1. © 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.

2. 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 2

3. CAVIAR Field Meeting, Imperial College -London(29 March2011) From last meeting: 3

4. CAVIAR Field Meeting, Imperial College -London(29 March2011) From last meeting 4

5. CAVIAR Field Meeting, Imperial College -London(29 March2011) Meetings with Tom Gardiner 5 Add more observations Calculate statistical errors on linear fits Filter observations using FTS DC signals

6. CAVIAR Field Meeting, Imperial College -London(29 March2011) Results-CO2 region (more obs+errors) 6

7. CAVIAR Field Meeting, Imperial College -London(29 March2011) New Results(CO2 region)-compared w previous 7

8. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 8 Method 1: Numerically Beers law: Assume that optical depth changes linearly with time of the day (solar zenith angle) in Beers law. k depends on assumed % change in optical depth, m is the range of airmass for the day and m 0 is the minimum airmass of the day.

9. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 9

10. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 10

11. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 11 Method 2: Analytically (Igor Ptashnik) Used the fact that there are 2 sources of deviation of y- intercept ( lnF ν 0 ) of Langley plot: change in slope and parallel shift of the linear fit. Assumption: changes in derieved F ν 0 between micro window and line centre is due to changing water vapour amount only.

12. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 12 Relative change in water vapour: Relative change in derived F ν 0 :

13. CAVIAR Field Meeting, Imperial College -London(29 March2011) Effect of changing WV amount on derived TOA 13 Using typical values of c, change in intercept( lnF ν 0 ) and τ, the relative change in retrieved F ν 0 due to changing water amount is less than 1 % in atmospheric micro windows. Relative change in line centres are higher.

14. CAVIAR Field Meeting, Imperial College -London(29 March2011) Summary and Future Work 14 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.