INFRARED MEASUREMENTS

THERMAL EMISSION MEASUREMENTS (IR, wave)

EARTH SURFACE

I(To)

Absorbing gas

To

T1

I(T1)LIMB VIEW

NADIRVIEW

Examples: MLS, IMG, MOPITT, MIPAS, TES, HIRDLS, IASI

Pros:• versatility (many species)• small field of view (nadir)• vertical profiling

Cons:• low S/N in lower troposphere• water vapor interferences

WHAT CAN WE RETRIEVE IN THE IR?

3.6 m15 m

Most easily retrieved (strongest features): CO (4.6m), O3 (9.6 m), CH4 (7.6 m)Other products more experimental, or require large perturbations (eg. SO2 in a volcano)

[Clerbaux et al., ACP 2009]

BAYESIAN INVERSION IS MOST COMMON RETRIEVAL USED FOR IR MEASUREMENTS

F( , ) y x b ε

Kx ε

RADIANCECONCENTRATIONS

Forward Model

Inverse Model

1y yˆ

1T 1 T 1

a a ax x K S K S K S y Kx

yˆ 1 T 1 1

aS K S K S

Here y is the vector of wavelength-dependent radiances (radiance spectrum); x is the state vector of concentrations; forward model y = Kx is the radiative transfer model

For profiles: b=surface emissivity, surface T and atmospheric T

1

ˆ ( )

( )

n a

a a

x Ax I A x Gε

A S K KS K S KT Ty

x̂

ˆ x x

AVERAGING KERNELS

Recall, the averaging kernel (A): describes the relative weighting of the ‘true’ mixing ratio (x) at each level to the retrieved value ( )

0.0 1.0

“Perfect” retrieval Typical retrieval

Averaging kernels are variable – particularly dependent on surface properties emissivity, i.e. surface T is critical An averaging kernel for each retrieval: provided with satellite retrieval products – or the error covariances to construct it – reminder: ˆ 1

aA I SS

ˆ x x

DOF trace( ) A

VERTICAL SENSITIVITY AND DEGREES OF FREEDOM

0.0 1.0

“Perfect” retrieval Typical retrieval

IR measurements have little sensitivity to the boundary layer (little temperature differential)!

Averaging Kernels tell us about the vertical sensitivity of the instrument. Peak of each averaging kernel gives the altitude of maximum sensitivity. It’s full width at half max can be interpreted as the vertical resolution of the retrieval.

Degrees of Freedom: m = ds+dn

m = number of measurementsds=degrees of freedom for signaldn=degrees of freedom for noise

ds is often just called the degrees of freedom (DOF) and is calculated as:

Tells us something about the vertical resolution & sensitivity of the instrument.

DOF=7 DOF=1.5

APPLES TO APPLES

The retrieved quantity is not a “real” concentration profile, but a reflection of what the instrument is sensitive to. Therefore one cannot compare directly with in situ observations (aircraft, sondes, models, etc.). This is an apples to oranges comparison!

smoothed low high low a,low x A x I A xSmoothed profile using low resolution measurement characteristics

Low resolutiona priori profile

Low resolutionAveraging kernels

High resolution profile (model/sonde)

For an apples to apples comparison, a pseudo-retrieval must be performed on the high-resolution dataset:

(Satellite Retrieval) xhigh (Aircraft) xsmoothed (Smoothed aircraft)

Example:MOPITT CO observations compared to aircraft profile over the Pacific

x̂

[Jacob et al., 2003]

USE OF AN A PRIORI

An a priori is needed because retrieval is underdetermined. While the a priori “comes out in the wash” when comparing satellite observations with other obs or models, it does impact the absolute satellite retrieved values.

ˆ ( ) n ax Ax I A x Gε

Constant Spatially/temporally variable

A priori is generally a climatological estimate of the state + uncertainty

Example: TES a priori from MOZART simulated mixing ratio in 10°x60° bins

Example: MOPITT CO a priori (v3) from 525 aircraft profiles (8 campaigns + 2 sites) weighted regionally

[Deeter et al., 2003] From Jennifer Logan

A SHORT NOTE ON SATELLITE DATA TYPES

Level 1: Raw Radiances (generally geo-located)

Level 2: Retrievals for each pixel (met, trace gases, etc)

Level 3: Gridded data products

Generally best to be looking at Level 2 data (you should generally do your own gridding and data filtering (see Lab #2!).

