Effect of variation of water vapour amount on derived TOA solar spectrum
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© University of Reading 2008 www.reading.ac.uk
Department of Meteorology
Effect of variation of water vapour
amount on derived TOA solar spectrum
Kaah Menang, Liam Tallis , Igor Ptashnik and Keith Shine.
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Outline
• Derived TOA solar spectra from Camborne
• Effect of Changing water vapour amount on derived TOA solar spectra
• Discussion and future work
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
From last meeting:
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
From last meeting
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Meetings with Tom Gardiner
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• Add more observations
• Calculate statistical errors on linear fits
• Filter observations using FTS DC signals
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Results-CO2 region (more obs+errors)
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
‘New’ Results(CO2 region)-compared w previous
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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• Method 1: Numerically• Beer’s law:
• Assume that optical depth changes linearly with time of the day (solar zenith angle) in Beer’s law.
• k depends on assumed % change in optical depth, m is the range of airmass for the day and m0 is the
minimum airmass of the day.
)exp(0 mFF
00 1 mmk
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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• Method 2: Analytically (Igor Ptashnik)
• Used the fact that there are 2 sources of deviation of y-intercept (lnFν0) of Langley plot:
change in slope and parallel shift of the linear fit.
• Assumption: changes in derieved Fν0 between
micro window and line centre is due to changing water vapour amount only.
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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• Relative change in water vapour:
• Relative change in derived Fν0:
c
FOH
0ln2
00
2Hc
F
F
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Effect of changing WV amount on derived TOA
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• Using typical values of c, change in intercept(lnFν0) and τ, the relative change in
retrieved Fν0 due to changing water amount is
less than 1 % in atmospheric micro windows.
• Relative change in line centres are higher.
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CAVIAR Field Meeting, Imperial College -London(29 March2011)
Summary and Future Work
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• Using observations that span the whole day has led to contamination of TOA solar spectrum by telluric lines.
• Changing water vapour not a likely cause for the observed differences in derived TOA solar spectrum.
• Differences may be caused by the passage of clouds during measurements and/or instability of the FTS.
• Filtering using FTS DC signals does not change results very much.
• Filter the water vapour lines in the derived TOA solar spectrum using optical depths derived from LBL simulations.