Genesis mechanism and structure of a supercell tornado in a fine-resolution numerical simulation

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1 Genesis mechanism and structure of a supercell tornado in a fine-resolution numerical simulation Akira T. Noda a, Hiroshi Niino b a Ocean Research Institute, The University of Tokyo, Minamidai, Nakano, Tokyo, Japan (Current affiliation: Frontier Research Center for Global Change, Kanazawa, Showa-machi, Yokohama, Kanagawa, Japan) b Ocean Research Institute, The University of Tokyo, Minamidai, Nakano, Tokyo, Japan KEYWORDS: supercell, tornadogenesis, fine-resolution numerical simulation, small-scale vortex in a gustfront ABSTRACT: A major tornado spawned by a supercell is reproduced by a fine-resolution threedimensional numerical simulation and its genesis mechanism is clarified. The tornado originates from one of the small-scale vortices on the gust front that forms between a warm moist environmental air and a rain-cooled air produced by the storm. Among several small-scale vortices, only the one that is close to the low-level updraft associated with the low-level mesocyclone develops into a major tornado. Several interesting characteristics of a threedimensional structure of the simulated tornado vortex are also reported. 1 INTRODUCTION It has been known that severe tornadoes are often spawned by a special type of cumulonimbus cloud called a supercell. The supercell develops in an environment in which the direction and speed of the wind varies rapidly with height. The supercell starts to acquire a circulation around the vertical axis roughly an hour after its initiation. The circulation has a typical diameter of about several kilometers, and is called a mesocyclone. It has been recognized that the mesocyclone plays an important role for tornadogensis. Because of difficulty for simulating a supercell tornado realistically, the detailed mechanism that explains comprehensively the stormscale motion to development of tornado and the vorticity source of supercell tornado, however, has not been satisfactorily understood. This paper reports the recent work done by Noda and Niino (2005) that reveals genesis and structure of supercell tornado using a high-resolution numerical simulation. 2 NUMERICAL MODEL The numerical model used for the simulation is ARPS Ver (Xue et al. 1995). The size of the three-dimensional calculation domain is 66.4 km times 66.4 km in the horizontal directions and is 15.1 km in the vertical direction. This domain is divided into 951 times 951 grid points in the horizontal direction and 45 grid points in the vertical. The horizontal grid interval is uniform, and is 70 m. The vertical grid interval varies from 10 m near the ground to 763 m near the top of the domain. Free-slip and adiabatic conditions are used for both top and bottom boundaries, and a radiation condition for lateral open boundaries. Rayleigh damping with the e-folding time of 300 s is introduced above 12 km. The simulation is started from a horizontally uniform state that represents the environment of the Del City storm, which caused a F1 tornado in Del City, Oklahoma on 20 May 1977 (Klemp et al. 1981). An ellipsoidal thermal bubble of maximum

2 temperature anomaly of 4 K with horizontal and vertical diameters of 10 km and 1.5 km, respectively, is given to initiate the storm. 3 RESULT 3.1 Development of Tornado The simulated storm evolves in a manner a typical supercell does (not shown). Figure 1 shows a time-evolution of the vertical distributions of the maximum updraft intensity, the maximum vertical vorticity and the minimum perturbation pressure in the calculation domain. The updraft (Fig. 1a) gradually intensifies with time after the initiation of the storm. After t=1700s, the updraft above 4 km AGL (Above Ground Level) always remains more than 30 m/s and occasionally accelerates over 60 m/s between 8 and 10 km AGL due to condensational heating of water vapor. After t=4000s, the updraft intensifies rapidly below 4 km AGL, and exceeds 40 m/s between 1 and 2 km AGL after t=4200s. In response to the rapid increase of the updraft, the vertical vorticity (Fig. 1b) starts to develop near the ground after t=4300s, indicating a development of a tornado. By t=4504s, the vertical vorticity reaches its maximum of 0.85 s -1. Figures 1a and 1c shows that the rapid intensification of the updraft is caused by a sequential evolution of the perturbation pressure field. After t=3300s, the perturbation pressure starts to drop at around 1.8 km AGL. This pressure drop accelerates the updraft below this level. After t=3800s, another pressure drop starts at around 1 km AGL. This eventually causes the extremely strong updraft between 1 and 2 km. The enhanced updraft below the pressure drop causes tilting of the horizontal vorticity in the low-level and subsequent stretching, resulting in a development of a low-level mesocyclone. The second pressure drop at about 1 km AGL occurs in response to this low-level mesocyclone, and produces the intense updraft exceeding 40 m/s between 1 and 2 km AGL, which, in turn, stretches the vertical vorticity near the ground to generate the tornado. Figure 1. Time-height cross section of (a) maximum updraft (m/s), (b) maximum vertical vorticity (1/s), and (c) minimum perturbation pressure (hpa) in the simulated supercell between 0 and 5000 s. (after Noda and Niino 2005)

3.2 Tornadogenesis Figure 2 shows the time-evolution of the vertical vorticity right near the ground (5 m AGL) and updraft at 200 m AGL between t=3900s and 4504s around the tornadic region.

