Numerical Simulation of an Intense Squall Line during 10–11 June 1985 PRE-STORM. Part II: Rear Inflow, Surface Pressure Perturbations and Stratiform Precipitation

Abstract

An intense rear-inflow jet, surface pressure perturbations, and stratiform precipitation associated with a squall line during 10?11 June 1985 are examined using a three-dimensional mesoscale nested-grid model. It is found that the large-scale baroclinity provides favorable and deep rear-to-front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow. However, latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow. The role of the rear inflow is generally to produce convergence ahead and divergence behind the system, and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions. In this case study, the rear-inflow jet appears to have caused the splitting of the surface wake low as well as the organized rainfall. As considerable mass within the rear inflow subsides, an intense surface wake low is formed at the back edge of the squall system. This result confirms previous observations that the surface wake low develops hydrostatically as a consequence of adiabatic warming and drying by the descending rear inflow. The wake low is shown to be an end product of complicated reactions involving condensate production, fallout cooling and induced subsiding motion. It does not have any significant effects on the evolution of atmospheric features ahead but contributes to vertical destabilization over the wake region. The simulation shows that the squall line initially leans downshear and later upshear as the low-level cold pool progressively builds up and the system moves into a convectively stable environment. During the mature stage, there are three distinct airflows associated with the squall system: a leading overturning updraft and an ascending front-to-rear (FTR) current that both are driven by high-?e, air from the boundary layer ahead of the line, and an overturning downdraft carrying low-?e, air from the rear. These features resemble previously published results using nonhydrostatic cloud models. Due to continuous deposit of FTR momentum at the upper levels, the FTR updraft is responsible for the rearward transport of high-?e, air mass for the generation of the trailing stratiform precipitation. Several sensitivity experiments are conducted. The generation of the descending rear inflow, and the surface and midlevel pressure perturbations are found to be most sensitive to the parameterized moist downdrafts, hydrostatic water loading, evaporative cooling and ice ice microphysics, in that order. Without any one of these model processes, neither the rear inflow reaches the surface nor the surface mesohigh and wake low become well developed. The results illustrate that the descending rear inflow is a product of the dynamic response to the latent-cooling-induced circulation. Different roles of the parameterized versus grid-resolved downdrafts in the development of the descending rear inflow are also discussed.