The Role of Convectively Generated Gravity Waves on Convective Initiation: A Case Study

A case study of a convection initiation (CI) event involving a mesoscale gravity wave is presented. This severe convection event occurred in east China on 5 June 2009. High-frequency automatic weather station (AWS) data, visible satellite data, and Doppler radar data were combined to depict the features of the gravity wave and the development of several convection centers. The gravity wave was manifested by a surface pressure dip and surface wind shift propagating westward away from the early convection. The pressure dip propagated at a speed of >30 m s(-1), which is comparable with that in previous observational studies of convectively generated gravity waves. A special focus is on the initiation of a deep convection cell in Anhui Province, which resulted in 25 deaths. Surface observations showed two precursors before CI, including a convergence line and wind shift at the eastern end of the convergence line. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to examine the structure of the gravity waves and forecast CI processes. The model reproduced the observed features of the gravity wave and the precursors before CI. Three-dimensional model results showed that CI occurred at the intersection between a convergence line and the gravity wave. The relationships between the wind shift and the pressure drop are consistent with polarization relation in ducted gravity waves. As the updraft of the gravity wave intersected with the convergence line, the low-level updraft strengthened and led to CI. The gravity wave, which had stronger updraft than downdraft, suggested a positive contribution to CI.