Multicell Squall-Line Structure as a Manifestation of Vertically Trapped Gravity Waves

Abstract

Abstract Two-dimensional and three-dimensional simulations of a midlatitude squall line with a high-resolution non-hydrostatic model suggest that the multicellular structure of the storm may be associated with gravity waves generated by convection. Time-lapse display of model output demonstrates that the commonly described “cut-off” process is actually a gravity wave phenomenon. The convective cells arise as gravity waves, which are forced by continuous strong low-level convergence at the storm's gust front. The waves propagate to both sides of the gust front. The stronger westward (front to rear) mode dominates at the mature stage of the squall line. Continuous low-level updraft is generated at the nose of the cold pool, which propagates at the speed of a density current. Updraft cells periodically break away from this persistent low-level gust-front updraft and move at phase speeds of their associated gravity waves, not at the surrounding airflow speeds as implied by the traditional multicell model. Lin...