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

Three Atmospheric Profiles

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

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