Synergetic use of IASI and TROPOMI space borne sensors for generating a tropospheric methane profile product

Abstract

Abstract. The thermal infrared nadir spectra of IASI (Infrared Atmospheric Sounding Interferometer) are successfully used for retrievals of different atmospheric trace gas profiles. However, these retrievals offer generally reduced information about the lowermost tropospheric layer due to the lack of thermal contrast close to the surface. Spectra of scattered solar radiation observed in the near and/or short wave infrared, for instance by TROPOMI (TROPOspheric Monitoring Instrument) offer higher sensitivity near ground and are used for the retrieval of total column averaged mixing ratios of a variety of atmospheric trace gases. Here we present a method for the synergetic use of IASI profile and TROPOMI total column data. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach is largely equivalent to applying the spectra of the different sensors together in a single retrieval procedure, but with the substantial advantage of being applicable to data generated with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors. We demonstrate the method exemplarily for atmospheric methane (CH4). We perform a theoretical evaluation and show that the a posteriori combination method yields a total column averaged CH4 product (XCH4) that conserves the good sensitivity of the corresponding TROPOMI product while merging it with the upper tropospheric and lower stratospheric (UTLS) CH4 partial column information of the corresponding IASI product. As consequence, the combined product offers in addition sensitivity for the tropospheric CH4 partial column, which is not provided by the individual TROPOMI nor the individual IASI product. The theoretically predicted synergetic effects are verified by comparisons to CH4 reference data obtained from collocated XCH4 measurements at six globally distributed TCCON (Total Carbon Column Observing Network) stations, CH4 profile measurements made by 24 individual AirCore soundings, and lower tropospheric CH4 data derived from continuous ground-based in-situ observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. The comparisons clearly demonstrate that the combined product can reliably detect XCH4 signals and allows to distinguish between tropospheric and UTLS CH4 partial column averaged mixing ratios, which is not possible by the individual TROPOMI and IASI products. We find indications of a weak positive bias of about +1 % of the combined lower tropospheric data product with respect to the references. For the UTLS CH4 partial columns we find no significant bias.

Design and description of the MUSICA IASI full retrieval product

“…Diekmann et al (2021) presents an an optimal estimation {H 2 O,δD} pair product, which among others makes use of such a posteriori constraint modification. Schneider et al (2021c) presents another possibility for MUSICA IASI data reuse. They apply the extensive information provided in the MUSICA IASI full retrieval product for optimally combining MUSICA IASI CH 4 data with the total column XCH 4 retrieval products of the sensor TROPOMI (TROPOspheric Monitoring Instrument) aboard the satellite Sentinel-5P (Lorente et al, 2020) without the need for running new retrievals.…”

“…This information is essential for being able to perform an a posteriori processing according to Diekmann et al (2021), or to optimally combine the data with other remote sensing data products (e.g. Schneider et al, 2021c). Section 4 can be skipped by readers that do not 2040s.…”

Analysis of stable water isotopes in tropospheric moisture during the West African Monsoon

“…This product is currently being evaluated by using the MUSICA IASI {H 2 O, D} pair data together with the ICON-ART iso simulations discussed here. As TROPOMI o ers high sensitivity near the ground, where the MUSICA IASI retrieval mostly lacks sensitivity, Schneider et al (2021b) present a method for a posteriori combining results from both sensors.…”

“…Despite a negative equator-to-pole gradient in the horizontal representativeness, there is still a satisfactory amount of reliable {H 2 O, D} pair data in higher latitudes, ranging during summer up to the polar regions. The entire subject is referenced in a corresponding publication (seeDiekmann et al, 2021b) and the full MUSICA IASI {H 2 O, D} pair dataset is referenced via the DOI 10.35097/415(Diekmann et al, 2021a).In addition, this section includes the development of a new MUSICA IASI retrieval simulator, which considers the full line-by-line radiative transfer calculations as assumed during the MUSICA IASI retrieval. By drastically reducing the originally considered set of spectroscopic line parameters from HITRAN2016 for all target species except for H 2 O and HDO, a computational speed-up of 50 % could be achieved.…”

Simulated Multispectral Temperature and Atmospheric Composition Retrievals for the JPL GEO-IR Sounder

“…Profiling of key gases including CO, CH4, and CO2 with sensitivity to planetary boundary layer (PBL) abundances was identified as a gap in current capability in the 2017 Decadal Survey, as was the promise of multispectral approaches for addressing this gap. In fact, combining radiances from the (thermal emission dominated) TIR and (solar reflection dominated) SWIR spectral regions has been shown to increase the vertical information content for these gases, providing improved information on near-surface variations relative to retrievals from the thermal alone (e.g., Christi and Stephens, 2004;Worden et al, 2010;Kuai et al, 2013;Worden et al, 2015;Fu et al, 2016;Zhang et al, 2018;Schneider et al, 2021). Such retrievals have the potential to extend the utility of satellite products for air quality forecasting, greenhouse gas monitoring and carbon cycle research.…”