Analyzing The Genesis Of A Supercell Embedded In A Squall Line With Radar Data And Numerical Simulation

An analysis of the genesis of a supercell embedded in a squall line is presented with radar observation and numerical simulation: A squall line is formed near Beijing with the advantageous synoptic forcing and environmental sounding, during the mature phase of this squall line, a new convective cell is generated in front of the southern end of the squall line, then this cell gets merged with the parent squall line, while developing into a supercell at the same time with a relative independence, thus forming the structure of a supercell embedded in a squall line. Main conclusions are as follows through the analysis above:(1) The development of the parent squall line: The parent squall line is a Line Echo Wave Pattern(LEWP), which consists of three bow echoes, the target embedded supercell is formed within the southern end of the southern bow echo; this parent bow echo has little trailing stratiform precipitation, with a narrow and discrete region of low level rear inflow, and there is a mesoscale vortices line in the convective precipitation region, indicating that the convective region of the parent bow echo has a relatively discrete structure, which provides a relatively independent environment for the development of the embedded supercell.(2) The initiation of a pre-squall-line convective cell: The embedded supercell is developed from a convective cell initiated in front of the squall line, radar observation shows that the initiation of this pre-squall-line cell is due to the convergence mainly caused by a downburst of the squall line, while this pre-squall-line cell is initiated near the mountain ridge. Further analysis with numerical simulation shows that there is a low level convergence line along the mountain ridge, and there is also a significant distinction of θe between the two sides of the mountain ridge, cold and dry to the west, while warm and moist to the east. Then the downburst gets through the squall line, collides with the low level convergence line along mountain ridge, leads to the enhancement of low level convergence, then the pre-squall-line cell is initiated in the warm and moist pre-squall-line region to the east of the mountain ridge.(3) The merger between the parent squall line and the pre-squall-line cell: After the initiation of the pre-squall-line cell, there is a merger between the parent squall line and the pre-squall-line cell from lower to higher levels, radar observation shows that, after merging with the squall line, the embedded cell keeps a relatively independent structure within the higher levels of the squall line, though it forms a narrow band structure of its low levels, mainly caused by the parent squall line forcing. Although the merger is not advantageous to the maintenance of the embedded cell structure, the pre-squall-line cell has already started moving right and developing into the supercell before the completion of merger, avoiding some disadvantages from the merger for the cell to develop independently. Further simulation analysis shows that the embedded cell keeps a quasi-supercell structure in the squall line with a relative independence, the θe of its rear flank downdraft is relatively high, indicating that it hasn’t been influenced by the entrainment from the rear inflow of the parent squall line, thus there is still a chance for the embedded cell to develop toward the supercell with a relative independence.(4) The development from the convective cell to the supercell: during the merger, this convective cell is developing toward the supercell with a relative independence, radar observation shows that some typical features of the supercell emerge, such as the mesocyclone, the hook echo and the precipitation core of the embedded cell, but the mesocyclone is relatively weaker and lower, maybe influenced by the parent squall line forcing, in addition, the embedded supercell is formed when moving almost down the mountain to the plain. Further simulation analysis shows that there is no difference between the nature of embedded supercell formation and isolated supercell formation, both of which are driven by the interaction between the buoyancy and the vertical wind shear, leading to the formation of a low perturbation pressure at the right flank of the supercell updraft in the middle levels, which is important to the maintenance and right moving of the supercell updraft, moreover, the east wind down the mountain makes the vertical shear shtronger, and the shear layer thicker, these are advantageous to the interaction between the buoyancy and the vertical shear, which may be contributed to the supercell formation almost down the mountain to the plain.(5) Two features of the generated supercell: The convective cell is developing toward the supercell during the merger with the parent squall line, finally forming the mature structure of a short-lived embedded supercell. Two features of the mature supercell are analyzed, the Descending Reflectivity Core(DRC) and the Rear Inflow Jet(RIJ) of the supercell, which are tied to further development of the supercell. Radar observation shows that the DRC at the right flank of the supercell promotes the formation of the hook echo; the DRC and the related hook echo are initially detached from the main echo at low levels, and subsequently connected to the main echo. Radar observation and further simulation analysis show that the structure of the supercell’s RIJ is based on the structure of the supercell’s Rear Flank Downdraft(RFD), and the genesis of the supercell’s RIJ is similar to the genesis of a bow echo’s RIJ; the supercell’s RIJ is not essential to the genesis of the supercell’s Deep Convergence Zone(DCZ), and the supercell’s RIJ has not enhanced the supercell’s precipitation core significantly.