Editorial Type:
Article
Article Type:
Research Article

Impact of Mesoscale Momentum Transport on Large-Scale Tropical Dynamics: Linear Analysis of the Shallow-Water Analog

Jun-Ichi Yano CRC-SHM, Monash University, Clayton, Victoria, Australia

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

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Print Publication:
01 Mar 1998
DOI:
https://doi.org/10.1175/1520-0469(1998)055<1038:IOMMTO>2.0.CO;2
Page(s):
1038–1050
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Abstract

The vertical transport of horizontal momentum by organized convection is a prominent process, yet its impact on the large-scale atmospheric circulation has not even been qualitatively assessed. In order to examine this problem in a simple framework the authors incorporate a nonlinear dynamical model of convective momentum flux into a linear model of the large-scale tropical atmosphere. This model has previously been used to investigate the WISHE (wind-induced surface-heat exchange) instability.

In order to implement the dynamically determined fluxes as a parameterization, a closure assumption is required to relate the relevant mesoscale parameters to the large-scale variables. The most straightforward method is to relate the low-level large-scale pressure (pL) to the mesoscale pressure perturbation (pM), which is linked to the mesoscale momentum flux by the dynamical model. The mesoscale momentum transport under this closure reduces the effective pressure gradient in the large-scale momentum equation and, consequently, the effective stratification. A sufficiently large pM may even cause an effectively unstable stratification (convective instability by mesoscale momentum transport), which is marginally realizable according to a scale analysis.

In general, the WISHE instability is suppressed by the mesoscale momentum flux under this closure because a larger effective stratification can provide a more efficient mass redistribution and, in turn, a larger potential energy for WISHE. This demonstrates that momentum transport by mesoscale convective systems can substantially modify the large-scale tropical dynamics through the WISHE mechanism.

Corresponding author address: Dr. Jun-Ichi Yano, CRC-SHM, Monash University, 3rd floor, Building 70, Wellington Rd., Clayton, Victoria 3168, Australia.

Abstract

The vertical transport of horizontal momentum by organized convection is a prominent process, yet its impact on the large-scale atmospheric circulation has not even been qualitatively assessed. In order to examine this problem in a simple framework the authors incorporate a nonlinear dynamical model of convective momentum flux into a linear model of the large-scale tropical atmosphere. This model has previously been used to investigate the WISHE (wind-induced surface-heat exchange) instability.

In order to implement the dynamically determined fluxes as a parameterization, a closure assumption is required to relate the relevant mesoscale parameters to the large-scale variables. The most straightforward method is to relate the low-level large-scale pressure (pL) to the mesoscale pressure perturbation (pM), which is linked to the mesoscale momentum flux by the dynamical model. The mesoscale momentum transport under this closure reduces the effective pressure gradient in the large-scale momentum equation and, consequently, the effective stratification. A sufficiently large pM may even cause an effectively unstable stratification (convective instability by mesoscale momentum transport), which is marginally realizable according to a scale analysis.

In general, the WISHE instability is suppressed by the mesoscale momentum flux under this closure because a larger effective stratification can provide a more efficient mass redistribution and, in turn, a larger potential energy for WISHE. This demonstrates that momentum transport by mesoscale convective systems can substantially modify the large-scale tropical dynamics through the WISHE mechanism.

Corresponding author address: Dr. Jun-Ichi Yano, CRC-SHM, Monash University, 3rd floor, Building 70, Wellington Rd., Clayton, Victoria 3168, Australia.

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