Post on 21-Dec-2015
Flight Data Simulations and the Extraction of the Continuum from
the Far-IR
Ralph BeebyPaul Green, Juliet Pickering, John Harries
Introduction
TAFTS: Tropospheric Airborne Fourier Transform Spectrometer
Far-IR: 17-200m (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?
Modelling• Attempt to simulate effects of RH error• Modelling line-by-line radiative transfer: LBLRTM
most suitable• Model inputs: altitude, RH, temperature, concn 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?
Microwindows Entire spectrum: too much information Mean radiance across transparent region - microwindow
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
Green: Micro-window (continuum) sensitivity as function of wavenumber, altitude and humidity
profile
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
Comparing radiance effects: continuum error and relative humidity
error
RH_radianceeffect.eps
Dry
Step
Wet
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-|zobs- zlayer|)±15% RH
±15%RH
Obs: ±2%RH
z = 0
z = 33kft
z = 15kft
Comparing radiance effects: continuum error and representation
error
scale_radianceeffect.eps
Dry
Step
Wet
% 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
% Equivalent Continuum Errors for Hygrometer and Representation Errors: variation with altitude and
wavenumber
Dry
Step
Wet
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