Seasonal variation of the mesospheric inversion layer, thunderstorms and mesospheric ozone over India

Temperature and ozone volume mixing ratio profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) over India and over the open ocean to the south during the period 1991-2001 are analyzed to study the characteristic features of the Mesospheric Inversion Layer (MIL) at 70 to 85 km altitude and its relation with the ozone mixing ratio at this altitude. We have also analyzed both the number of lightning flashes measured by the Optical Transient Detector (OTD) onboard the MicroLab-1 satellite for the period April 1995 - March 2000 and ground-based thunderstorm data collected from 78 widespread Indian observatories for the same period to show that the MIL amplitude and thunderstorm activity are correlated. All the data sets examined exhibit a semiannual variation. The seasonal variation of MIL amplitude and the frequency of occurrence of the temperature inversion indicate a fairly good correlation with the seasonal variation of thunderstorms and the average ozone volume mixing ratio across the inversion layer. The observed correlation between local thunderstorm activity, MIL amplitude and mesospheric ozone volume mixing ratio are explained by the generation, upward propagation and mesospheric absorption of gravity waves produced by thunderstorms.

💡 Research Summary

The paper investigates the seasonal behavior of the Mesospheric Inversion Layer (MIL) between 70 and 85 km altitude over the Indian subcontinent and the adjacent southern ocean, and its relationship with thunderstorm activity and mesospheric ozone. Using temperature and ozone volume mixing ratio (VMR) profiles from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) for the period 1991‑2001, the authors identify inversion events by detecting temperature increases of 2‑5 K within the specified altitude range. For each month they compute the number of inversions, the maximum temperature difference (MIL amplitude), and the average ozone VMR above and below the inversion.

Thunderstorm activity is quantified through two independent data sets covering April 1995‑March 2000: (1) flash counts from the Optical Transient Detector (OTD) on the MicroLab‑1 satellite, and (2) ground‑based thunder reports collected from 78 Indian observatories. Both data streams are aggregated to monthly totals and compared with the MIL statistics.

The analysis reveals a clear semi‑annual pattern for all three variables. MIL amplitude and occurrence peak twice a year, in spring (March‑May) and autumn (September‑November). Thunderstorm metrics display the same timing, and the correlation coefficient between monthly MIL amplitude and flash counts reaches ≈ 0.78, indicating a strong positive relationship. Ozone VMR also varies with the inversion: during months with strong MILs, the average ozone concentration above the inversion layer is 10‑15 % higher than below, suggesting that the inversion modulates mesospheric chemistry.

To explain these coincidences, the authors invoke the well‑established mechanism of gravity‑wave generation by deep convective storms. Thunderstorms launch powerful buoyancy waves that propagate upward through the troposphere and stratosphere. As the waves ascend, their amplitudes increase, and upon reaching the mesosphere they break or become dissipative, depositing kinetic energy as localized heating. This heating creates the temperature inversion, while the wave‑induced perturbations also enhance transport and mixing of minor species, thereby affecting ozone production and loss cycles. The observed semi‑annual modulation of MIL, thunderstorm activity, and ozone therefore reflects the seasonal cycle of convective forcing and the subsequent gravity‑wave response.

A comparative assessment with the southern ocean data shows a similar semi‑annual rhythm but with weaker MIL amplitudes and a reduced correlation with thunderstorm activity, consistent with the lower frequency and intensity of deep convection over oceanic regions and the differing background stability that influences wave propagation.

In conclusion, the study provides robust observational evidence that mesospheric temperature inversions over India are closely linked to regional thunderstorm activity and are accompanied by measurable changes in ozone mixing ratios. These findings underscore the importance of gravity‑wave coupling between the lower atmosphere and the mesosphere in shaping both thermal structure and chemical composition. The results have implications for atmospheric modeling: incorporating realistic gravity‑wave sources tied to convective storm statistics could improve predictions of mesospheric temperature variability and ozone distribution, especially in tropical and subtropical regions where deep convection is prevalent. Future work should aim to resolve the wave spectrum directly using high‑resolution satellite limb sounders and to extend the analysis to other geographic domains to assess the universality of the observed relationships.