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Satellite-retrieved vertical profiles of methane over the Indian region: impact of synopticscale meteorology
- Kavitha, M., Nair, Prabha R.
- International journal of remote sensing 2019 v.40 no.14 pp. 5585-5616
- aircraft, altitude, greenhouse gases, meteorology, methane, mixing, mixing ratio, remote sensing, satellites, seasonal variation, spatial distribution, troposphere, winter
- The altitude distribution of methane (CH₄) is the least addressed topic in the greenhouse gas assessment over the Indian region. In the absence of the in-situ measurements, the satellite-based retrievals of the vertical distribution of CH₄ using Atmospheric Infrared Sounder (AIRS) measurements during the period 2003–2015 were made use in this study for the first time to understand the 3D distribution (latitude-longitude-altitude) of CH₄ over Indian region. Significant regional and seasonal variations are observed in the vertical distribution of CH₄, even though it is a long-lived greenhouse gas and known to be well-mixed. Over most of the regions, the highest mixing ratio is observed during post-monsoon months and minimum in the pre-monsoon/monsoon season. The presence of a ‘high altitude peak’ in CH₄ (around 1880 ppbv) around 300 hPa–250 hPa was noted in post-monsoon which is caused by the monsoon-associated convective updrafts and the anti-cyclonic system. The vertical profiles show seasonal variations which are region as well as altitude-dependent. Over the oceanic region, the highest seasonal amplitude of CH₄ mixing ratio was observed over North–Arabian Sea due to the proximity of the source rich land regions. During the winter and pre-monsoon months, the latitudinal differences are absent throughout the troposphere. A consistent increasing trend in CH₄, ranging from 1 ppbv year–¹ to 6 ppbv year–¹ is seen at all the tropospheric altitudes, with faster growth rates at higher altitudes, maximizing at 300 hPa–150 hPa. An approximate estimate of direct forcing due to CH₄ lies in the range 0.80 W m–²–0.83 W m–². The paper also presents a comparison of the in-situ measured upper tropospheric CH₄ mixing ratio from CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flight data and AIRS retrievals.