Figure 6. Vortex emergence and evolution for two different initial vorticity spatial scales, r* = 1 (5—10 km scale) and r* = 4 (30-40km scale).
core vortices are used in the experiments. The parameter y is now taken to be the ratio of the vorticity in the core vortex to the peak turbulent background vorticity, which is set to 3x 10-3s-1. This is the same amplitude used for the companion vortex in the binary vortex experiments. We have avoided the use of stronger turbulent background vorticity in order to retain the basic nature of the problem we are investigating.
In their analysis of this problem, Rozoff et al. (2006, hereafter R06) used a core vortex that approximates a Category 5 hurricane (75 ms-1 tangential winds near 25 km radius) embedded within turbulent background vorticity elements having horizontal scales between 20 km and 40 km and a peak amplitude of 1 x 10-3s-1. The vorticity in the R06 core vortex is 6.5x 10-1s-1, so that the corresponding vorticity strength ratio is y = 6.5, as in the present article. Their integrations (e.g. their Fig. 8) result in a concentric structure, but with a rather weak vorti-city halo due to the vorticity mixing that occurs. We have used the formula of R06 in the specification of the characteristic spatial scale of the turbulent background. In addition, we have extended the R06 experiments by including a vor-ticity clear zone (moat) near the core vortex. In real hurricanes the moat can be viewed as being the result of rapid filamentation (R06) and of strong subsidence induced by surrounding convection (e.g. Dodge et al., 1999).
Figure 7 shows results from the turbulent background experiments for the (1) straining-out case (r* = 1, y = 2), (2) merger case (r* = 4, y = 2), (3) tripole case (r* = 4,
Y = 6). A vorticity clear zone (moat) of 10 km width is introduced near the core vortex for the last two experiments. Note that the outer vor-ticity bands in the concentric case in Fig. 7, unlike the results in R06 which are a magnitude weaker, are of similar strength compared to the binary vortex interaction. Figure 7 indicates that the core vortex is able to stir and to mix the turbulent background vorticity into a concentric eyewall end state. Of particular interest in Fig. 7 is the tripole case. A tripole is a linear arrangement of three regions of distributed t=0hr t=12hr t=18hr t=24hr
Figure 7. Turbulent background experiments for the (1) straining-out case (r* = 1, 7 =2), (2) merger case (r* = 4, 7 = 2), (3) tripole case (r* =4, 7 = 2), and (4) concentric case (r* =4, 7 = 6). A vorticity clear zone (moat) of 10 km is introduced near the core vortex for the last two experiments.
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