Characteristic Analysis and Influence of Environmental Variability on Idealized Simulations for a Long-lived Bow Echo with High Percentage of Positive Cloud-to-Ground Lightning

On June 5, 2009, a long-lived squall line with a bow echo occurred under a dry and unstable environment, and brought widely strong straight-line winds for Jiangsu Province, Shanghai, and Zhejiang Province in the East China region. The horizontal and vertical structure of this bow echo is analyzed using weather Doppler radars. Vertical cross sections of the along the core of the bow echo showed a strong wind gust front, a mid altitude radial convergence (MARC), and a sloping-downward rear inflow jet (RIJ). The bow echo was dominated by positive could-to-ground (CG) lightning flashes (percentage over 66%) rather than negative CG lightning flashes at the early stages. At its mature stage, burst of negative CG flashes were found in the major convection areas. The dominant positive CG lightning polarity was associated with rapid updraft intensification stages of the bow echo, and dominant negative CG with updraft weakening. The reasons of high positive-CG flash rate in its early stages are discussed. In order to investigate the environmental mechanisms of the development and evolution of this squall line, idealized simulations using a WRF meso-scale numerical weather prediction (NWP) model are made according to five schemes on modifying the convective parameters of the near-storm environmental sounding data, such as temperature lapse rate, low-level and mid-level humidity, and vertical wind shear. The simulation results show: 1) the increasing lapse rate caused by the strong cold advection aloft over the warm advection at low levels was the crucial factor of a large convective energy or strong updrafts for this convection system. Warmer ambient air could enlarge the temperature difference between downdrafts and environment. 2) The rapid increase of boundary humidity was the major cause of this deep convection case by providing a lower LFC (level of free convection) and a larger CAPE (convective available potential energy). 3) Dry layers at mid-levels and low-levels provided evaporative cooling potential to enlarge downdrafts. 4) Along the squall line’s motion axe, strong surface-high level vertical wind shear interacted with the cold pool to enhance the lifting at leading edge of the convective system, which made the squall live longer. On the other hand, the mid-to-weak vertical wind shear at low levels made a sloping-downward rear inflow jet (RIJ) which spread out along the surface and caused widely strong wind gusts.