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

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. Linear theory shows that the multicellular structure is associated with vertically trapped gravity waves in the troposphere. The waves become trapped in the mid- to upper troposphere because of the strong decrease of Scorer parameter with height as a result of strong vertical wind shear and the reduced static stability aloft. Waves are trapped in lower levels because of the rigid ground. The basic characteristics of these trapped tropospheric gravity waves are wavelengths of 16?20 km, storm-relative phase speeds of 20?25 m s?1, and periods of 11?17 min, which are consistent with the generation periods of precipitation cells at the mature stage in the leading portion of the storm. In the trailing stratiform region, these tropospheric gravity waves become more diffuse with weaker amplitudes, and their wavelengths become longer (25?35 km) with greater storm-relative phase speeds (30?40 m s?1), as described by the dispersion relationship of internal gravity waves. The tropospheric gravity waves differ from disturbances above the tropopause, which are mechanically forced by convective cells impinging on the tropopause. These waves in the lower stratosphere have the structure of vertically propagating (rather than trapped) gravity waves.