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ACE Comparisons. Kaley Walker, Ashley Jones, Chris Boone, Chris Sioris, Felicia Kolonjari, Sean McLeod, Peter Bernath and Tom McElroy MOHAVE-2009 #2 Workshop - Bern, Switzerland - 20 October 2010. Talk Overview. Recap of ACE satellite and measurement technique

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ace comparisons

ACE Comparisons

Kaley Walker, Ashley Jones, Chris Boone, Chris Sioris, Felicia Kolonjari, Sean McLeod, Peter Bernath and Tom McElroy

MOHAVE-2009 #2 Workshop - Bern, Switzerland - 20 October 2010

talk overview
Talk Overview
  • Recap of ACE satellite and measurement technique
  • ACE occultation measurements during MOHAVE
  • Comparisons with:
    • MIPAS and MLS
    • STROZ and ALVICE
ace on scisat 1
Launch date: August 12, 2003

Orbit: 74° inclination at 650 km

Measurement mode: solar occultation


FTIR spectrometer, 2-13 microns at 0.02 cm-1 resolution

2-channel visible/NIR imager, 0.525 and 1.02 microns


dual UV / visible / NIR grating spectrophotometer, 285 to 1030 nm at ~1-2 nm resolution

Pointing: suntracker in ACE-FTS


Atmospheric Chemistry Experiment (ACE) Satellite Mission:

Mission to measure atmospheric composition: profiles of trace gas species, cloud and aerosol extinction and temperature/pressure

technique solar occultation
Technique: Solar Occultation


  • Radiance of sun gives higher signal-to-noise than emission
  • Limb view gives longer path length ~500 km (lower detection limits) than nadir
  • “Self-calibrating” so excellent long-term accuracy and precision


  • Modest global coverage
  • Samples only free troposphere
occultation sequence
Sunset 2245

12 Jan. 2004 9:50:23 UTC

Lat: 67°S

Lon: 168°W

Occultation sequence
ace latitude coverage 2005 2006
ACE Latitude Coverage 2005/2006

Orbit allows repeat of measurement locations each year

beta angle of measurement
Beta Angle of Measurement
  • Beta is the angle between the orbit plane of the satellite and the Earth-Sun vector
  • The larger the beta angle the longer the occultation measurement and thus more frequent altitude sampling

High beta

Low beta

Diagram shows beta angle as viewed from Sun

ace measurements for mohave
Seven ACE occultations were measured near TMF during the MOHAVE-2009 campaign

All are “Opportunity Science” observations - beta angle > 60°

With higher beta angle, occultations are longer and cover larger ground tracks

Plot shows ground-tracks of ACE occultations from 0 to 150 km in altitude (stretch over 10 º lat.!)

Label location of occultation using 30 km point (geometric)

ACE Measurements for MOHAVE


ace occultations
ACE Occultations
  • ACE Measurement times, locations and distances from TMF
    • Distance from TMF (34.4 N, 117.7 W) to location of ACE 30 km tangent altitude (as calculated geometrically)
    • Have profiles for all occultations from ACE-FTS and for two from MAESTRO (ss33239 and ss33254)
ace fts data processing
ACE-FTS data processing
  • Raw data to atmospheric transmission spectra (Level 0 to 1)
    • Interferograms transmitted to Earth are Fourier transformed and transmission spectra are calculated using exo-atmospheric spectra
  • Spectra to atmospheric profiles (Level 1 to 2)
    • Temperature and pressure profiles are determined from global (non-linear least-squares) fit of CO2 transitions – relative line intensities give temperature and absolute line intensities give pressure
    • Then concentration profiles of atmospheric species are retrieved using microwindow approach
    • ACE-MAESTRO retrievals also use ACE-FTS pressure and temperature profiles
ace water vapour
ACE Water Vapour
  • ACE-FTS profiles (version 2.2 + O3, N2O5 & HDO updates):
    • Cloud tops to ~90 km depending on abundance
    • ~3-4 km vertical resolution from FTS field of view
    • H2O MWs
      • v2.2: ~60 from 950-975 cm-1 and 1360-2000 cm-1
      • v3.0: ~40 from 935-945 cm-1 and 1195-1945 cm-1
  • MAESTRO profiles:
    • H2O - research product - not part of current v1.2 distribution
    • Cloud tops to tropopause is best region for retrievals
    • ~1 km vertical from MAESTRO
    • Spectral range: 926 - 969.7 nm
ace h 2 o validation
ACE H2O Validation

