A Comparison of Two-Dimensional and Quasi-Three-Dimensional Simulations of a Tropical Squall Line

Abstract

Results of a simulation of a tropical squall line which allows three dimensionality on the scale of convective elements, shows many similarities with those of a two-dimensional simulation. Differences are 1) The quasi-three-dimensional model produces less front-to-rear acceleration of updraft air and rear-to-front acceleration of downdraft air; 2) The horizontally averaged vertical mass flux and momentum flux profiles show sharper low-level peaks in two-dimensions; 3) The ratio of the maximum to minimum vertical velocities is larger in the quasi-three-dimensional simulation; 4) There is more of a cellular structure in the vertical plane perpendicular to the line in two-dimensions; and 5) The ratio of ice to liquid water is greater in the quasi-three-dimensional simulation. An unexpected result is that very little of the air feeding the rear low-level downdraft originates from ahead of the system, even in the quasi-three-dimensional simulation. Strong vertical mixing of the inflow air occurs so that the equivalent potential temperature of the mid-level air increases and it ascends rather than feeds the cold pool. The strong vertical mixing is associated with overturning cells, which become more prevalent as the updraft branch of the circulation tilts. Results indicate that at the upper regions of the main downdraft, the pressure force is playing a role in accelerating air downwards. The major mechanism responsible for the downdraft appears to be diabatic cooling.