Numerical Simulated Analysis And Nowcasting Of Squall Lines Over Beijing-Tianjin-Hebei Region

The dissertation focuses on analyses of temporal and spatial evolution characteristics of thermo-dynamical structures and storm-related environment in different positions, and numerical nowcasting of squall lines occurred on the mountains and the plains over BeijingTianjin-Hebei region. One was appeared on 30 July 2013, referred as "730", and the other was on 4 July 2013 referred as "704". The study is based on a four-dimensional variational radar data analysis system established by radar data rapid-refresh 4DVar(RR4DVar), using 6 CINRAD radar observations combined with regional automatic weather stations. The primary results as the follows:(1) The "730" and "704" squall lines are forced under favorable large-scale background conditions based on analysis on observations. Troughs at 500 hPa level and cold fronts on surface crossing are the main synoptic systems influencing the occurrence and development of the ones; Specific humidity over12g/kg provides abundant vapor; Potential energy(CAPE) is up to 2445J/kg which provides much unstable energy for the "730"; Strong vertical wind shear at lower levels provides very favorable convective condition for the "704".(2) There is remarkable environment difference in different positions and developmental stages of the "730" squall line based on results from the four-dimensional variational radar data analysis system. 1) Forming stage: low-level strong storm outflow remains stably long time in northern section, cutting off warm moist inflow, making the vertical wind shear weakened, and the lower is under the control of negative vorticity, which is not conducive to organization and development of convective cells in the northern section of the squall line. The front of middle and southern sections are under the charge of strong vertical wind shear with low-level warm air convergence and updraft, where negative and positive vorticities reach approximate balance according with "RKW" theory completely, conducive to newborn convective cells along the outflow boundary and feed the cells into squall line in the middle and southern sections, which are primary mechanism of initiation of convective cells and organization of the squall line in the sections. 2) Enhancing stage: strong vertical wind shear in front of the middle and southern sections produces strong positive vorticity that intensifies low-level dynamic instability, and southerly warm flow remains stably, which are important factors on influencing organization and strengthening of convection in the sections. 3) Bow echo phase: while the middle section goes down the mountains, intersection between the cold pool outflow and the southerly warm flow leads to strong convergence and updraft, maintenance of strong vertical wind shear is prone to intensify the low-level instability, and gradient of turbulent temperature increases gradually that produces frontogenesis. All of the above are main reasons of initiation of the bow echo in the section of the squall line.(3) Numerical nowcasts of the "704" squall line over the plains indicates significant superiority and some faults in convective-scale numerical nowcasting based on a 3-D numerical cloud model initialized by the four-dimensional variational radar data analysis system. 1) The model can nowcast accurately organization and development of the squall line over the plains; 2) The model can predict robustly movement and evolution characteristics of monomer appearance in early stage of the squall line; 3) The model is capable of better forecasting of heavy rain center and rain-band position that formed by the squall line; 4) The model has higher nowcasting ability for evolution features of internal structure of the squall line. 5) While the model has higher deviations of wind forecasting in front of the mountains due to the model is short of a reasonable processing scheme of complex terrain in the region.