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On a Type of Strongly Divergent Steady State

Alexander E. MacDonaldNational Weather Service, Salt Lake City, Utah 84147

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Abstract

The atmosphere is idealized into two kinematic steady states. The first, common in the earth's atmosphere, is rotational and quasi‐nondivergent; the second is divergent and quasi‐irrotational (with respect to space). A motion parameter is introduced which reveals a symmetry between these two types of steady state. When the motion parameter is positive, a rotational steady state is possible, with the absolute vorticity given by the square root of the motion parameter and the divergence equal to zero. When the motion parameter is negative, a divergent steady state is possible, with the divergence given by the square root of the negative of the motion parameter and the absolute vorticity equal to zero.

An observational study using objectively analyzed data reveals that synoptic-scale areas with negative motion parameter, large divergence and small absolute vorticity are 1) common in the subtropical upper troposphere during summer, 2) are generally associated with heavy wet convection, 3) have a mean geographical location which is similar to that of the large upper tropospheric continental anticyclones, and 4) exist in the upper troposphere in conjunction with a tropical storm and some large tornado outbreaks.

Abstract

The atmosphere is idealized into two kinematic steady states. The first, common in the earth's atmosphere, is rotational and quasi‐nondivergent; the second is divergent and quasi‐irrotational (with respect to space). A motion parameter is introduced which reveals a symmetry between these two types of steady state. When the motion parameter is positive, a rotational steady state is possible, with the absolute vorticity given by the square root of the motion parameter and the divergence equal to zero. When the motion parameter is negative, a divergent steady state is possible, with the divergence given by the square root of the negative of the motion parameter and the absolute vorticity equal to zero.

An observational study using objectively analyzed data reveals that synoptic-scale areas with negative motion parameter, large divergence and small absolute vorticity are 1) common in the subtropical upper troposphere during summer, 2) are generally associated with heavy wet convection, 3) have a mean geographical location which is similar to that of the large upper tropospheric continental anticyclones, and 4) exist in the upper troposphere in conjunction with a tropical storm and some large tornado outbreaks.

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