Abstract. The present study examines the role of tropical cyclones in the enhancement of tropospheric ozone. The most significant and new observation reported is the increase in the upper-tropospheric (10-16 km) ozone by 20-50 ppbv, which has extended down to the middle (6-10 km) and lower troposphere ( < 6 km). The descent rate of enhanced ozone layer during the passage of tropical cyclone is 0.8-1 km day −1 , which is three times that of a clear-sky day (non-convective). Enhancement of surface ozone concentration by ∼ 10 ppbv in the daytime and 10-15 ppbv in the night-time is observed during a cyclone. Potential vorticity, vertical velocity and potential temperature obtained from numerical simulation, reproduces the key feature of the observations. A simulation study indicates the downward transport of stratospheric air into the troposphere. Space-borne observations of relative humidity indicate the presence of sporadic dry air in the upper and middle troposphere over the cyclonic region. These observations quantitatively constitute experimental evidence of redistribution of stratospheric ozone during cyclonic storms.

This article discusses different plausible mechanisms to account for the intrusion of dry ozone-rich stratospheric air into the troposphere during the passage of the tropical cyclone Nilam-2012. The present study shows that the overshooting convection associated with Nilam is found to be the prime candidate for the generation of turbulence in the vicinity of tropopause (VOT). In addition, the presence of strong updraughts and downdraughts in the VOT weakened the stability of the tropopause which allowed the dry ozone-rich stratospheric air to penetrate downwards to the troposphere. Significant modulation of the tropopause structure is observed which is found to be accompanied by structures linked with shear instability. Concurrent ozone measurements also indicate the intrusion of stratospheric air mass into the upper and middle troposphere.

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