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| Titel |
Wavelet Analysis of the Bivariate Time Series of Transmittance and Reflectance of an Atmospheric Column |
| VerfasserIn |
H. M. Deneke, W. H. Knap, C. Simmer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027555
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| Zusammenfassung |
| In this investigation, collocated time series of narrowband 0.6μm atmospheric flux
transmittance at the surface and bidirectional reflectance at the top-of-atmosphere are
decomposed into distinct frequency bands, to investigate the time scale dependences of their
variance and correlation. To this goal, we apply a multiresolution analysis based on the
maximum overlap discrete wavelet transform and the Haar wavelet to 5 minute
resolution measurements from two multifilter rotating shadowband radiometers
operated at Cabauw, the Netherlands, and Heselbach, Germany, and to observations of
the geostationary METEOSAT8 SEVIRI satellite imager operating in rapid scan
mode.
Both time series are best correlated when the satellite data are shifted by about 1 pixel or
6 km to the North, which is likely attributable to the parallax effect caused by the location of
cloud tops above the surface and the slant satellite viewing geometry. While variations in
transmittance and reflectance with periods longer than an hour are found to be highly
anti-correlated, the correlation breaks down for shorter periods. For periods below
one hour, the transmittance time series also exhibits significantly higher variance
than the reflectance. The larger extent of the satellite pixel (6 Ã 3km2) versus the
point-nature of the ground measurements is proposed as an explanation. Due to
the small contributions of high frequency variability to the total variance of the
reflectance, aliasing effects due to the 5 minute repeat cycle of SEVIRI are expected to be
small.
Our findings have important implications for the evaluation of satellite estimates of
surface solar irradiance with surface measurements. Temporal averaging of the surface
measurements over a period of at least 40 minutes is recommended to exclude frequencies
with higher variance in transmittance than in reflectance. Estimates from geostationary
satellites should be averaged over an period equal to that used for averaging the surface
measurement to obtain an optimal agreement. |
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