1996 2000 2002 2005 2007

IMG/ADEOS

MOPITT/TERRA

TES/AURA

IASI/METOP

AIRS/AQUA

CO, O3, HNO3 profiles, HDOFew days of measurementsHigh spectral resolution

CO profilesGlobal coverage 3 daysAtm. Chemistry

CO, O3 profilesHDO, NH3, CH3OHGlobal coverage 14 daysHigh Spectral resolutionAtm. Chemistry

CO, O3, CO2, CH4, H2O, volcanic SO2

Global coverage dailyCoarse spectral res.NWP

ATMOSPHERIC MEASUREMENTS FROM NADIR IR SOUNDING

CO, O3 profiles, CH4, HNO3

NH3, CH3OH, HCOOH, PAN, C2H2, volcanic SO2

Global coverage 1-2 daysHigh Spectral resolution

NWP/Atm. Chemistry

* SCIAMACHY (ENVISAT, 2002) as well, but limited IR channels, so far high retrieval uncertainties on CO & CH4

MEASUREMENT OF POLLUTION IN THE TROPOSPHERE (MOPITT)

Launched Dec. 1999

HORIZONTAL COVERAGE: 22 km x 22 km nadir footprint with 612 km cross-track scanningGlobal coverage in ~3 days

OVERPASS TIME: ~10:30, 22:30 equator cross-over

1 day of dataLaunched onboard EOS-Terra (NASA)

Priorities: first instrument targeting tropospheric composition

MOPITT INSTRUMENTLaunched Dec. 1999

PRODUCTS:CO (7 level profile)CO column

MEASUREMENT TECHNIQUE:Correlation Radiometer–4 channels: thermal emission (4.6 m)–2 channels: solar reflected (2.3 m)

MOPITT OPERATIONALLY…

Dec 18, 1999: LaunchAs of Mar 2000: Operational (coolers on, in Science mode)May 7, 2001: Side B cooler failure (channels 1-4 no longer usable)May 7-Aug 24, 2001: Standby mode, instrument testing (no retrievals)Aug 25, 2001: Return to Science modeJul 28-Sept 29, 2009: Anomaly (no retrievals)To present… Still operational

Use only clear-sky radiances (MOPITT & MODIS cloud detection used)Atmospheric T & water vapour profiles obtained from NCEP re-analysisSurface T & emissivity are retrieved by MOPITT

Version 3: available March 2000-July 31, 2009Version 4: current operational product (re-analysed back to March 2000)

updates in apriori , uses log(vmr), 10 levels, changes in file information

Pre-anomaly3 channels: 1, 3, 7

N=4ds=1.21

Post-anomaly2 channels: 5, 7

N=3ds=1.15

MOPITT SOLAR CHANNELS

Solar channels (near IR: 2.2 m) were included in the instrument design to constrain the total CO (and methane) column. Low SNR (also issues with sun glint & polarization over water and reflectivity over land), in these channels has precluded use in operational retrievals to date.

Efforts are on-going…

As a result there are no operational methane retrievals from MOPITT [Deeter et al., 2009]

ATMOSPHERIC INFRARED SOUNDER (AIRS)

Launched May 2002

HORIZONTAL COVERAGE:100 km x 60 km nadir footprint with 1650 km cross-track scanningDaily Global coverage

OVERPASS TIME: ~13:30, 1:30 equator cross-over

1 day of dataLaunched onboard EOS-Aqua (NASA)

Priorities: climate research and weather prediction

AIRS INSTRUMENTLaunched May 2002

STANDARD PRODUCTS:Met (T, H2O, OLR, clouds)CO (9 layer profile)O3 (28 layer profile)CH4 (7 layer profile)SO2

MEASUREMENT TECHNIQUE:IR grating spectrometer–Broad spectral coverage (3.7-15.4 m), but only select windows–Medium spatial resolution (~2 cm-1 near CO feature)

AIRS OPERATIONALLY…

May 4, 2002: LaunchAug 30, 2003: Operational retrievalsJan 9-26, 2010: Anomaly (no retrievals)To present… Operational

Clouds, surface properties, atmospheric T and water vapour profiles retrieved by AIRSUses radiances from AMSU (also on Aqua), although products w/o AMSU data have been produced (in case of AMSU failure)Cloud clearing to retrieve in partially cloudy scenes.

Version 5: current retrieval version, retrieval process underwent significant updating from Version 4, now includes HNO3(strat mainly), N2O and SO2 (volcanic plumes)

Note: AIRS retrievals use log(vmr) and a single a priori of partial columns

TROPOSPHERIC EMISSION SPECTROMETER (TES)

Launched July 2004

HORIZONTAL COVERAGE:5 km x 8 km nadir footprint with no cross-track scanningGlobal coverage in ~16 days

OVERPASS TIME: ~13:45, 1:45 equator cross-over

(1 day of data)Launched onboard EOS-Aura (NASA)

Priorities: targeting tropospheric composition

TES INSTRUMENTLaunched July 2004

STANDARD PRODUCTS:Met (T, H2O / HDO)CO (67 level profile)O3 (67 level profile)CH4 (67 level profile)(NH3)

