Information-based mid-upper tropospheric methane derived from Atmospheric Infrared Sounder (AIRS) and its validation

Abstract. Atmospheric Infrared Sounder (AIRS) measurements of methane (CH₄) generally contain about 1.0 degree of freedom and are therefore dependent on a priori assumptions about the vertical methane distribution as well as the temperature lapse rate and the amount of moisture. Thus it requires that interpretation and/or analysis of the CH₄ spatial and temporal variation based on the AIRS retrievals need to use the averaging kernels (AK). To simplify the use of satellite retrieved products for scientific analysis, a method based on the information content of the retrievals is developed, in which the AIRS retrieved CH₄ in the layer from 50 to 250 hPa below the tropopause is used to characterize the mid-upper tropospheric CH₄ in the mid-high latitude regions. The basis of this method is that in the mid-high latitude regions the maximum sensitive layers of AIRS to CH₄ have a good correlation with the tropopause heights, and these layers are usually between 50 and 250 hPa below the tropopause. Validation using the aircraft measurements from NOAA/ESRL/GMD and the campaigns INTEX-A and -B indicated that the correlation of AIRS mid-upper tropospheric CH₄ with aircraft measurements is ~0.6–0.7, and its the bias and rms difference are less than ±1% and 1.2%, respectively. Further comparison of the CH₄ seasonal cycle indicated that the cycle from AIRS mid-upper tropospheric CH₄ is in a reasonable agreement with NOAA aircraft measurements. This method provides a simple way to use the thermal infrared sounders data to approximately analyze the spatial and temporal variation CH₄ in the upper free tropospere without referring the AK. This method is applicable to derive tropospheric CH₄ as well as other trace gases for any thermal infrared sensors.