3 3.2 Tornadogenesis Figure 2 shows the time-evolution of the vertical vorticity right near the ground (5 m AGL) and updraft at 200 m AGL between t=3900s and 4504s around the tornadic region. By t=3900s, the cold air generated by evaporation of precipitation particles inside the storm flows out from the west side of the figure and collides with the warm moist environmental air from the northeast side of the figure. This convergence line between the westerly and northeasterly flows is called a gust front and is associated with a significant vertical vorticity due to the horizontal wind shear. Several vortices (hereafter denoted by Vortices A, B, C and D) are generated along the gust front by a barotropic instability of the horizontal shear flow. Vortices A, B, C and D move southward by the advection of the environmental northerly wind, and go out of the domain of Fig. 2 without showing noticeable amplification. At t=3968s, a new vortex (Vortex E) appears near the southern end of the updraft region. Though Vortex E exhibits a considerable development until its vertical vorticity exceeds 0.3 s -1, it eventually dissipates without developing into a major tornado. (These vortices may correspond to gustnadoes (Doswell et al. 1993)). This occurs because the strong low-level rotation generates a downward pressure gradient, causes a vortexscale downdraft, and results in compressing the vertical vortex tube. By t=4069s, another new Vortex F emerges near the updraft center of the low-level mesocyclone. The updraft stretches the vertical vorticity of Vortex F to develop into the tornado. Figure 2. Vertical vorticity (contoured at 0.01 s -1, 0.03 s -1 and every 0.05 s -1 between 0.05 s -1 and 0.80 s -1 ) and storm-relative horizontal wind vector (drawn for every 10 grids) at 5 m AGL between t=3900s and 4504s. The updraft region at 200 m AGL is shaded according to the gray scale in the right hand side. Figure 3. Vertical cross sections of the tornado at t=4504s along y=23.35km for (a) horizontal wind (drawn for every 5 m/s) and (b) perturbation pressure (drawn for every 3 hpa). The vertical vorticity is shown by the gray scale. The zero contour lines are omitted in (a).

4 3.3 Tornado vortex We shall show some out of the interesting features of the tornado vortex. Figures 3a and 3b show vertical cross sections of the horizontal wind speed and the perturbation pressure near the tornado, respectively. It is seen that the vertical vorticity is strongest near the ground. The vorticity gradually decreases with increasing height. The axis of the maximum vorticity tilts westward with height. In the present simulation, the vertical vorticity of the tornado originates from the gust front, and develops toward the low pressure associated with the mesocyclone which is located to the northwest of the tornado (cf. Fig. 2). This seems to result in the westward tilt of the tornado. If the isotaches of 32 m/s is assumed to represent the width of the tornado, the radius of the tornado is 400 m near the ground, increases with height, and becomes more than 700 m at 1 km AGL (Fig. 3a). The axis of the minimum pressure coincides with that of the maximum vertical vorticity (Fig. 3b). 4 CONCLUSIONS The most important finding of the present study is that the immediate source of the vertical vorticity of the major tornado spawned by a supercell storm is the vertical vorticity associated with the gust front. Several gustnado-like vortices caused by shear instability appear on the gust front though they do not develop into the major tornado. In order for an initial vortex on the gust front to develop into a major tornado, a strong low-level updraft associated with a low-level mesocyclone is necessary. The existence of small-scale vortices near the tornado as observed in our simulation are actually acquired by a conventional mobile Doppler radar (Bluestein et al. 2003). It is known that even if a mesocyclone is detected, only 20 percent of the mesocyclones spawn a tornado (Burgess 1997), suggesting that an existence of a mesocyclone alone is not sufficient for generating a tornado. It is also pointed out that apparently very similar morphology of the mesocyclones does not assure the tornado genesis (Wakimoto and Cai 2000). These observational facts and the results of the present study strongly suggest that the timing between the developments of the vortices along the gust front and the intensification of the low-level updraft associated with the low-level mesocyclone development is important for tornadogenesis. 5 ACKNOLEDGEMENTS The present work was partly supported by Grant-in-Aids for Scientific Research (B)(2) No , the Ministry of Education, Culture, Sports, Science and Technology. This simulation was made using the Advanced Regional Prediction System(ARPS) developed by the Center for Analysis and Prediction of Storms (CAPS), University of Oklahoma. CAPS is supported by the National Science Foundation and the Federal Aviation Administration through combined grant ATM REFERENCES

A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget

Journal of the Meteorological Society of Japan, Vol. 88, No. 2, pp. 135--159, 2010. 135 DOI:10.2151/jmsj.2010-203 A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget Akira T.