MEASUREMENT TECHNIQUE:Fourier Transform Spectrometer (FTS)–Broad spectral coverage (3.6-15.5 m)–High spectral resolution (0.1 cm-1)

TES OPERATIONALLY…

Jul 15, 2004: LaunchAug 22, 2004: First global surveys taken (Science mode)Dec 2004: Regular, complete observation scheduleApr 11-May 20, 2005: No dataApril 10, 2005: Limb view taken out of operational rotation

(coverage improved – obs separated by 1.6° instead of 5°)Jun 2005: No dataSep 15-29, 2005: No dataAs of Dec 20, 2009: Retrievals suspended (pointing system error), expected to

resume in Jan 2010

Use only clear-sky radiances (identified in Level 1 retrieval)Surface T, atmospheric T profile, surface emissivity and water vapour profile are retrieved by TES

Current retrieval: v4

Note: tes retrievals use log(vmr) and varying a priori (10°x60° means from MOZART)

TES OBSERVING SCHEDULE

http://tes.jpl.nasa.gov/data/datacalendar/

Global Surveys: 16 orbits over 26 hoursPixels separated by ~1.6°

Special observations:Stare: specific nadir point Transect: 850 km Step & Stare: nadir obs separated by 35km

Data Stream:Global Surveys followed by 20 hours of calibration/ special observations opportunities

THE A-TRAIN

The Afternoon Constellation

See the same scene will minimal time separation – possibility for integrating measurements (the holy grail for NASA!)

INFRARED ATMOSPHERIC SOUNDING INTERFEROMETER (IASI)

Launched Oct. 2006

MetOp

MetOP

HORIZONTAL COVERAGE:12 km x 12 km pixel at nadir with 2200 km of cross-track scanningDaily Global coverage

OVERPASS TIME: ~9:30, 21:30 equator cross-over

(1 day of data)Launched onboard METOP-A (ESA)

Priorities: NWP as well as chemistry/climate

Infrared Atmospheric Sounding Interferometer (IASI)

IASI

IASI INSTRUMENTLaunched Oct. 2006

PRODUCTS:Met products (T, H2O, surface, clouds, etc)CO (profiles)O3 (profiles)CH4 (pseudo-column)HNO3 (column)(SO2, NH3, CH3OH, HCOOH, others) Includes a cloud fraction flag (FLG_CLDSUM)

MEASUREMENT TECHNIQUE:Fourier Transform Spectrometer (FTS)–Broad spectral coverage (3.6-15.5 m)–Moderate spectral resolution (0.5 cm-1)–High radiometric performance

IASI OPERATIONALLY…

Oct 19, 2006: LaunchNov 30, 2006: First data availableJuly 18, 2007: Level 1 data formally operationalTo present: Fully operational

Use only clear-sky radiances – cloud properties (cloud cover, liquid water content, cloud top T) are retrievedSurface T, atmospheric T profile, clouds, surface emissivity and water vapour profile are retrieved by IASI

Some operational products, many research products…Currently, operational products distributed by EUMETSAT

COMPARING NADIR IR INSTRUMENTS: SPECTRAL RANGE

Tra

nsm

ittan

ce

MOPITT (2 channels)

3.6 m15 m

4.7 m 2.3 m

SCIAMACHY(UV-vis-IR)

AIRS(UV-vis-IR)

TES(IR)

IASI(IR)

0.4-.94m

3.7-4.6 m6.2-8.2 m8.8-15.4 m

0.24-2.4 m

3.2-15.4 m

3.6-15.5 m

COMPARING NADIR IR INSTRUMENTS: SPATIAL COVERAGE

MOPITT (22 km x 22 km)

SCIAMACHY (60 km x 30 km)

AIRS (100 km x 60 km)

TES (5 km x 8 km)

IASI (12 km x 12 km)

Smaller pixel sizes reduce

cloud contamination

MOPITT (612 km)SCIAMACHY (960 km)

AIRS (1650 km)

TES (0)

IASI (2200 km)

NADIR PIXEL SIZE

CROSS-TRACK SWATH

Cross-track reduces time for global coverage

ROLE OF THERMAL CONTRAST

An example from IASI (May 2008)

Thermal contrast is higher (hence improved retrieval sensitivity):- during the day- over land- over dry, sparsely vegetated regions

Day Night

[Clerbaux et al., 2009]

Ability to probe PBL depends on location, temperature, type of surface and time of day.

IR MEASUREMENTS CANNOT SEE THROUGH CLOUDS

Feb 23, 2001 Feb 24, 2001 Feb 26, 2001

Example of a CO plume transported across the Pacific “observed” by MOPITT

Plume likely co-located with clouds (frontal outflow)

For polar-orbiting satellites (twice daily sampling), clouds can severely limit our ability to capture plumes.