M. Carleer et al., ACPD, 8, 4499-4559 (2008), still in revision

  • Comparisons with SAGE II, HALOE, POAM III, MIPAS and SMR
    • ACE-FTS v2.2 fitting errors generally better than 5% from 7 to 70 km and increasing above
    • In all comparisons except for POAM III, ACE-FTS typically biased high on order of 3-10 % from 15 to 70 km
    • Approximately constant wet bias of 0.4 ppmv for ACE-FTS versus SMR from 50 to 90 km
    • Largest differences for ACE-FTS at lowest altitudes - MAESTRO results seem better in upper troposphere
      • Vertical resolution effect?
ace data used for comparisons
ACE Data Used for Comparisons
  • Using newest versions of ACE-FTS (v3.0) and MAESTRO (research) profiles (available in MOHAVE database)
  • ACE-FTS: now all seven occultations have processed
    • Challenges with large number of measurements in each occultation (because of high beta angle)
    • For highest beta, analyse every other measurement to deal with large amount of data (sr33350, ss33283, ss33254)
    • For slightly lower high beta, every measurement is analysed but small distance between them (< 1.5 km) can cause oscillations to occur (ss33269 and ss33365)
  • ACE-MAESTRO: two occultations available (ss33239 and ss33254)
    • Limited by number of spectra processed with new L1 code
maestro versus ace fts

ss33254, 2009-10-16 01:12UTC; (29.72N, 116.71W); beta 61.93; 528.7 km

ACE-FTS gives fitting errors while MAESTRO calculates uncertainty for each altitude

maestro versus ace fts1

Large error given for this value!

ss33239, 2009-10-15 00:44UTC; (20.34N, 107.79W); beta 60.77; 1842.0 km

comparison details
Comparison Details
  • Starting with each ACE occultation, search for coincidences within:
    • 6 hours and 500 km for MLS (~same resolution)
    • 6 hours and 700 km for MIPAS (~same resolution)
    • 6 hours and 500 km for lidars (STROZ, ALVICE)
      • Employed smoothing technique to make vertical resolution of measurements comparable
      • Created 3 km wide weighting functions at ACE-FTS observation altitudes and convolved these with the individual lidar profiles - used in v2.2 validation!
  • Focus only on water vapour for these comparisons...
ace fts vs mipas

sr33365; 2009-10-23 14:23UTC; (36.10N, 121.23W); beta 60.97; 372.1 km

Example of oscillations

in profiles

1 ACE-FTS versus 3 MIPAS profiles

See average comparisons in Gabi’s MIPAS talk!

Non-LTE effects for MIPAS

above 50 km

ace fts versus mls
ACE-FTS versus MLS

Example of minimal oscillations

in profiles

1 ACE-FTS vs. 6 MLS profiles

Consistent with ACE-MLS v2.2 comparisons

±5% with no bias

sr33351; 2009-10-22 15:33UTC; (45.02 N, 135.84W); beta 62.02; 1942.4 km

ace fts versus stroz

ss33254, 2009-10-16 01:12UTC; (29.72N, 116.71W); beta 61.93; 528.7 km

Using 1 hour integ. STROZ

Within (just) 6 hours!

Smoothing generally makes profiles more comparable.

ace fts versus stroz1

ss33254, 2009-10-16 01:12UTC; (29.72N, 116.71W); beta 61.93; 528.7 km

Using 1 hour integ. STROZ

Focusing on UTLS part of profile

Challenge of comparing limb and vertical profiles!

ace fts versus stroz2

ss33254, 2009-10-16 01:12UTC; (29.72N, 116.71W); beta 61.93; 528.7 km

Using 1 hour integ. STROZ

Now within ~3 hours

UTLS more comparable when closer in time.

ace fts versus alvice

ss33254, 2009-10-16 01:12UTC; (29.72N, 116.71W); beta 61.93; 528.7 km

Using 1 hour integ. ALVICE within ~3 hours

See larger differences than with STROZ

Would be useful to look at T, O3

  • Seven occultations were measured within 2000 km of TMF
    • ACE-FTS v3.0 and new MAESTRO profiles available from archive
  • All “Opportunity Science” Observations
    • Challenging to process because of high beta angle
  • Limited opportunities for statistical comparisons (unlike MIPAS, MLS)
    • So, how do we best to use ACE in MOHAVE comparisons?

Funding for ACE provided by:

  • Canadian Space Agency (CSA)
  • Natural Sciences and Engineering Research Council of Canada (NSERC)
  • Environment Canada
ace fts versus haloe v19
ACE-FTS versus HALOE v19
  • 36 coincidences within 2 hours and 500 km; most northern polar summer occultations
  • Hygropause at same altitude but more rapid increase in H2O at lower altitudes for POAM III

C. Randall (in ACE H2O validation paper)