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| Titel |
Performance of the Line-By-Line Radiative Transfer Model (LBLRTM) for temperature, water vapor, and trace gas retrievals: recent updates evaluated with IASI case studies |
| VerfasserIn |
M. J. Alvarado, V. H. Payne, E. J. Mlawer, G. Uymin, M. W. Shephard, K. E. Cady-Pereira, J. S. Delamere, J.-L. Moncet |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 14 ; Nr. 13, no. 14 (2013-07-15), S.6687-6711 |
| Datensatznummer |
250018761
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| Publikation (Nr.) |
 copernicus.org/acp-13-6687-2013.pdf |
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| Zusammenfassung |
| Modern data assimilation algorithms depend on accurate infrared spectroscopy
in order to make use of the information related to temperature, water vapor
(H2O), and other trace gases provided by satellite observations.
Reducing the uncertainties in our knowledge of spectroscopic line parameters
and continuum absorption is thus important to improve the application of
satellite data to weather forecasting. Here we present the results of a
rigorous validation of spectroscopic updates to an advanced radiative
transfer model, the Line-By-Line Radiative Transfer Model (LBLRTM), against
a global dataset of 120 near-nadir, over-ocean, nighttime spectra from the
Infrared Atmospheric Sounding Interferometer (IASI). We compare calculations
from the latest version of LBLRTM (v12.1) to those from a previous version
(v9.4+) to determine the impact of spectroscopic updates to the model on
spectral residuals as well as retrieved temperature and H2O profiles.
We show that the spectroscopy in the CO2 ν2 and ν3
bands is significantly improved in LBLRTM v12.1 relative to v9.4+, and
that these spectroscopic updates lead to mean changes of ~0.5 K in
the retrieved vertical temperature profiles between the surface and 10 hPa, with the sign of the change and the variability among cases depending
on altitude. We also find that temperature retrievals using each of these
two CO2 bands are remarkably consistent in LBLRTM v12.1, potentially
allowing these bands to be used to retrieve atmospheric temperature
simultaneously. The updated H2O spectroscopy in LBLRTM v12.1
substantially improves the a posteriori residuals in the P-branch of the
H2O ν2 band, while the improvements in the R-branch are more
modest. The H2O amounts retrieved with LBLRTM v12.1 are on average
14% lower between 100 and 200 hPa, 42% higher near 562 hPa, and 31%
higher near the surface compared to the amounts retrieved with v9.4+ due
to a combination of the different retrieved temperature profiles and the
updated H2O spectroscopy. We also find that the use of a fixed ratio of
HDO to H2O in LBLRTM may be responsible for a significant fraction of
the remaining bias in the P-branch relative to the R-branch of the H2O
ν2 band. There were no changes to O3 spectroscopy between the
two model versions, and so both versions give positive a posteriori
residuals of ~ 0.3 K in the R-branch of the O3 ν3 band. While the updates to the H2O self-continuum employed by
LBLRTM v12.1 have clearly improved the match with observations near the
CO2 ν3 band head, we find that these updates have
significantly degraded the match with observations in the fundamental band
of CO. Finally, significant systematic a posteriori residuals remain in the
ν4 band of CH4, but the magnitude of the positive bias in the
retrieved mixing ratios is reduced in LBLRTM v12.1, suggesting that the
updated spectroscopy could improve retrievals of CH4 from satellite
observations. |
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