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Interactions Between Water Vapor, Potential Vorticity and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems

Ángel F. Adames 1
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  • 1 Department of Climate and Space Science and Engineering, University of Michigan, Ann Arbor, Michigan
Published-online:
22 Dec 2020
DOI:
https://doi.org/10.1175/JAS-D-20-0205.1
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Abstract

A linear two-layer model is used to elucidate the role of prognostic moisture on quasi-geostrophic (QG) motions in the presence of a mean thermal wind (u¯T). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture-vortex instability —an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component— exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically-stacked structure. When moisture is prognostic and u¯T is easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture-vortex instability at the expense of baroclinic instability is due to the baroclinic (u¯T) component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterly u¯T advect anomalous moisture and the associated convection towards the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture-vortex instability. On the other hand, a westerly u¯T reinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture-vortex instability is an important instability in humid regions of easterly u¯T such as the South Asian and west African monsoons.

Corresponding author address: Ángel F. Adames, A0156 - 1225 W Dayton St, Madison, WI 53706-16123. E-mail: angel.adamescorraliza@wisc.edu

Current Affiliation: Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI.

Abstract

A linear two-layer model is used to elucidate the role of prognostic moisture on quasi-geostrophic (QG) motions in the presence of a mean thermal wind (u¯T). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture-vortex instability —an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component— exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically-stacked structure. When moisture is prognostic and u¯T is easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture-vortex instability at the expense of baroclinic instability is due to the baroclinic (u¯T) component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterly u¯T advect anomalous moisture and the associated convection towards the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture-vortex instability. On the other hand, a westerly u¯T reinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture-vortex instability is an important instability in humid regions of easterly u¯T such as the South Asian and west African monsoons.

Corresponding author address: Ángel F. Adames, A0156 - 1225 W Dayton St, Madison, WI 53706-16123. E-mail: angel.adamescorraliza@wisc.edu

Current Affiliation: Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI.

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