Balanced atmospheric response to squall lines

• Published in: Journal of the atmospheric sciences 1989-08, Vol.46 (15), p.2478-2483
• Main Authors: SCHUBERT, W. H, FULTON, S. R, HERTENSTEIN, R. F. A
• Format: Article
• Language: English
• Subjects: Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/1520-0469(1989)046<2478:BARTSL>2.0.CO;2

When a squall line propagates through the atmosphere, it not only excites transient gravity-inertia wave motion but also produces more permanent modifications to the large-scale balanced flow. The authors calculate this balanced response by using the isentropic-geostrophic coordinate version of semigeostrophic theory. This approach results in a simple mathematical form in which the horizontal ageostrophic velocities are completely implicit and the entire dynamics reduces to a predictive equation for the potential pseudodensity and an invertibility relation. For a two-dimensional squall line, the potential pseudodensity equation is simple enough to be solved analytically. The solutions illustrate how the squall line leaves in its wake a region of low potential pseudodensity in the lower troposphere and a region of high potential pseudodensity in the upper troposphere. The solutions also show that the character of the potential pseudodensity modification by the squall line depends upon the ratio of the convective overturning time to the squall line passage time. This makes it possible to distinguish intensely raining, wide, slowly moving squall lines from weakly raining, narrow, fast-moving squall lines. After the potential pseudodensity is determined, it can be used in the invertibility relation to yield balanced wind and mass fields that capture some of the observed large-scale features associated with squall lines.