1. Flight Data Simulations and the Extraction of the Continuum from the Far-IR Ralph Beeby Paul Green, Juliet Pickering, John Harries

2. Introduction TAFTS: Tropospheric Airborne Fourier Transform Spectrometer Far-IR: 17-200  m (50-600cm -1 ), resolution 0.1cm -1, differential FTS Aim: use data from TAFTS to improve water vapour foreign-broadened continuum Many sources of error  uncertainty in relative humidity (RH) How does this affect our ability to measure the continuum?

3. Modelling Attempt to simulate effects of RH error Modelling line-by-line radiative transfer: LBLRTM most suitable Model inputs: altitude, RH, temperature, conc n of other species – focus on RH Generate spectra for errors in RH, compare with spectra for changes in continuum strength To what accuracy can we expect to measure the continuum?

4. Microwindows  Entire spectrum: too much information  Mean radiance across transparent region - microwindow

5. Previously… Previous modelling by Green: sensitivity of foreign continuum as function of wavenumber, humidity and altitude Modelled at 6 different observer altitudes Used LBLRTM, atmospheric profiles from Camborne radiosondes Continuum sensitive to distribution of RH Net radiance gives better sensitivity

6. Three Atmospheric Profiles

7. Green: Micro-window (continuum) sensitivity as function of wavenumber, altitude and humidity profile

8. 1 – Hygrometer Error What about measurement uncertainty in profiles? How sensitive are radiance measurements to uncertainty in RH and in continuum? Sonde hygrometer uncertainty: ±2%RH Change continuum strength and RH profile Compare resulting changes in radiance

9. Comparing radiance effects: continuum error and relative humidity error RH_radianceeffect.eps Dry Step Wet

10. 2 – Representation Error  Scene changes greatly as aircraft and atmosphere move  Increase error linearly  “Error cone” centred on observer  Profiles is 1-D, calculate RH error according to:  RH =  H (1-|z obs - z layer |) ± 15% RH Obs: ±2%RH z = 0 z = 33kft z = 15kft

11. Comparing radiance effects: continuum error and representation error scale_radianceeffect.eps Dry Step Wet

12. % Equivalent Continuum Error To what accuracy can we measure foreign continuum? Apply linear fit to data for radiance effect of continuum error Calculate continuum error equivalent to hygrometer and representation error

13. % Equivalent Continuum Errors for Hygrometer and Representation Errors: variation with altitude and wavenumber Dry Step Wet

14. Conclusions Best to fly through areas of high RH: - error in RH proportionally smaller - stronger signature of continuum Continuum sensitive to structure of profile – particularly smaller wavenumbers Greater profile/representation error reduces continuum accuracy

15. Summary  Modelled errors in RH measurements – hygrometer uncertainty and representation error  Greater continuum accuracy in regions of high RH  Uncertainty is sensitive to structure of atmospheric profile  Next step: realistic change in scene, use sonde profiles to simulate