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Profiles of CH4, HDO, H2O, and N2O with improved lower tropospheric vertical resolution from Aura TES radiances

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Author(s): J. Worden | S. Kulawik | C. Frankenberg | V. Payne | K. Bowman | K. Cady-Peirara | K. Wecht | J.-E. Lee | D. Noone

Journal: Atmospheric Measurement Techniques Discussions
ISSN 1867-8610

Volume: 4;
Issue: 6;
Start page: 6679;
Date: 2011;
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ABSTRACT
Thermal infrared (IR) radiances measured near 8 microns contain information about the vertical distribution of water vapor (H2O), one of its minor isotopologues (HDO) and methane (CH4), key gases that can be used to investigate the water and carbon cycles. Here, we show improvements in vertical resolution and reduction in uncertainties for estimates of these trace gases made from the Aura Tropospheric Emission Spectrometer (TES). The improvements are achieved by utilizing more of the inherent information available in the TES measurements. In previous versions of the TES profile retrieval algorithm, a "spectral-window" approach was used that attempted to minimize uncertainty from interfering specie. However, this approach can also reduce the vertical resolution of the retrieved species. Here we document the vertical sensitivity and error characteristics of retrievals in which H2O, HDO, CH4 and nitrous oxide (N2O) are jointly estimated (together with temperature, surface emissivity, and cloud properties) using the spectral region between 1100 cm−1 and 1330 cm−1. The TES retrieval constraints are also modified to maximize the use of this information. The H2O estimates show greater vertical resolution in the lower troposphere and boundary layer, while the new HDO/H2O estimates can now profile the HDO/H2O ratio between 925 hPa and 450 hPa in the tropics and during summertime at high latitudes. The new retrievals are now sensitive to methane in the free troposphere between 800 and 150 mb with peak sensitivity near 650 hPa. However, there is a bias in the upper troposphere of approximately 10 % that is likely related to temperature uncertainties and/or to errors in the methane spectroscopy. We discuss approaches for correcting this bias either through averaging or through correcting the estimated methane using co-estimated N2O profiles. While these new CH4, HDO/H2O, and H2O estimates are consistent with previous TES retrievals in the regions of overlap, future comparisons with independent profile measurement will be required to validate these new retrievals.

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