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Figure 13. Axisymmetric structure of (a) tangential winds, (b) radial winds, (c) angular momentum, and (d) perturbation pressure, at 0417 LST. Solid (dashed) lines represent positive (negative) values. The thick, solid line is the zero contour. Axisymmetric reflectivity factors are in color. Vectors in (b) are composed of axisymmetric radial winds and vertical velocities. (Adapted from Lee et al., 2000.)

increasing altitude. This feature stands in contrast to the inner core structure of a mature tropical cyclone in which the tilt is outward with increasing altitude. The mean radial wind [Fig. 15(d)] outside (inside) the RMW is inward (outward) with an intensity of 5ms-1 (2ms-1) and convergence near the RMW. It has been demonstrated in the above presentation that the GBVTD technique is a powerful tool for retrieving the kinematic structure of mesoscale vortices.

5.3. Structure of tornados

The first GBVTD analysis of a tornado, near Bassett, Nebraska, was presented by

Bluestein et al. (2003), who illustrated coherent tornado structures over a 3.5-minute period. The W-band radar (Bluestein et al., 1995) can only scan in one elevation angle; therefore, only horizontal structure can be deduced from this dataset. The Doppler velocity pattern and the corresponding GBVTD analysis of the Bassett tornado are deduced (Fig. 9 in Bluestein et al., 1995). However, with a scan update time of approximately 12 seconds, unprecedented evolution of the tornado was revealed. The axi-symmetric tangential and radial winds over the 3.5 minutes are portrayed in Fig. 16. The MATW is -28ms-1 at R - 170m. The axisymmetric radial winds are positive (outflow) inside R = 200 m and negative (inflow) outside

Distance East of Radar (KM)

Figure 14. (a) The surface stations report hourly data in northern Taiwan. Typhoon locations at 0502 and 0602 LST are indicated by the typhoon symbols. The terrain is in gray shades. (b) The thick lines are the GBVTD-derived axisymmetric perturbation pressure (P') at the surface. Circles are the perturbation pressures computed from five surface stations. (Adapted from Lee et al., 2000.)

Distance From Typhoon Center

Figure 14. (a) The surface stations report hourly data in northern Taiwan. Typhoon locations at 0502 and 0602 LST are indicated by the typhoon symbols. The terrain is in gray shades. (b) The thick lines are the GBVTD-derived axisymmetric perturbation pressure (P') at the surface. Circles are the perturbation pressures computed from five surface stations. (Adapted from Lee et al., 2000.)

R = 200 m which suggests a two-cell circulation. The switch-over point of the inflow and outflow is outside the RMW. This may be an artifact of the additional centrifugal effects experienced by the debris (targets of the radar), not by the winds (Dowell et al., 2005). The structure of another tornado, near Stockton,

Kansas, collected by the same radar, has been presented by Tanamachi et al. (2006). The near-stationary wave number 2 structure was attributed to the superimposition of an axisymmetric vortex onto a near-stationary deformation field (Bluestein et al., 2003). Recent studies by Lee et al. (2006a) and Tanamachi et al. (2006) indicated that this near-stationary wave number 2 asymmetry might be an artifact due to the spatial aliasing of a fast-moving tornado and a relatively slow scan rate of the W-band radar in this particular situation. Research is underway to systematically examine this issue.

In contrast, DOWs are capable of scanning multiple elevation angles at a slower volume update rate minute), but the 3D structures of tornados can be retrieved. Lee and Wurman (2005) produced the first comprehensive study of tornado circulations collected by DOWs on 4 May 1999 near Mulhall, Oklahoma, USA. The axisymmetric structure of the Mulhall tornado at 0310:03 (hhmm:ss) UTC is presented in Fig. 17. The axisymmetric tangential wind [Fig. 17(a)] (hereafter, all quantities are axisymmetric unless stated otherwise) profiles resemble a miniature, intense TC (about 1/15 in length scale) and possess characteristics commonly seen in the inner core region of a mature TC (e.g. Marks et al., 1992; Lee et al., 1994; Roux and Marks, 1996). The depth of the inflow layer reached 1 km but the most intense inflow was clearly confined near the surface. The downdraft magnitude of 30ms_1 is comparable to the estimated downdraft speed in the Dimmitt tornado with similar intensity (Wurman and Gill, 2000). The secondary (meridional) circulation [Fig. 17(b)] provided the first observational evidence that a classical two-cell circulation, commonly seen in tornado vortex chamber and numerical simulations with a swirl ratio greater than 1 (e.g. Ward, 1972; Rotunno, 1979), does exist in intense tornados. The swirl ratios of the Mulhall tornado computed from the GBVTD-derived axisymmetric circulation during the entire observational period

Figure 15. GBVTD-derived horizontal vortex circulations at (a) 2km and (b) 4km altitudes. The lower panels show (c) the axisymmetric tangential winds and (d) radial winds of the mesocyclone.

(~14 minutes) are all above 2 (Table 1), consistent with the two-cell structure in the secondary circulation and the multiple vortices observed in the Mulhall tornado. A striking example is seen in Figs. 3(c) and 3(d), where six small vortices can be identified on the west side of the tornado, especially in the velocity field, at 1316:28 (Wurman, 2002; Lee and Wurman, 2005).

The pressure deficit from advection terms [Fig. 17(c)] is consistent with the secondary circulation, a component rarely, if ever, resolved in past observational studies of tornadoes. The total pressure deficit [Fig. 17(g)] at z = 50 m

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