Effect of variation of water vapour amount on derived TOA solar spectrum

1. Effect of variation of water vapour amount on derived TOA solar spectrum Kaah Menang, Liam Tallis , Igor Ptashnik and Keith Shine.

2. Outline • Derived TOA solar spectra from Camborne • Effect of Changing water vapour amount on derived TOA solar spectra • Discussion and future work

3. From last meeting:

4. From last meeting

5. Meetings with Tom Gardiner • Add more observations • Calculate statistical errors on linear fits • Filter observations using FTS DC signals

6. Results-CO2 region (more obs+errors)

7. ‘New’ Results(CO2 region)-compared w previous

8. Effect of changing WV amount on derived TOA • 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,mis the range of airmass for the day and m0 is the minimum airmass of the day.

9. Effect of changing WV amount on derived TOA

10. Effect of changing WV amount on derived TOA

11. Effect of changing WV amount on derived TOA • 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.

12. Effect of changing WV amount on derived TOA • Relative change in water vapour: • Relative change in derived Fν0:

13. Effect of changing WV amount on derived TOA • Using typical values of c, change in intercept(lnFν0) and τ, the relative change in retrievedFν0due to changing water amount is less than 1 % in atmospheric micro windows. • Relative change in line centres are higher.

14. Summary and Future Work • 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.