TAFTS: Comparing Uncertainties in Atmospheric Profiles with the

Water Vapour Continuum

Ralph Beeby, Paul Green, Juliet Pickering, John Harries

• Context• Case study: B400• Atmospheric profiles / Jacobians• Comparing continuum with measurement errors

Context

• TAFTS (Tropospheric Airborne Fourier Transform Spectrometer) used to measure in-situ far-infrared atmospheric radiances

• Aim: to use TAFTS measurements to measure the water vapour continuum as part of CAVIAR

• Need to compare TAFTS spectra with simulations based on existing line databases

• LBLRTM (Line by Line Radiative Transfer Model) used: can easily adjust strength of continuum absorption within this model

• Can also incorporate measurements of atmospheric conditions from, e.g., radiosondes as model input

ContextOnboard instruments and

sondes: auxiliary measurements

Radiometers to measure spectra – TAFTS1, ARIES2

Generate best estimate of atmospheric profile

Use profile as input to LBLRTM4

LBLRTM calculates expected water vapour spectrum according

to current HITRAN5/CKD6 parameters

Field campaigns: Camborne, Jungfraujoch

Calibrate radiometers at NPL3

Calibrate data to obtain water vapour spectrum

Compare LBLRTM with TAFTS so that monomer lines agree within errors

Compare differences in “micro-window” regions to estimate error in water vapour continuum

1 Tropospheric Airborne Fourier Transform Spectrometer

2 Airborne Research Interferometer Evaluation System 3 National Physical Laboratory 4 Line By Line Radiative Transfer Model5 High-resolution Transmission molecular absorption database 6 Clough, Kneizys, Davies

Flight B400

Atmospheric Profiling• Combines radiosondes

from Camborne, dropsondes from aircraft and model fields from ECMWF

• Divide SLR into sections over land and over the sea

• Over the sea: use nearest dropsonde as most reliable source

• Use nearest radiosonde from top of dropsonde profile up to 100mb

• ECMWF up to top of model atmosphere

• Look at variation in ECMWF with time and space, apply similar variation to sonde profiles where necessary

Pre

ssure

/

mb

Water vapour / %RH

Temperature / K

Uncertainty in Profiles

• Jacobians in LBLRTM used to assess sensitivity of spectra to uncertainties in temperature and relative humidity

• Analytic Jacobians: calculate dR/dx across spectral range where R = radiance and x = atmospheric parameter

• Indicates the change in radiance that would be caused by a change in a given atmospheric parameter (e.g., temperature, relative humidity)

• Integrate over column to obtain estimate uncertainty

Wavenumber / cm-1

dR

/d log

[vm

r(H

2O

)] /

m

Wm

2sr

.cm

-1/log

[vm

r]

Comparing Continuum with Measurement Errors

• Looking for areas of spectra where sensitivity to the continuum is high (Rcont) relative to realistic uncertainties in the atmospheric (temperature and water vapour) profile used in the simulated data (Rsim) – so where

Rcont <Rsim – will not be able to define continuum

Rcont ≈Rsim – possible to define continuum depending on ability to separate contributions

Rcont >Rsim – will be able to measure continuum

• Adjust strength of foreign-broadened continuum in simulation• Use Jacobians to calculate changes in radiance due to errors in RH,

T and “representation error”• Compare R for both cases – at which wavelengths can we define

continuum?

Comparing Continuum with Measurement Errors

• Example: simulation of 20,000ft run, DW view

• Two window regions

• LBLRTM simulation with normal continuum (black) compared with continuum strength adjusted by ±25% (red)

• Measurement uncertainties calculated from Jacobians (blue) assume an error equivalent to ±10%RH from a combination of instrument and representation error

Comparing Continuum with Measurement Errors

• Change in radiance due to profile uncertainty (black)

• Change in radiance due to change in continuum strength (blue)

Conclusions

• Model simulation used to compare effects on radiance of- Uncertainty in atmospheric profile (RH, T)- Changes to foreign-broadened continuum

• Example: 20,000ft flight level• In some windows, change in continuum shows greater change in

radiance can measure continuum in these

Coming Soon

• Similar simulations for other flight levels – determine which other windows can be used to measure continuum at different altitudes

• Calibrating TAFTS spectra

• Compare TAFTS spectra with simulations to measure continuum