The Synoptic Charactistics And Numerical Simulation On The Damaging Straight-line Winds And Their Associating Convective Systems In Beijing-Tianjin Area

Straight-line Winds are classificated by the American Meteorology Society and include the convective gusts, outflows and downbursts, all of which are sure to bring severe wind damage to surface. For this reason it is important to deepen research on such systems, which are triggered by the convective systems from mesoscale to fine-scale, sush as Quasi-Linear Convective System (QLCS, a.k.a. general Squall Lines and Bow Echoes) and Downbursts, and Low Leveled Mesovortices also may contribute to Straight-line Winds. The observation on such systems are mainly from Doppler RADAR, in China mainly as Single-Doppler RADAR station, and if lucky the Observation Towers when the Winds passing directly through the Tower. For such reasons, Numerical Simulation is widely used as a replacement to lacking observations. However, as the meso-to-fine scale currents in the PBL are affected by strong turburlence effects, the correspounding Numerical Simulations need to reflect such traits to provide sufficient results for research. Large-Eddy Simulation (LES) is one of the solutions, which is used in WRF-ARW to provide turburlence traits. In this paper three cases of Straight-line Winds are analyzed, all of which passing through the Observation Tower in Tianjin, and the first-handed data preserved.(1) The analysis of the mesoscale Environment, with data from Surface Stations, Soundings, and RADAR. The three cases are with three different environmental settings, and as a result three different causes to the Straight-line Winds. In Case A it is an outflow gust front from a QLCS with high CAPE and moderate Vertical Wind Shear. In Case B it is also a gust front from a QLCS, but with very low CAPE, strong middle-leveled Vertical Wind Shear and weak low-leveled Vertical Wind Shear, and is also affected by a single mesovortex and the sea-breeze front. In Case C the cause is a downburst. The propagation of the outflow follows some same patterns even if the causes are different:With multiple Meso Highs and Lows it is easy to form deformation fields and in the fields a baroclinic zone. The outflow propagates through the baroclinic zone, with some angular rotation to the right due to the Coriolis force. Yet in the situation that finer scale structures occurs, like meso vortices, their rotational effects would lead the outflow to the correspounding direction, which is the reason why the propagation In Case B show a roration to the left of the temperature gradient.(2) The analysis of the fine-sclae structure in the PBL outflows is made, using the20-seconds data from the Observation Tower in Tianjin when the three cases passing right through the Tower. The period of the correspounding Kelvin-Helmholtz Waves (K-H Waves) and the length of the head sectionof the outflows are calculated. Using Moving Average Method of different time length (10-minute,5-minute,2-minute,1-minute) shows different scales of internal structures of an outflow, also with their differences due to environment and the systems. The turburlent-affected fine-scale structures are segregated using the Wavelet Method, and their Integral flux is looked into. After that the Scheme of the fine-scale structure of outflow gust fronts in different environments are provided with Straight-line Winds Areas:In Case A the surging section of a gust front is contributed to strong CAPE and Vertical Wind Shear, leading the propagation of the K-H wave upstreams back to the cold pool.In Case B the surging section is hardly observed, as low CAPE and low Vertical Wind Shear In low levels inhibits the propagation of K-H waves while the strong Vertical Wind Shear strengthens the convection triggered by the K-H billow up the head section of the outflow, which lead to a stronger downdraft at the end of the head section that result in stronger Straight-line Winds, which shows charactistics of a downburst like that in Case C that makes the Severe Wind Area fragmentated.(3) High-resolution simulation on Case B is made using LES as the PBL scheme, with the gradually scaling down method used in order to minimine the inter-nesting differences. Five-nesting is used with the last two using LES and the finest a resolution of300m. The result shows that the grids with LES scheme provides a better view of both internal structure of the gust front and the sub-generated finer-scale systems as the mesovortices and the see-breeze front. The circulation and the Winds Area are compared with the observation result.