AIRS applies “cloud clearing” algorithm to improve coverage: extrapolate from MODIS (smaller footprint ) clear radiance vs. cloudy radiance measurements (“remove” radiance contribution from cloud emission)

[Heald et al., 2003]

Perfect vertical resolution

10 AK

z

i

Example of smoothing (typical)

10 AK

z

i

CO profile19 levels

1. Uncertainty on the radiances (radiometric noise): measurement error only error accounted for in theoretical retrieval error

2. Uncertainty on the atmospheric and surface parameters (e.g. emissivity, temperature and water vapor profiles)

3. Lack of vertical resolution: smoothing error

characterized by the averaging kernel and the derived degrees of freedom of signal: )(

ˆ

Ax

xA

traceDOFS

RETRIEVAL ERROR

CARBON MONOXIDE: A TRACER OF POLLUTION

Current retrievals based on 4.6 m feature.

Design of MOPITT (and SCIAMACHY) was to also use the 2.3 m feature (solar backscatter) to provide a constraint on total column (and thus by subtraction, the BL). Retrievals from this wavelength are challenging (noise).

Sink: CO + OH → CO2 + HO2

CO Lifetime = 2 monthsCH4

NMHC

OHO2

GLOBAL CO DISTRIBUTION FROM IASI

[Clerbaux et al., 2009]

COMPARING CO RETRIEVALSAugust 2008

[George et al., 2009]

COMPARING CO RETRIEVAL SENSITIVITYAugust 1, 2008

[George et al., 2009]

EXAMPLE OF DOFS FOR RETRIEVED CO (MOPITT)

DOFs < 0.5 in polar regions, peak in the tropics

[Deeter et al., 2004]

TROPOSPHERIC OZONE: PRIMARY CONSTITUENT OF SMOG, AND 3RD MOST IMPORTANT GHG

O3

OH HO2

h, H2ONO

H2O2

NO2

HNO3

CO, HC, NOx

IR retrieval relies on 9.6 m feature.

Tropospheric IR retrievals only from TES and IASI instruments currently (as we’ll see, O3 can also be retrieved in the UV-visible…)

GLOBAL OZONE DISTRIBUTIONS FROM TES

2006 ozone at 500 hPa averaged on 4ox5o resolution

MAM

SON DJF

JJA

Courtesy of Lin Zhang (Harvard)

TES OZONE RETRIEVAL

DOFs for tropospheric O3 (July 2006)

Typical Averaging Kernel

Courtesy of Lin Zhang (Harvard)

Note that O3 profile is weighted to stratosphere:

Example ozonesonde @ Ascension

So ozone retrievals often done in logarithm

[Worden et al., 2007]

ˆln( ) ln( ) ( ) ln( ) n ax A x I A x

Spectral range selected: 1240-1290 cm-1

Global distribution from 4 to 8 October 2008 averaged on a 4x4° grid

Averaging Kernels (tropical example)

Averaged vmr most representative of the 4 to 12 km layer[Razavi et al., 2009]

METHANE RETRIEVALS FROM IASI

SO2 RETRIEVALS FOR VOLCANIC PLUMES(possible with high spectral resolution)

IASI: Sept 2007 Jebel eruption

Spectra (blue inside plume, red outside)

Retrieved SO2 on September 30, 2007

Peak altitude for observed

SO2 ~15-17 km

[Clarisse et al., 2008]

TES: 4 eruptions in 2005-2006

Spectra (blue with SO2, red without)

Retrieved SO2 profiles for 2 plumes

TES can discern high and low altitude plumes, but has poor spatial coverage.

[Clerbaux et al., 2008]

FIRST MEASUREMENTS OF AMMONIA (NH3) AND METHANOL (CH3OH) IN TROPOSPHERE FROM SPACE

[Beer et al., 2008]

Averaging Kernels

GLOBAL “RETRIEVALS” OF AMMONIA WITH IASI

[Clarisse et al., 2009]

Did not use a full MAP retrieval (used brightness T to NH3 scaling)Monthly gridded averages calculated to reduce noise.

Annual average NH3

Species Vertical Res. (DOFS)

Error(%)

Comment References

Short-lived species (lifetime a few hours to a few days)NH3 NA Detected in fires and

over agricultural regions

Coheur et al., this issueClarisse et al., 2009

CH3COOH NA Detected in fires Coheur et al., this issue

HCOOH NA Detected in fires Coheur et al., this issue

C2H4 NA Detected in fires Coheur et al., this issue

SO2 volcans NA Detected in volcanic plumes for concentrations above 2 DU

Clarisse et al., this issue

Aerosols

Dust (sand), volcanic ash, ice clouds

~1 NA

IASI RETRIEVALS OF SHORT-LIVED SPECIES

[Clerbaux et al., 2009]

IASI RETRIEVALSTsurf CH4H2O

NH3Trop O3CO

SO2O3HNO3

[Clerbaux et al., ACP, 2009]