Validation and error estimation of AIRS MUSES CO profiles with HIPPO, ATom, and NOAA GML aircraft observations

Abstract

Abstract. Single-footprint retrievals of carbon monoxide from the Atmospheric Infrared Sounder (AIRS) are evaluated using aircraft in situ observations. The aircraft data are from the HIAPER Pole-to-Pole Observations (HIPPO, 2009–2011), the first three Atmospheric Tomography Mission (ATom, 2016–2017) campaigns, and the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML) Global Greenhouse Gas Reference Network aircraft program in years 2006–2017. The retrievals are obtained using an optimal estimation approach within the MUlti-SpEctra, MUlti-SpEcies, MUlti-SEnsors (MUSES) algorithm. Retrieval biases and estimated errors are evaluated across a range of latitudes from the subpolar to tropical regions over both ocean and land points. AIRS MUSES CO profiles were compared with HIPPO, ATom, and NOAA GML aircraft observations with a coincidence of 9 h and 50 km to estimate retrieval biases and standard deviations. Comparisons were done for different pressure levels and column averages, latitudes, day, night, land, and ocean observations. We found mean biases of +6.6±4.6 %, +0.6±3.2 %, and -6.1±3.0 % for three representative pressure levels of 750, 510, and 287 hPa, as well as column average mean biases of 1.4±3.6 %. The mean standard deviations for the three representative pressure levels were 15 %, 11 %, and 12 %, and the column average standard deviation was 9 %. Observation errors (theoretical errors) from the retrievals were found to be broadly consistent in magnitude with those estimated empirically from ensembles of satellite aircraft comparisons, but the low values for these observation errors require further investigation. The GML aircraft program comparisons generally had higher standard deviations and biases than the HIPPO and ATom comparisons. Since the GML aircraft flights do not go as high as the HIPPO and ATom flights, results from these GML comparisons are more sensitive to the choice of method for extrapolation of the aircraft profile above the uppermost measurement altitude. The AIRS retrieval performance shows little sensitivity to surface type (land or ocean) or day or night but some sensitivity to latitude. Comparisons to the NOAA GML set spanning the years 2006–2017 show that the AIRS retrievals are able to capture the distinct seasonal cycles but show a high bias of ∼20 % in the lower troposphere during the summer when observed CO mixing ratios are at annual minimum values. The retrieval bias drift was examined over the same years 2006–2017 and found to be small at <0.5 %.

TROPESS/CrIS carbon monoxide profile validation with NOAA GML and ATom in situ aircraft observations

“…Figure 4 shows the bias (% relative difference) of the CrIS CO retrieved profiles with respect to NOAA GML aircraft profiles ( xval ). A similar pattern of positive bias in the lower to middle troposphere and negative bias in the upper troposphere is observed for MUSES/AIRS profiles compared to NOAA GML flights (Hegarty et al, 2022). However, MOPITT (version 9, TIR-only data) comparisons to NOAA GML (Deeter et al, 2022) have almost the opposite vertical bias pattern with a negative bias (−1.6 %) in the lower to middle troposphere and a positive bias (0.6 %) in the upper troposphere.…”

“…The a priori profile is scaled to match the CO abundance of the aircraft measurement at the highest altitude. The choices of model and approach for extending the aircraft profiles are examined more in Tang et al (2020) and Hegarty et al (2022), with similar conclusions that the impacts apply mostly to bias estimates in the middle to upper troposphere. Martìnez-Alonso et al ( 2022) compute the un- certainty introduced by this extension explicitly using NOAA AirCore in situ balloon profiles that sample into the stratosphere (Karion et al, 2010).…”

“…The Atmospheric Tomography Mission (ATom) was designed to study air masses in the most remote regions of the Pacific and Atlantic Ocean in each season (Thompson et al, 2022), which also makes the data valuable for validating satellite CO observations over a range of latitudes, with mostly background CO concentrations, except for where transported pollution plumes were encountered Martínez-Alonso et al, 2020;Hegarty et al, 2022). We use CO profiles from the quantum cascade laser spectrometer (QCLS) on ATom campaigns 1-4 (see Table 1).…”

“…We then find all eligible CrIS and aircraft profile pairs within 9 h and 50 km distance. This has been a standard coincidence distance criterion for several validation studies (e.g., Deeter et al, 2019Deeter et al, , 2022Hegarty et al, 2022). Tang et al (2020) found very little sensitivity in MOPITT CO validation results for 25, 50, 100, and 200 km coincidence except for the cases with a 25 km radius that resulted in an insufficient number of matches for meaningful statistics.…”

“…This is important for evaluation of retrieval errors using in situ profiles since the validation comparison removes the effect of smoothing in the retrieval by applying the retrieval averaging kernel and a priori to the in situ profile before differencing (Rodgers and Connor, 2003). Similar comparisons were performed in the recent validation study for the MUSES single-FOV CO retrievals from the Aura Atmospheric Infrared Sounder (AIRS) of Hegarty et al (2022).…”