The Role of Gravity Waves in the Meso-.BETA.-Scale Cycle of Squall-Line Type Convective Systems

Abstract

Observed or simulated mesoscale convective systems (MCSs) often show pulsations with time periods of a few hours. In this paper, results from a two-dimensional cloud model are used to determine the dynamical processes responsible for such self-modulating oscillations (meso-β-scale cycle, MBC) in squallline type convective systems. The simulated storms have weak cold pools and include notable pulsations with time periods of 3∼4 hours. These pulsations are manifested as periodic organizations of convective-scale cells into upsheartilted meso-β-scale cells (MCs). A rearward-propagating, dissipating MC and disturbances on the downshear side have the properties of vertically propagating gravity waves, with storm-relative time periods that are about 1.5 hour less than those of the MBC. Dry model simulations driven by prescribed thermal forcings show that the waves arise in response to the reduction of latent heating in the MC in its stationary stage. The evolution of storm structure following a convective burst is related to the phasing of the uppertropospheric wave disturbances relative to the lower-tropospheric disturbances. The storm exhibits multicellular structure so long as the downward and frontward acceleration phase of gravity waves exerts suppressing effects on convections behind the gust front. As the phase propagates to the rear, it in turn enhances the convergence at the terminus of the steady lower-tropospheric front-to-rear flow, aiding the cell reinvigoration there. The time period of MBC increases as MCs tilt more horizontally in their stationary growth stage, and as the storm-relative rear-to-front upper-tropospheric wind increases. The former effect increases the intrinsic period of excited waves, and the latter increases the storm-relative period of the waves by Doppler-shifting the phase speed. These results support the argument that gravity waves generated by tilted MCs are prominent in the dynamics of MBCs.