Mesoscale Disturbances in the Tropical Stratosphere Excited by Convection: Observations and Effects on the Stratospheric Momentum Budget

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  • 1 NASA/Ames Research Center, Moffett Field, California
  • | 2 San Jose State University, San Jose, California
  • | 3 Synernet, Fremont, California
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Abstract

The importance of the momentum flux of topographically generated mesoscale gravity waves to the extratropical middle atmosphere circulation has been well established for over a decade. Estimates of the zonal forcing due to tropical mesoscale gravity waves, however, are hampered by lack of data on their primarily convective sources. The advent of aircraft measurements over tropical convective systems now makes such estimates possible without the use of ad hoc assumptions about amplitudes and phase speeds.

Aircraft measurements from NASA's 1980 Panama and 1987 STEP/Australia Missions show that convectively generated disturbances observed just above the tropopause have horizontal scales comparable to those of the underlying anvils (about 50–100 km) with peak-to-peak isentropic surface variations of about 300–400 m. Satellite imagery of tropical anvil evolution indicates a typical lifetime of about five hours. Assuming that each convective system's impact on the stratosphere can be modeled as a time-dependent “mountain” with the preceding spatial and time scales, the excited spectrum of gravity waves can be calculated. A suitable quasilinear wave-mean flow interaction parameterization and satellite-derived cloud area statistics can then be used to evaluate the zonal acceleration as a function of altitude induced by gravity waves from mesoscale convective systems.

The results indicate maximum westerly accelerations due to breaking mesoscale gravity waves of almost 0.4 m s−1/day in the upper stratosphere (in the region of the semiannual oscillation) during September, comparable to but probably smaller than the accelerations induced by planetary-scale Kelvin waves. Calculated easterly accelerations due to breaking mesoscale gravity waves in the QBO region below 35 km are smaller, accounting for about 10% of the required zonal acceleration.

Abstract

The importance of the momentum flux of topographically generated mesoscale gravity waves to the extratropical middle atmosphere circulation has been well established for over a decade. Estimates of the zonal forcing due to tropical mesoscale gravity waves, however, are hampered by lack of data on their primarily convective sources. The advent of aircraft measurements over tropical convective systems now makes such estimates possible without the use of ad hoc assumptions about amplitudes and phase speeds.

Aircraft measurements from NASA's 1980 Panama and 1987 STEP/Australia Missions show that convectively generated disturbances observed just above the tropopause have horizontal scales comparable to those of the underlying anvils (about 50–100 km) with peak-to-peak isentropic surface variations of about 300–400 m. Satellite imagery of tropical anvil evolution indicates a typical lifetime of about five hours. Assuming that each convective system's impact on the stratosphere can be modeled as a time-dependent “mountain” with the preceding spatial and time scales, the excited spectrum of gravity waves can be calculated. A suitable quasilinear wave-mean flow interaction parameterization and satellite-derived cloud area statistics can then be used to evaluate the zonal acceleration as a function of altitude induced by gravity waves from mesoscale convective systems.

The results indicate maximum westerly accelerations due to breaking mesoscale gravity waves of almost 0.4 m s−1/day in the upper stratosphere (in the region of the semiannual oscillation) during September, comparable to but probably smaller than the accelerations induced by planetary-scale Kelvin waves. Calculated easterly accelerations due to breaking mesoscale gravity waves in the QBO region below 35 km are smaller, accounting for about 10% of the required zonal acceleration.

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