Tornadogenesis resulting from the transport of circulation by a downdraft: Idealized numerical simulations

• Published in: Journal of the atmospheric sciences Vol. 60; no. 6; pp. 795 - 823
• Main Authors: MARKOWSKI, Paul M, STRAKA, Jerry M, RASMUSSEN, Erik N
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 15.03.2003
• Subjects: Atmosphere; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Rain; Relative humidity; Storms, hurricanes, tornadoes, thunderstorms; Tornadoes; Updraft; Relative humidity; Angular momentum transfer; Rain; Tornadogenesis; Intensity; Thermodynamic parameter; Numerical simulation; Tornado; Concentration; Downdraft
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/1520-0469(2003)060<0795:trftto>2.0.co;2

Idealized numerical simulations are conducted in which an axisymmetric, moist, rotating updraft free of rain is initiated, after which a downdraft is imposed by precipitation loading. The experiments are designed to emulate a supercell updraft that has rotation aloft initially. In the idealized simulations, the rain curtain and downdraft are annular, rather than hook-shaped, as is typically observed. The downdraft transports angular momentum, which is initially a maximum aloft and zero at the surface, toward the ground. Once reaching the ground, the circulation-rich air is converged beneath the updraft and a tornado develops. The intensity and longevity of the tornado depend on the thermodynamic characteristics of the angular momentum-transporting downdraft, which are sensitive to the ambient low-level relative humidity and precipitation character of the rain curtain. For large low-level relative humidity and a rain curtain having a relatively small precipitation concentration, the imposed downdraft is warmer than when the low-level relative humidity is small and the precipitation concentration of the rain curtain is large. The simulated tornadoes are stronger and longer-lived when the imposed downdrafts are relatively warm compared to when the downdrafts are relatively cold, owing to a larger amount of convergence of circulation-rich downdraft air. The results may explain some recent observations of the tendency for supercells to be tornadic when their rear-flank downdrafts are associated with relatively small temperature deficits.