Analytical Models of Narrow Cold-Frontal Rainbands and Related Phenomena

Mitchell W. Moncrieff National Center for Atmospheric Research, Boulder, Colorado

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Abstract

Simplified versions of the steady analytical models of density current developed by Moncrieff and So are shown to represent archetypes of cold-frontal rainbands (NCFRs) by making comparisons with the limited amount of published observational data that describe the phenomena. An overturning (or in a special case, stagnation) of the upper-level, system-relative flow of ahead of the rainband and vortidty within the cold air are important effects not included in conventional density current dynamics. The bands conserve mass energy and domain-averaged total momentum flux and involve a balance between inertial and pressure gradient effects. This is distinct from a semigeostrophic mechanism that requires a base-state baroclinity and an ageostrophic adjustment towards thermal-wind balance.

The theory also represents two-diimensional squall lines in the limiting cam when the convective available potential energy is negligible so squall lines could, in principle, be maintained solely by the kinetic energy of the mean flow and the work done by the pressure field. Furthermore, it is shown that squall lines with a deep inflow from ahead of the line and an anafront type of structure should be more prevalent than those having shallow inflows.

The basic dynamics of squall lines and narrow cold-frontal rainbands are formally shown to be analogous and their archetypal behavior can be represented by simple hydrodynamical models.

Abstract

Simplified versions of the steady analytical models of density current developed by Moncrieff and So are shown to represent archetypes of cold-frontal rainbands (NCFRs) by making comparisons with the limited amount of published observational data that describe the phenomena. An overturning (or in a special case, stagnation) of the upper-level, system-relative flow of ahead of the rainband and vortidty within the cold air are important effects not included in conventional density current dynamics. The bands conserve mass energy and domain-averaged total momentum flux and involve a balance between inertial and pressure gradient effects. This is distinct from a semigeostrophic mechanism that requires a base-state baroclinity and an ageostrophic adjustment towards thermal-wind balance.

The theory also represents two-diimensional squall lines in the limiting cam when the convective available potential energy is negligible so squall lines could, in principle, be maintained solely by the kinetic energy of the mean flow and the work done by the pressure field. Furthermore, it is shown that squall lines with a deep inflow from ahead of the line and an anafront type of structure should be more prevalent than those having shallow inflows.

The basic dynamics of squall lines and narrow cold-frontal rainbands are formally shown to be analogous and their archetypal behavior can be represented by simple hydrodynamical models.

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