© 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.

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

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

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.

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

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.

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.