1. “Brief” update on ACE water vapour
Kaley Walker, Patrick Sheese and Chris Boone
ACE-FTS Team, U. Toronto & U. Waterloo
SPARC WAVAS II Workshop on Satellite Data Quality Assessment
JPL, Pasadena – 4 December 2013

2. Outline Quick review of ACE-FTS
Intercomparison examples – building on last meeting
Looking at v3.0/3.5 versus v2.2 and other data sets for main isotopologue (range: 5 – 101 km for v3.5)
Current work from Patrick S.
There are also HDO, H218O, and H217O profiles (range: ~5 – 50 km with some variations with latitude and species)
Looking ahead to ACE-FTS v4.0 water vapour
Recent work from Chris B.

3. ACE on SCISAT-1
Atmospheric Chemistry Experiment (ACE) Satellite Mission:
Launch date: 12 August 2003
Orbit: 74° inclination at 650 km
Measurement mode: solar occultation
ACE-FTS:
FTIR spectrometer, 2-13 microns at 0.02 cm-1 resolution
2-channel visible/NIR imager, and 1.02 microns
MAESTRO:
dual UV / visible / NIR grating spectrophotometer, 285 to 1030 nm at ~1-2 nm resolution
Pointing: suntracker in ACE-FTS
Mission to measure atmospheric composition: profiles of trace gas species, cloud and aerosol extinction and temperature/pressure

4. ACE Mission Status
Now into 11th year in orbit – designed for 2 year lifetime
Starting to see some degradation in ACE-FTS performance and MAESTRO continues to “age gracefully”
Since launch, satellite and instrument operations nominal
Routine operations began on 21 February 2004
On 2 Dec. 2013, SCISAT completed its 55,500th orbit!
~50% of occultations occur in polar regions (> 60 degrees)
Issue with pT inputs after Sept impacting all versions
Reprocessing is being completed – ACE-FTS v2.5/3.5
Operation of ACE mission approved until end of March 2014
CSA will be conducting a review in the coming months

5. ACE-FTS Version Differences
Comparison of v2.2 and v3.0
Difference is calculated as (v3.0-v2.2) and is relative to v2.2
Number of sunrise occs: ~9600
Number of sunset occs: ~10000

6. Intercomparison Data Sets
Data sets and versions (being updated currently!)
Using ACE-FTS 1 km gridded data and interpolating comparison data set to this vertical grid
ACE-FTS v3.0 ( )
MLS v3.3
ESA MIPAS v6
IMK MIPAS v5
HALOE v19
SAGE III v4
POAM III v4

7. Total Number of Comparisons
Coincidence criteria: ΔUT < 2 h; Δlat < 5°; Δlon < 10°
In instances of multiple profiles per coincidence, profiles closest in distance were used
Sunrise
Sunset

8. Sunset comparisons: v3.0
Wet bias in ACE-FTS at lower altitudes in v2.2 is reduced in v3.0

9. Sunrise comparisons: v3.0
There are fewer comparisons for ACE sunrises with occultation sensors

10. Differences for v3.0
Sunset
Sunrise

11. Comparison of Time Series
First cut at looking at this:
Coincidence criteria: ΔUT < 2 h; Δlat < 5°; Δlon < 10°
In instances of multiple profiles per coincidence, profiles closest in distance were used
Did not separate by sunset/sunrise
Looking at MIPAS (currently only ESA) and MLS since longest time series
Data shown on each plot represents km averages for each month in different latitudinal bands
Only showing where there is more than one coincidence in spatial/temporal region

14. H2O in ACE-FTS Chris B. is revisiting water vapour for version 4
In version 3.0/3.5 there are bad residuals for H2O lines in the troposphere, where the line shape is larger than the ILS.
Especially bad in the tropics, where H2O levels are high – there is large absorption from weak lines.
What is the cause: Bad Voigt broadening parameters? Speed dependent Voigt? Artifact of the rapid change of H2O VMR as a function of altitude?
Use H2O gas cell spectra (from Manfred Birk and Georg Wagner) to evaluate.

15. Using Laboratory Spectra
4 pure H2O spectra
23 air-broadened spectra
Fitting intensities, self-shift, and self T-dependence exponent
Residuals comparable in magnitude to those observed with the ACE-FTS.

16. New Linelist Improved (for some)
HITRAN 2004 (ACE v3.0/3.5)
HITRAN 2012 (ACE v4)

17. Speed-dependent Voigt
Speed-dependent Voigt (2 lines). Adjusted Voigt for other lines.
HITRAN 2012 (ACE v4)
Some H2O lines require speed-dependent Voigt to reduce residuals to the noise level.
For many (most?) lines, residuals can be reduced close to the noise simply by adjusting the Voigt parameters.
HITRAN 2012 an improvement for H2O in general, but little change for the weak H2O lines of most interest to us.

18. H2O Conclusions
H2O lines in gas cell spectra exhibit similar residuals to those seen in ACE-FTS spectra.
In general ACE-FTS residuals not an artifact of rapidly changing VMR, but there will be some occultations with enhanced residuals.
Speed dependent Voigt required to reduce residuals to the noise level.
Major improvements for most lines achieved by simply changing the Voigt parameters.
Determine parameters from ACE-FTS spectra?

19. Example: Determining from ACE
Occultation spectra
Residual spectra
ACE V3 line list uses HITRAN 2004 parameters. Little change in HITRAN 2012 for this line (unlike some other H2O lines).
Fit for pressure broadening
Some concern with the rapid change of H2O VMR as a function of altitude in the troposphere. Does this introduce systematic errors in the width retrieved from the ACE-FTS spectra?
Residual spectra

20. Summary New comparisons shown for v3.0/3.5 of ACE-FTS water vapour
Looking at both coincident profile and time series analyses for evaluation
Wet bias in lower atmosphere is reduced from v2.2
Preview of some of the developments being made for water vapour for ACE-FTS V4
Looking at large absorptions from weak lines in troposphere where there are large residuals