Andreas Richter Institute of Environmental Physics University of Bremen tel. ++49 421 218 4474

Satellite observations of the atmosphere and the ocean surface

Heraeus Summer School “Physics of the Environment”

Andreas Richter

Institute of Environmental Physics

University of Bremen

tel. ++49 421 218 4474

e-mail: richter@iup.physik.uni-bremen.de

http://www.iup.physik.uni-bremen.de/doas

A. Richter, Heraeus-Summerschool, 3.9.2005 - 2 -

Lecture Contents

1. What is Remote Sensing?

2. Which Quantities can be Measured?

3. What are the Underlying Physical Principles?

4. Examples:a.Tropospheric Aerosols

b.Stratospheric Ozone

c.Tropospheric NO2

d.Stratospheric Aerosols

e.Temperature Profiles

f.Wind Speed and Direction

g.Sea Surface Temperature

h.Sea Ice

5. Summary

What is Remote Sensing?

“Remote sensing is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation“ (Lillesand and Kiefer 1987)

“The art of dividing up the world into little multi-coloured squares and then playing computer games with them to release unbelievable potential that's always just out of reach.” (Jon Huntington, Commonwealth Scientific and Industrial Research Organisation Exploration, Geoscience, Australia)

The Eye as a Remote Sensing Instrument

• eye: remote sensing instrument in the visible wavelength region (350 - 750 nm)

• signal processing in the eye and in the brain• colour (RGB) and relative intensity are used to

identify surface types • large data base and neuronal network used to

derive object properties

• eyes are scanning the environment with up to 60 frames per second

• 170° field of view, 30° focus

• stereographic view, image processing, and a large data base enables detection of size, distance, and movement

• passive remote sensing instrument, relies on (sun) light scattered from the object

• no sensitivity to thermal emission of objects

8-14 microns image of a cat

• active remote sensing by use of artificial light sources

Why should we use Remote Sensing?

• not all measurement locations are accessible (atmosphere, ice, ocean)• remote sensing facilitates creation of long time series and extended

measurement areas• for many phenomena, global measurements are needed• remote sensing measurements usually can be automated• often, several parameters can be measured at the same time• on a per measurement basis, remote sensing measurements usually are

less expensive than in-situ measurements

Why NOT to use Remote Sensing:

• remote sensing measurements are always indirect measurements• the electromagnetic signal is often affected by more things than just the

quantity to be measured• usually, additional assumptions and models are needed for the

interpretation of the measurements• usually, the measurement area / volume is relatively large• validation of remote sensing measurements is a major task and often not

possible in a strict sense• estimation of the errors of a remote sensing measurement often is difficult

Schematic of Remote Sensing Observation

Validation

Sensor

Measurement

Object

Changed Radiation

Radiation

Data Analysis

Final Result

A priori information

Forward Model

Classification of Remote Sensing Techniques

• active / passive• platform• wavelength range• spectral resolution

low / medium / high• spatial resolution

low / high• detection technique

absorption, emission or extinction spectroscopyspectral reflectance

Active vs. Passive Remote Sensing

Active Remote Sensing:

Artificial source of radiation, the reflected or scattered signal is analysed:• sound: SONAR• radio waves: RADAR (RAdio Detection And Ranging)• laser light: LIDAR (LIght Detection And Ranging)• white light: long path DOAS (Differential Optical Absorption Spectroscopy)

Passive Remote Sensing:

Natural sources of radiation, the attenuated, reflected, scattered, or emitted radiation is analysed:

• solar light• lunar light• stellar light• thermal emission

Remote Sensing Platforms

• ground-based measurementscontinuous, high accuracy, easy accessibilitylocal measurement

• air-borne measurements (up to 15 km)fast moving, long distanceexpensive, sporadic

• sonde / balloon measurements (up to 30 km)high altitudelogistically difficult, often expensive

• rocket measurements (up to 80 km)very high altitudeexpensive, sporadic

• Space Shuttle / Space Station measurements global coverage, limited time coverage, good accessibility

• satellite measurements global coveragepoor accessibility, expensive

Wavelength Ranges in Remote Sensing

UV: some absorptions + profile informationaerosols

vis: surface information (vegetation)some absorptionsaerosol information

IR: temperature informationcloud informationwater / ice distinctionmany absorptions / emissions+ profile information

MW: no problems with cloudsice / water contrastsurfacessome emissions + profile information

Which Quantities are Measured?

• absolute intensities in dedicated wavelength intervals• intensities relative to the intensity of a reference source• ratios of intensities at different wavelengths• variations of intensities• degree of polarisation• phase and delay of signal

Which Quantities can be Determined?

Surfaceheightalbedovegetation typesurface (water) temperaturefiressurface roughnesswind speedwater turbidity / chlorophyll

concentrationsice coverice type

Meteorologypressuretemperaturecloud covercloud top heightcloud typelightning frequency

Chemical constitution of the atmosphere

aerosol burdenaerosol typetrace species

The Electromagnetic Spectrum

• nearly all energy on Earth is supplied by the sun through radiation• wavelengths from many meters (radio waves) to nm (X-ray) • small wavelength = high energy• radiation interacts with atmosphere and surface

absorption (heating, shielding)excitation (energy input, chemical reactions)re-emission (energy balance)

Wavelength λ

I I i I I I I I I I I I I I 1km 100m 10m 1m 0.1m 10cm 1cm 1mm 0.1mm 10μm 1μm 0.1μm 10nm 1nm Radiowaves Microwaves thermal X-ray Infrared Visible Ultraviolet Interaction of electromagnetic Rotation Vibration Electron radiation with matter Transition

Radiative Transfer in the Atmosphere

Contributions:• Direct Solar Ray• Reflection on the Surface• Reflection from Clouds• Scattering in the Atmosphere

Rayleigh ScatteringMie ScatteringRaman Scattering

• Absorption in the Atmosphere• Emission in the Atmosphere• Emission from the Surface• Emission from Clouds

Radiative Transfer in the Atmosphere

Absorption

Scattering

Aerosol / Molecules

Atmosphere

Absorption on the ground

Scattering / Reflection on the

ground

Emission from the ground

Emission

Scattering from a cloud

Transmission through a

Scattering / reflection oh a

Scattering within a cloud

Emission from a cloud

Scattering in the Atmosphere

Depending on the ratio of the size of the scattering particle (r) to the wavelength () of the radiation:

Mie parameter = 2 r / ,

different regimes of atmospheric scattering can be distinguished.

