic rotation Anticyclonic rotation

Simplified vorticity equation for coordinate system where x axis is perpendicular to the gradient of temperature (baroclinic zone) aloft in the mesocyclone What is the source of rotation in the mesocyclone? x y Time rate of change of vorticity following a parcel Coriolis effects D = divergence  = specific volume Others = usual meaning Vortex stretching Vortex tilting Pressure/density solenoidal term

Lemon and Doswell estimated the terms and concluded that Tilting of the ambient shear dominated vorticity production, although stretching of the vortex and solenoidal effects were also important