A Numerical Study of the Stratiform Region of a Fast-Moving Squall Line. Part II: Relationship between Mass, Pressure, and Momentum Fields

Abstract

In a companion paper, a two-dimensional simulation of a fast-moving tropical squall line was successfully compared to observations performed during the COPT81 experiment over West Africa. The full ice phase parameterization is shown to be crucial in the simulation of trailing anvil precipitation. Different diagnostic tools are applied to the simulated fields to further our understanding of the scale interactions within a squall line-type mesoscale convective system. The pressure organization is characterized by two marked features important for explaining the inner circulation: first, a front-to-rear midlevel pressure gradient and, second. the surface pressure mesohigh extending from the gust front to the rear of the most active part of the trailing stratiform region. Based on the hydrostatic approximation, an original method of decomposition of the pressure field is proposed, whereby dynamical and buoyant contributions depend only on the horizontal and vertical, respectively. The mean pressure increase through the whole system is in part related to the horizontal momentum changes occurring in the system. Concerning the mass contribution, the midlevel system-scale pressure gradient is mainly due to the widespread rear anvil injecting a large amount of water vapor behind the system and to the adiabatic warming underneath the rear anvil. The line-normal momentum budget in the stratiform region shows that the nidlevel pressure mesohigh, induced by the system at its rear, can prevent the progression by advection of the nidlevel front-to-rear flow coming from the convective part and can force the mesoscale ascent in the anvil and the unsaturated, warm mesoscale descent underneath. The mesoscale ascent in the stratiform part transports front-to-rear momentum to the upper troposphere, whereas the mesoscale subsidence leads to a rear-to-front momentum vertical flux underneath. Its impact at the system scale is important due to its widespread extension. The effects of the convection on the cross-line momentum field at large scale is quantified by computing the apparent source of line-normal momentum Qu. It is not negligible and the stratiform contribution can be significant.