Methane vertical profiles over the Indian subcontinent derived from the GOSAT/TANSO-FTS thermal infrared sensor

Abstract. We examined CH₄ variability over different regions of India and the surrounding oceanic regions derived from thermal infrared (TIR) band observations by the Thermal And Near-infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS) onboard the Greenhouse gases Observation SATellite (GOSAT) and simulated by the updated MIROC4.0-based Atmospheric Chemistry Tracer Model (MIROC4-ACTM) for the period 2009–2014. This study attempts to understand the sensitivity of the vertical profile retrievals at different layers of the troposphere and lower stratosphere, arising from the averaging kernels and a priori assumptions. We stress that this is of particular importance when the satellite derived products are analyzed using a different ACTMs from that is used as retrieval a priori. A comparison of modeled and retrieved CH₄ vertical profiles shows the 22 vertical levels of GOSAT/TANSO-FTS TIR retrievals provide critical information about transport from the top of the boundary layer to the upper troposphere and lower stratosphere in a consistent manner. The mean model-GOSAT TIR CH₄ mismatch is within 50 ppb, excepting 150 hPa and upward, where the sensitivity of GOSAT/TANSO-FTS TIR observations becomes very low. Convolution of the modeled profiles with GOSAT/TANSO-FTS TIR averaging kernels reduce the mismatch to below uncertainty. Distinct seasonal variations of CH₄ have been observed at the upper atmospheric boundary layer (800 hPa), free troposphere (500 hPa), and upper troposphere (200 hPa) levels over northern and southern regions of India corresponding to the southwesterly monsoon (July–September) and post-monsoon (October–December) seasons. Analysis of the transport and emission contributions to CH₄ suggests that the CH₄ seasonal cycle over the Indian subcontinent is governed by both the heterogeneous distributions of surface emissions and the influence of the global monsoon divergent wind circulations. GOSAT/TANSO-FTS TIR observations provide additional information about CH₄ observations in this region compared to what is known from in situ data, which is important for improving the accuracy of emission flux optimization. Based on two emission sensitivity simulations, we suggest that the emissions of CH₄ from the India region is 51.2 ± 1.6 Tg yr⁻¹ during the period of 2009–2014.