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| Titel |
Validation of two independent retrievals of SCIAMACHY water vapour columns using radiosonde data |
| VerfasserIn |
A. Piesanie, A. J. M. Piters, I. Aben, H. Schrijver, P. Wang, S. Noël |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 10 ; Nr. 6, no. 10 (2013-10-31), S.2925-2940 |
| Datensatznummer |
250085098
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| Publikation (Nr.) |
 copernicus.org/amt-6-2925-2013.pdf |
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| Zusammenfassung |
Two independently derived SCIAMACHY total water vapour column (WVC) products
are compared with integrated water vapour data calculated from radiosonde
measurements, and with each other. The two SCIAMACHY WVC products are
retrieved with two different retrieval algorithms applied in the visible and
short-wave infrared wavelength regions respectively. The first SCIAMACHY WVC
product used in the comparison is ESA's level 2 version 5.01 WVC product
derived with the Air Mass Corrected Differential Optical Absorption Spectroscopy
(AMC-DOAS) retrieval algorithm applied in the visible wavelength range
(SCIAMACHY-ESA). The second SCIAMACHY WVC product is derived using the
iterative maximum likelihood method (IMLM) in the short-wave infrared
wavelength range and developed by Netherlands Institute for Space Research
(SCIAMACHY-IMLM). Both SCIAMACHY WVC products are compared with collocated
water vapour amounts determined from daily relative humidity radiosonde
measurements obtained from the European Centre for Medium-Range Weather
Forecasts (ECMWF) radiosonde network. The SCIAMACHY-ESA WVC product is
compared with radiosonde-derived WVC amounts for an 18-month period from
February 2010 to mid-August 2011, and the SCIAMACHY-IMLM WVC amounts are
compared with radiosonde WVC amounts for the two individual years of 2004
and 2009. In addition the WVC amounts from SCIAMACHY-ESA and SCIAMACHY-IMLM
are also compared with each other for a 1-month period for June 2009.
The AMC-DOAS method used to retrieve SCIAMACHY-ESA WVC is able to correct for
water vapour present below the clouds and can be used during cloudy
conditions over both land and ocean surfaces. Results indicate a good
agreement between the WVC amounts of SCIAMACHY-ESA and that of radiosondes,
with a mean difference of −0.32 g cm−2 for all collocated
cases. Overall the SCIAMACHY-ESA WVC amounts are
smaller than the radiosonde WVC amounts, especially over oceans. For cloudy
conditions the WVC bias has a clear dependence on the cloud top height and
increases with increasing cloud top heights larger than approximately 2 km.
A likely cause for this could be the different vertical profile shapes of
water vapour and O2 leading to different relative changes in their
optical thickness, which makes the air mass factor (AMF) correction method used in the
algorithm less suitable for high clouds. The SCIAMACHY-IMLM product's water
vapour measurements are best used over land surfaces during cloud-free
conditions, and in these cases a good agreement is found when compared to
radiosonde WVC amounts, with a mean difference of 0.08 g cm−2.
It is shown that over ocean surfaces during cloudy conditions the partial
SCIAMACHY-IMLM water vapour column above the cloud can be well estimated by
using the simultaneously retrieved methane column to calculate the cloud top
height. Comparing the two satellite WVC products with each other indicates
that SCIAMACHY-ESA consistently measures higher WVC amounts than those of
SCIAMACHY-IMLM. Furthermore, the importance of the choice of cloud product is
highlighted, as intercomparisons between the two SCIAMACHY WVC products
indicate that using different cloud products to screen water vapour data for
cloud-free conditions influences the data selection and may ultimately lead to
a variation in results.
In the last section of the paper, various options for filtering the two
SCIAMACHY WVC data sets are discussed and best selection criteria suggested. |
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