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The Advanced Regional Prediction System (ARPS) (Xue et al., 2000, 2001, 2003) was used to simulate the supercell thunderstorm. As in SX08, the Lin-type single moment microphysical scheme (referred to as LFO83, Lin et al., 1983) was chosen to examine the DSD impacts on the supercell and tornadogenesis. In LFO83, mixing ratios of six-category water substances (graupel and hail were treated as one category here, using "hail" as the category name) were explicitly predicted. Non-precipitating hydrometeors were assumed to be monodisperse. For all the precipitating hydrometeors (rain, snow and hail), an exponential DSD was assumed: nx(D)=n0xexp(-#cod#923;xDx), where x denotes the species of hydrometeor, Dx is the particle diameter, nx(D) is the number of particles per unit volume per unit size interval, and n0x and #cod#923;x are the intercept and slope parameters, respectively (LFO83; SX08). The intercept parameter was specified as a constant value and the slope parameter was a function of the intercept parameter, density, and mixing ratio of the hydrometeors. In LFO83, the default values of the intercept parameters for rain, hail and snow were 8#cod#215; 106,4#cod#215; 104 and 3#cod#215; 106 m-4, respectively. DSDs with a larger (smaller) intercept have a larger (smaller) slope and thereby favor smaller (larger) particles in clouds.
Figure 1. (a) Real-data simulation results at 1200 UTC: rain mixing ratio (shaded; g kg-1) and horizontal wind vectors (m s-1) at 500 m; (b) skew T-logP plots for soundings observed at "AQ" (red) and model extracted from "LA" (blue); (c) skew T-logP plots for soundings of May20 (red) and LML (blue) used for 1-km-grid simulations.
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The observed sounding nearest to the tornado event in time and location was first applied to initiate the simulation with ARPS. The 1200 UTC sounding at Anqing (AQ), #cod#8764;150 km away from WW (Fig. 1a), is presented in Fig. 1b (red), which was observed a few hours before the tornado outbreak. This sounding shows a large convective available potential energy (CAPE) of 2855 J kg-1, a strong vertical wind shear of 24 m s-1 from the surface to 6 km (16 m s-1 in the lowest 1.5 km), and a relatively low lifting condensation level (LCL) below 600 m, all indicating a favorable environment for the supercell formation. Unfortunately, the observed sounding was unable to reproduce a sustained supercell during the simulation. Multiple reasons might be responsible for the failure in simulating the supercell storm, e.g., the coarse vertical resolution and the wet mid-layer air condition in the sounding, and the long distance between the storm location and sounding station.
Following (Dawson et al., 2010), an extracted sounding from the ARPS 3-km-grid real-data simulation instead of the observed one was used to initiate the simulation. A real-data simulation was conducted from 0600 UTC to 1800 UTC 8 July 2003 with full physics including surface physics and a 1.5-order TKE-based subgrid-scale turbulence closure (Xue et al., 2001). The LFO83 microphysics scheme was chosen and the cumulus parameterization was turned off. The model domain was 1080#cod#215; 1080#cod#215; 20 km3 in size located within (26#cod#x000b0;-36#cod#x000b0;N, 112#cod#x000b0;-121#cod#x000b0;E) over eastern China, with a horizontal resolution of 3 km and 51 vertical levels of 20 m grid spacing near the ground and 770 m near the model top. The initial and lateral boundary conditions were derived from 1#cod#x000b0;#cod#215; 1#cod#x000b0; National Centers for Environmental Prediction (NCEP) reanalysis data at 6-h intervals. The model sounding was extracted at the grid point most representative of the unstable inflow region of the simulated storms (marked "LA" in Fig. 1a) at 1200 UTC. The model extracted sounding had similar temperature and wind profiles as observed (blue in Fig. 1b). However, the CAPE was 2135 J kg-1, a little smaller than observed, and the 0-6 km (0-1 km) vertical wind shear was about 21.3 (6.6) m s-1 with the wind hodograph turning clockwise under 1 km. The dewpoint profiles were also different, i.e., the mid-troposphere was much direr for the extracted sounding with a 600-hPa relative humidity of 40% (65% for the observed). The mid-level humidity condition may have large impacts on producing a supercell and its tornadogenesis through affecting the entrainment processes within the storm (Gilmore and Wicker, 1998; James and Markowski, 2010).
