Mid-upper tropospheric methane retrieval from IASI and its validation

[摘要] Mid-upper tropospheric methane (CH₄), as an operational product atNOAA's (National Oceanic and Atmospheric Administration) Comprehensive LargeArray-data Stewardship System (CLASS), has been retrieved from the InfraredAtmospheric Sounding Interferometer (IASI) since 2008. This paper provides adescription of the retrieval method and the validation using 596 CH₄vertical profiles from aircraft measurements by the HIAPER Pole-to-PoleObservations (HIPPO) program over the Pacific Ocean. The number of degreesof freedom for the CH₄ retrieval is mostly less than 1.5, and itdecreases under cloudy conditions. The retrievals show greatest sensitivitybetween 100–600 hPa in the tropics and 200–750 hPa in the mid- to highlatitude. Validation is accomplished using aircraft measurements (convolvedby applying the monthly mean averaging kernels) collocated with all theretrieved profiles within 200 km and on the same day, and the results showthat, on average, a larger error of CH₄ occurs at 300–500 hPa. The biasin the trapezoid of 374–477 hPa is −1.74% with a residual standarddeviation of 1.20%, and at layer 596–753 hPa the bias is −0.69% with a residual standard deviation of1.07%. The retrieval error is relatively larger inthe high northern latitude regions and/or under cloudy conditions. The mainreasons for this negative bias include the uncertainty in the spectroscopynear the methane Q branch and/or the empirical bias correction, plusresidual cloud contamination in the cloud-cleared radiances. It is expectedfor NOAA to generate the CH₄ product for 20 + years using a similaralgorithm from three similar thermal infrared sensors: AtmosphericInfrared Sounder (AIRS), IASI and the Cross-track Infrared Sounder (CrIS).Such a unique product will provide a supplementary to the currentground-based observation network, particularly in the Arctic, for monitoringthe CH₄ cycle, its transport and trend associated with climate change.

[学科分类] 几何与拓扑