Difference between Upshear and Downshear Propagating Waves Associated with the Development of Squall lines

Abstract

During the development of squall lines, low-frequency gravity waves exhibit contrasting behaviors behind and ahead of the system, corresponding to its low-level upshear and downshear sides, respectively. This study employed idealized numerical simulations to investigate how low-level shear and tilted convective heating influence waves during two distinct stages of squall line evolution. In the initial stage, low-level shear speeds up upshear waves, while it has contrasting effects on the amplitudes of different wave modes, distinguishing from the Doppler effect. Downshear deep tropospheric downdraft (n=1 wave) exhibit larger amplitudes, resulting in strengthened low-level inflow and upper-level outflow. However, n=2 wave with low-level ascent and high-level descent has higher amplitude upshear, and exhibit higher altitude of peak w values downshear, leading to the development of a more extensive upshear low-level cloud deck and higher altitude of downshear cloud deck. In the mature stage, as the convective updraft greatly tilts rearward (upshear), stronger n=1 waves occur behind the system, while downshear-propagating n=2 waves exhibit larger amplitudes. These varying wave behaviors subsequently contribute to the storm-relative circulation pattern. Ahead of the squall line, stronger n=2 waves and weaker n=1 waves produce intense outflow concentrated at higher altitudes, along with moderate mid-level inflow and weak low-level inflow. Conversely, behind the system, the remarkable high-pressure in the upper troposphere and wake low are attributed to more intense n=1 waves. Additionally, the cloud anvil features greater width and depth rearward and is situated at higher altitudes ahead of the system due to the joint effects of n=1 and n=2 waves.