(Grams et al., 2012) analyzed the thermodynamic conditions of 448 significant tornado events across the contiguous United States from 2000 to 2008 and found that the mean environmental mixed-layer CAPE was around 1500 J kg^-1 for supercells over the South Great Plain in spring and 2100 J kg^-1 over the North Great Plain in summer, which is comparable with the case in the present study (2135 J kg^-1). However, the present case was characterized by a higher melting level and a deeper warm layer than those over the U.S. Great Plains. The average 500-hPa (700-hPa) temperature was -3#cod#x000b0;C (10#cod#x000b0;C) for the present case, but around -11.5#cod#x000b0;C (6#cod#x000b0;C) for the 448 tornado events in the U.S., and the melting level for the present extracted sounding was #cod#8764;5.2 km AGL, versus close to 4 km AGL (below 600 hPa) for most of the Great Plains cases (e.g., SX08; Dawson et al., 2010; Grams et al., 2012). Such thermodynamic conditions over the subtropics may allow precipitating hydrometeors to remain in storms longer, resulting in more melting/evaporation and therefore greater sensitivity of tornadogenesis to the variation of DSDs relative to the U.S. Great Plains is suggested.
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SX08 pointed out that supercell tornadogenesis is very sensitive to the intercept values of rain and hail DSDs, while the impact of snow DSD is relatively small (Snook and Xue, 2006). In this study, we further investigate the role of rain and hail intercept parameters in the tornadogenesis within supercells over the subtropics. Ten sensitivity experiments were performed with various intercept parameters. The specifications of the intercept parameters for each experiment are summarized in Table 1. The first experiment (referred to as CNTL) was conducted with the intercept parameters as the default values in LFO83, and the following eight experiments were ones with perturbed intercept parameters for rain or hail but a default value for snow. For example, the hail and rain intercepts were 4#cod#215; 106 and 8#cod#215; 107 m-4 in experiment H6R7 (4#cod#215;102 and 8#cod#215; 105 m-4 in H2R5), respectively, which will favor smaller (larger) hailstones and raindrops. In SX08, the snow intercept parameter was set as 8#cod#215;106 m-4 instead of the default value (3#cod#215; 106 m-4). Therefore, an additional experiment, S8, with identical parameter configuration as CNTL except for a different snow intercept parameter was conducted to test the sensitivity of model results to the snow size distribution.
The values of intercept parameters (n0) applied in different sensitivity experiments on a 100-m grid and features of the tornadic vortices in experiments (denoted by *) that produced a sustained tornado vortex. Name n 0(m-4) Characteristics of tornadic vortices #cod#160; Rain Hail Snow Duration a (min) Max. #cod#950; Max. winds Rank CNTL* 8#cod#215; 106 4#cod#215; 104 3#cod#215; 106 13 (5160-5880 s) 0.39 (5520 s) 45.7 (5580 s) EF1 H2 8#cod#215; 106 4#cod#215; 102 3#cod#215; 106 - - - - H6 8#cod#215; 106 4#cod#215; 106 3#cod#215; 106 - - - - R5* 8#cod#215; 105 4#cod#215; 104 3#cod#215; 106 4 (6720-6900 s) 0.35 (6840 s) 34.4 (6720 s) EF0 R7 8#cod#215; 107 4#cod#215; 104 3#cod#215; 106 - - - - H2R5 8#cod#215; 105 4#cod#215; 102 3#cod#215; 106 - - - - H6R7 8#cod#215; 107 4#cod#215; 106 3#cod#215; 106 - - - - H2R7 8#cod#215; 107 4#cod#215; 102 3#cod#215; 106 - - - - H6R5 8#cod#215; 105 4#cod#215; 106 3#cod#215; 106 - - - - S8* 8#cod#215; 106 4#cod#215; 104 8#cod#215; 106 13 (8160-8880 s) 0.36 (8580 s) 38.8 (8340 s) EF1 aDuration is the continuous time with max. near surface winds #cod#62;29 m s-1 (EF0) and max. vertical vorticity (#cod#950;) #cod#62;0.1 s-1. For all experiments, a high horizontal resolution of 100 m was used to explicitly resolve the tornado-scale characteristics within a supercell (Grasso and Cotton, 1995; Wicker and Wihelmson, 1995; Lee and Wilhelmson, 1997; Finley et al., 2001; Noda and Niino, 2005; Lerach et al., 2008; SX08). The domain was 64#cod#215; 64#cod#215;20 km3 in size with 81 vertical layers stretched from 10 m near the ground to roughly 500 m at the model top. Convection was initialized with a warm thermal bubble of 4 K maximum perturbation centered at point x=48 km, y=20 km, and z=1.5 km with horizontal and vertical radii of 10 and 1.5 km, respectively. Before initialization, a constant wind of u=10 m s-1 and v=6 m s-1 was subtracted from the sounding to keep the simulated storm within the domain, as in some previous studies (Xue et al., 2001; Caya et al., 2005; Gao and Xue, 2008; Dawson et al., 2010). All simulations were integrated for 3 h with a time step of 0.2 s.
Three additional sets of simulations but at 1 km resolution (for saving computation time) were respectively conducted with the present model-extracted sounding, the one used in SX08 (red in Fig. 1c), and a modified one similar to the present extracted one except for a lower melting level (blue in Fig. 1c), so as to further explore the role of melting level on DSD impacts.