Validation of ENVISAT/SCIAMACHY columnar methane by solar FTIR spectrometry at the Ground-Truthing Station Zugspitze R. Sussmann^{1}, W. Stremme^{1}, M. Buchwitz^{2}, and R. de Beek^{2} ^{1}IMK-IFU, Forschungszentrum Karlsruhe, Garmisch-Partenkirchen, Germany ^{2}Institute of Environmental Physics (iup), University of Bremen FB1, Bremen, Germany

Abstract. Methane total-vertical column retrievals from ground-based solar FTIR
measurements at the Permanent Ground-Truthing Station Zugspitze (47.42° N,
10.98° E, 2964 m a.s.l.), Germany are used to validate column averaged
methane retrieved from ENVISAT/SCIAMACHY spectra by WFM-DOAS (WFMD) version
0.4 and 0.41 for 153 days in 2003. Smoothing errors are estimated to be
below 0.10% for FTIR and 0.14% for SCIAMACHY-WFMD retrievals and can
be neglected for the assessment of observed bias and day-to-day-scatter. In
order to minimize the altitude-difference effect, dry-air column averaged
mixing ratios (XCH_{4}) have been utilized. From the FTIR-time series of
XCH_{4} an atmospheric day-to-day variability of 1% was found, and a
sinusoidal annual cycle with a ≈1.6% amplitude. To obtain the
WFMD bias, a polynomial fitted to the FTIR series was used as a reference.
The result is WFMD v0.4/FTIR=1.008±0.019 and WFMD v0.41/FTIR=1.058±0.008.
WFMD v0.41 was significantly improved by a
time-dependent bias correction. It can still not capture the natural
day-to-day variability, i.e., the standard deviation calculated from the
daily-mean values is 2.4% using averages within a 2000-km radius, and
2.7% for a 1000-km radius. These numbers are dominated by a residual
time-dependent bias in the order of 3%/month. The latter can be
reduced, e.g., from 2.4% to 1.6% as shown by an empirical
time-dependent bias correction. Standard deviations of the daily means,
calculated from the individual measurements of each day, are excluding
time-dependent biases, thus showing the potential precision of WFMD daily
means, i.e., 0.3% for a 2000-km selection radius, and 0.6% for a
1000-km selection radius. Therefore, the natural variability could be
captured under the prerequisite of further advanced time-dependent bias
corrections, or the use of other channels, where the icing issue is less
prominent.

Citation: Sussmann, R., Stremme, W., Buchwitz, M., and de Beek, R.: Validation of ENVISAT/SCIAMACHY columnar methane by solar FTIR spectrometry at the Ground-Truthing Station Zugspitze, Atmos. Chem. Phys. Discuss., 5, 2269-2295, doi:10.5194/acpd-5-2269-2005, 2005.