Characteristics of Spatial and Temporal Distributions of Methane over the Tibetan Plateau and Mechanism Analysis for High Methane Concentration in Summer

Observations of global methane inversion (CH₄) products measured middle-upper by the Atmospheric Infrared Sounder (AIRS) on the Aqua satellite and the NCEP reanalysis data are used to investigate characteristic changes in CH₄ over the Tibetan Plateau from 2003 to 2014. The relationship between the CH₄ change and the monsoon is explored. The results indicate that, taking the Tibetan Plateau as whole, the concentration of CH₄ decreases with the increase in altitude. The seasonal variation of CH₄ in the middle-upper troposphere is significant. Monthly average concentration of CH₄ is the highest from July to September, followed by that in June and October, and the value is low in other months. The concentration of CH₄ increased year by year during 2003-2014, showing an overall increasing trend with an annual growth rate of about 4.66ppb (10^-9). Based on analysis of the spatial distribution of CH₄ over the plateau, the concentration of CH₄ is higher in the north than in the south of the plateau. During the summer monsoon period, CH₄ concentration in the mid-upper troposphere significantly increases and continues to accumulate due to strong convective transport and influences of South Asia high blocking over the plateau. The maximum CH₄ concentration appears at the end of August to early September. Analysis of the monsoon index suggests that strong convection under the influence of summer monsoon is one of the major factors contributing to the high value of CH₄ over the Tibetan Plateau. The maximum CH₄ concentration appears about one month later than the Monsoon Index. With the decline of the monsoon, the high value of CH₄ also quickly decreases.