Numerical Research Of A Squall Line In East China And Its Sensitivity To Terrain Resolution And Microphysical Parameterization Scheme

By assimilating the FY satellite data and Doppler radar data with the Local Analysis and Prediction System (LAPS), a severe squall line that occurred in east China on 12 July 2014 was simulated by the Weather Research and Forecasting (WRF) model version 3.6.1. Based on the high-resolution simulation results, the dynamic, thermodynamic and microphysical structures as well as the propagation and organization mechanisms of the squall line were analyzed in detail. A conceptual model was given at last. The main results were presented as follows:(1) The squall line was formed in an environment with stable and well-developed cold vortex in Northeast China and a quasi-stationary cold front in the Yangtze and Huaihe region. The large convective available potential energy (CAPE) and strong vertical wind shear in low levels also favored the longevity of squall line. (2) Convective cells were formed along the surface convergence line. In the mature phase of squall line, the surface pressure field presented thunderstorm high, pre-squall low and wake low. (3) The storm-relative airflow was mainly featured by the warm-moist front to rear (FTR) inflow in low levels, descending rear inflow in middle levels and slightly ascending front-to-rear flow at upper levels. The increasingly intensified rear inflow cut off the low-level front inflow and eventually resulted in the decay of squall line. (4) The analysis of the hydrometeor contents revealed a large amount of ice-phase particles were generated during the development of squall line, especially the snow. (5) New convective cells were triggered along the cold outflow boundary because of the deepening and spreading of cool pool and squall line propagated in this way. (6) The merging and organization of convective cells might be responsible for the enhancement of squall line.Based on the analysis above, we carried out the sensitivity studies of terrain resolution and bin and bulk microphysical scheme. Some preliminary results were obtained:(1) The terrain resolution have the influence on the distribution of surface rainfall. The low resolution terrain simulated easter rainfall centers, while the 30s terrain results which capured the mesoscale terrain in the southern part of Anhui province were more consistent with the observation. (2) The traditional bulk scheme seems to have a better performance than newly-incorporated bin scheme in WRF model in our case study. The bulk scheme produced a well-organized radar reflectivity structure while the bin scheme simulated a less-organized structure with relatively isolated convective cells in the leading edge. The strong rear to front (RTF) inflow and updraft were identified in bulk scheme by comparison with weak RTF and relatively large downdraft area in bin scheme. In addition, bin simulated a deeper cold pool with the heating layer much thinner than bulk. The statistics of hydrometeor mixing ratio revealed a large amount of ice particles were produced in bulk scheme, while snow particles dominated at upper levels in bin scheme.