On May 6th 2012, a strong storm process attacked Kanto plain in Japan in which three tornadoes was observed and caused serious damages. Together with this tornadogenesis storm process, extreme local gust occurred. Considering the wind environment for the man-made structure stability and wind load caused structure response, the wind environment in the severe storm condition is different from normal cases like typical wind tunnel experiments or even a typhoon wind environment, which makes it important to understand the process of this strong convective process and the behavior of local gust. Authors have simulated this process by mesoscale meteorological model Weather Research and Forecast (WRF). Two nested domains have been employed downscaling the resolution from 1.25 km to 0.25 km. The results will firstly be verified by introducing the successfully reproduced mesocyclone in a strong convective storm and a tornado-like structure. Considering the sensitivity to Surface layer (SL) schemes and Planet Boundary Layer (PBL) schemes, the characteristics of near ground local gust with extremely high speed will be discussed in detail.
2 Simulation design and brief verification
Numerical simulation of this severe local storm and tornado case is conducted by WRF-ARW (Advance Research WRF) model. From the lowest level to the top level (100mb), the level height intervals are increasing smoothly from about 18 m to about 310 m. The SL scheme is Monin-Obukhov (Janjic Eta) scheme and the PBL scheme is Mellor-Yamada-Janjic (MYJ) schemes (referred as Eta case). The near ground wind velocity is strongly affected by these schemes, therefore, the performance of another scheme combination as MM5 scheme for SL and YSU (Yonsei Univeristy) scheme for PBL is also tested (referred as MM5 case).
Figure 1 shows the mesocyclone structure at different heights in which different quantities are used to depict the structure. From the hydrometer on 1 km high, It could be figured out that cumulus cloud has been forced sucked approaching the low-pressure area that forms a hook-like shape. Vertical velocity on 2 km high and vorticity on 3 km high show a rotating updraft area whose maximum vertical velocity exceeds 32 m/s. This implies the strong convection within mesocyclone system and the sucking force which forces the air moving upwards. The rotation around the low-pressure center is stretching throughout these three heights so that it could be regarded as not only a mid-level mesocyclone but also a low-level mesocyclone, which implies the possibility of tornadogenesis.
Figure 2 shows a tornado-like structure which lasts about 6 minutes even though its rotation intensity is not strong enough comparing to a typical tornado. The wind direction changes rapidly around the low-pressure center whose deficit is about 10 hPa (smaller than a typical tornado case). A speed converged area also appears where air from two directions meets and causes updraft. To be mentioned, a downdraft zone can also be observed near this low-pressure center on the south side, which is referred as RFD-like zone. This RFD-like zone is located on the same area where horizontal wind direction is altering, which implies its function of titling vorticity from horizontal to vertical direction.
3 Results and Discussions of Local Gust of Different Schemes
Figure 3 depicts the horizontal velocity distribution on 200 m high of gust which stretches several kilometers long. Similar to downburst condition, the front of cold air will move fast when meet with warm air and caused gust front which is very close to the ground. The instabilities of atmosphere are always responsible for severe meteorological phenomena.
Figure 4 shows that the wind speed near the ground increases rapidly for both cases, while in Eta case speed rises from 16 m/s to 63 m/s in a short time of about 30 s, which is rapider than MM5 case. It should be mentioned that the high frequency fluctuation of wind speed can hardly be resolved, which will be an important work for the future research in wind-sensitive structure analysis. The wind speed profile during gust is apparently different from it before the gust. The wind profile near the ground before gust comes is more corresponding to a traditional profile caused by wind shear near the ground, while the profile during gust shows extremely high speed just near the ground. The vertical gradient of horizontal speed in MM5 case is smoother than Eta case, which may be explained by that YSU is a non-local scheme that the profile tends to be modified smoother than the local scheme of MYJ, and MYJ scheme also tends to exaggerate the flux in unstable condition. The turbulence profile near the ground during gust time is much larger than before gust time. Gust factors on different heights have also shown that the gust factor in Eta case is larger than MM5 case. Comparing to previous researches, the gust front is larger than a normal gust case (Cao, 2009) and even storm gust experiments (Oriwig, 2007).
4 Conclusions and Future Research
To obtain the realistic wind environment for the man-made structure analysis in a severe storm condition, the authors employ WRF to a real storm case and focus on the near ground wind characteristic it generated. The simulation by WRF on tornado-genesis storm is very few recently, this research could provide a test for its performance in reproducing extreme unstable process. This extreme local gust has rapidly increasing wind velocity and large gust factor which is much different from normal gust. Considering the large wind speed near the ground in an unstable atmosphere condition, the Eta scheme (with MYJ for PBL) will lead to sharper vertical gradient and larger turbulence intensity and gust factor than a non-local scheme. Meanwhile, the validity of traditional surface layer scheme on this intensive friction and unstable condition needs further discussion in future. In future research more nesting will be conducted for downscaling and CFD method could be an approach to generate more high frequency wind fluctuation which is hard to be generated in mesoscale model.
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