A NUMERICAL STUDY OF THE STRATIFORM REGION OF A FAST-MOVING SQUALL LINE .2. RELATIONSHIP BETWEEN MASS, PRESSURE, AND MOMENTUM FIELDS

In a companion paper, a two-dimensional simulation of a fast-moving tr opical squall line was successfully compared to observations performed during the COPT81 experiment over West Africa: The full ice phase par ameterization is shown to be crucial in the simulation of trailing anv il precipitation. Different diagnostic tools are applied to the simula ted fields to further our understanding of the scale interactions with in a squall line-type mesoscale convective system. The pressure organi zation is characterized by two marked features important for explainin g the inner circulation: first, a front-to-rear midlevel pressure grad ient and, second, the surface pressure mesohigh extending from the gus t 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 an d buoyant contributions depend only on the horizontal and vertical, re spectively. The mean pressure increase through the whole system is in part related to the horizontal momentum changes occurring in the syste m. Concerning the mass contribution, the midlevel system-scale pressur e gradient is mainly due to the widespread rear anvil injecting a larg e amount of water vapor behind the system and to the adiabatic warming underneath the rear anvil. The line-normal momentum budget in the str atiform region shows that the midlevel pressure mesohigh, induced by t he system at its rear, can prevent the progression by advection of the midlevel front-to-rear flow coming from the convective part and can f orce the mesoscale ascent in the anvil and the unsaturated, warm mesos cale descent underneath. The mesoscale ascent in the stratiform part t ransports 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 wid espread extension. The effects of the convection on the cross-line mom entum held at large scale is quantified by computing the apparent sour ce of line-normal momentum Q(u). It is not negligible and the stratifo rm contribution can be significant.