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Titel |
Evaluation of MODIS-Derived Cloud Fraction Using Surface Observations at
Low-, Mid- and High Latitude DOE ARM sites |
VerfasserIn |
Yang Wang, Chuanfeng Zhao |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250131541
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Publikation (Nr.) |
EGU/EGU2016-11964.pdf |
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Zusammenfassung |
Clouds play essential roles in the Earth’s energy and water cycle, and Cloud Fraction (CF) is
one of the most important cloud parameters. The CF from Moderate Resolution Imaging
Spectroradiometer (MODIS) has been widely used, whereas the time representation of these
instantaneous CF values is not clear. In this study, we evaluate MODIS-derived CF by using
continuous, day-and-night radar/lidar CF from the Atmospheric Radiation Measurement
(ARM) program Active Remote Sensing of CLouds (ARSCL) product and the total sky cover
(TSC) day-time CF datasets. Inter-comparisons between MODIS and surface CFs for time
period from 2000 to 2011 are performed for three climate regimes as represented by the ARM
sites of Southern Great Plains (SGP), Manus, Papua New Guinea (PNG) and North Slope of
Alaska (NSA).
We first choose both the TSC and ARSCL CFs averaged over 1 hour around the two
passing time of satellite, which are around 10:30 AM and 1:30 PM local time. Then two kind
of analyses have been done. One is the spatial variation analysis and the other is temporal
variation analysis. For the spatial variation analysis, we compare the 1-hour averaged cloud
fractions from TSC and ARSCL around 10:30 AM and 1:30 PM with the instantaneous cloud
fractions from MODIS but with different spatial resolution. By obtaining the RMS
errors and ratio of average values of CFs for these inter-comparisons, the optimal
CF-matching spatial resolutions for MODIS regarding to TSC and ARSCL are
obtained which are both 30 km radius of circle. We also find that the optimal matching
spatial resolution increases when the ground observation average time increases. For
the temporal analysis, we first analyze the diurnal variation of the cloud fraction
based on the surface CFs from TSC and ARSCL from which we can see the daily
representation of cloud fraction observed at 10:30 AM and 1:30 PM. Then we make a
statistical comparison of daily and monthly cloud fraction between using all time
observation and using the 1-hour averaged observations at both 10:30 AM and 1:30 PM.
Comparison results will be shown in our paper. It shows a high correlation coefficient of
0.95 (0.93) for observations from TSC (ARSCL). The ratios of daily (monthly)
averaged cloud fraction between using all time and using the time satellite passes
are 0.87(0.92) and 0.86(0.97) for TSC and ARSCL, respectively. This suggests
that considerable errors could be introduced while using the cloud fraction at two
fixed time points (10:30 AM and 1:30 PM) to represent the daily cloud fraction. |
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