=> different wavelengths probe different parts of the atmosphere / surface

What is the Optimal Instrument?

A compromise must be found to get the optimum amount of information out of the limited number of photons available under the given boundary conditions:

spatial resolution

spectral resolution

vertical resolution

time resolution

spatial coverage

spectral coverage

time coverage

instrument size and price

satellite orbit

measurement quantity

data rate

measurement error

Satellite Orbits

(Near) Polar Orbit:• orbits cross close to the pole• global measurements are possible• low earth orbit LEO (several 100 km)• ascending and descending branch• special case: sun-synchronous orbit:

overpass over given latitude always at the same local time, providing similar illumination

for sun-synchronous orbits: day and night branches

Geostationary Orbit:• satellite has fixed position relative to the Earth• parallel measurements in a limited area from low to

middle latitudes• 36 000 km flight altitude, equatorial orbit

http://www2.jpl.nasa.gov/basics/bsf5-1.htm

http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter2/chapter2_2_e.html

How can Vertical Information be Derived?

In many atmospheric application, vertical profiles of quantities are needed.

Approaches:• Vertical Scanning

sequential of parallel measurements at different altitudes=> e.g. SCIAMACHY limb profiles

• Pressure / Temperature dependence of signal (e.g. line shape)inversion of signal using a priori information on e.g. vertical p-profile=> e.g. microwave sounding

• Saturation Effects at different wavelengths (frequencies)using spectral regions with different penetration depths=> e.g. SBUV ozone profile measurements

• Time Resolved measurementsusing pulsed signals and photon flight time information=> e.g. LIDAR

• Combination of different types of measurements, instruments or models=> e.g. GOME tropospheric NO2 measurements

Approaches:• Vertical Scanning

sequential of parallel measurements at different altitudes

Nadir: observation of scattered and reflected light, total column determination (and O3 profile), good spatial resolution, global coverage, good SNR

Limb: observation of scattered light, stratospheric and upper atmosphere profiles, poor spatial resolution, near global coverage, SNR decreases with altitude

Occultation: direct observation of sun or moon at horizon, stratospheric profiles, poor spatial resolution, limited coverage (close to terminator), high SNR but low UV sensitivity

Limb Nadir Matching: combination of nadir and limb measurements to estimate the tropospheric column of a trace gas

http://www.sciamachy.de

Approaches:• Pressure / Temperature dependence of signal (e.g. line shape)

http://www.ram.uni-bremen.de/index_ram.html

pressure broadening:

high p

T-profile

Measured Spectrum

p-profile

trace gas profile

inversion

Approaches:• Saturation Effects at different wavelengths (frequencies)

Example: ozone profiling in the UV (e.g. SBUV, GOME)

Ozone absorption is increasing by orders of magnitude over 50 nm in the UV, and virtually no photons reach the surface below 300 nm. By measuring ozone at different wavelengths, different sub-columns are determined => profile

331 nm

306 nm

297 nm

How can the desired signal be isolated?

In most measurements, several effects on the signal interfere and need to be corrected.

Example: retrieval of NO2 by UV/vis absorption spectroscopy of scattered sun light• NO2 absorption• absorption by other species (O3, O4, H2O, ...)

=> use of measurements at many wavelengths and characteristic absorption spectrum for correction

• colour of the surface (e.g. ocean colour)=> use of measurements at many wavelengths and characteristic absorption spectrum for correction

• scattering by aerosols=> fit of broad band contribution

• elastic scattering by air molecules=> fit of broad band contribution

• inelastic scattering by air molecules=> explicit correction by modelling the effect

=> in many cases, measurements at several wavelengths / frequencies help

Validation of Remote Sensing Measurements

Remote Sensing measurements are indirect measurements, and need validation!

The perfect validation measurements should• measure the same quantity• integrate over the same volume• measure at the same time• use an independent technique• have higher accuracy and precision than the measurement to be validated• cover a large range of geophysical conditions• have no location bias such as measurements

only over land, only during clear weather or mostly in the Northern Hemisphere

• not be too expensive

=> such measurements do usually not exist!

Problems for Validation

Example: Stratospheric NO2 measurements from SCIAMACHY:

Amount of data: SCIAMACHY provides about 150 000 NO2 measurements per day or more than 50 000 000 measurements per year. To validate even a small part of these data necessitates a large number of validation measurements

Global coverage: hardly any validation measurements are truly global in coverage but usually biased over land in NH mid-latitudes

Averaging volume: even a “small” SCIAMACHY ground pixel is 30 x 60 km2 large and at high sun vertically integrated over the whole atmosphere. Sampling this volume at 3 km resolution horizontally and vertically (up to 20 km) would take many hours in an aircraft.

Inhomogeneity in time and space: many validation measurements do not coincide exactly in time and space with the remote sensing measurement. Horizontal variability as well as changes over time often are the largest uncertainty in validation

Errors of validation measurements: validation measurements often have themselves relatively large random and systematic errors, in particular if they are remote sensing measurements (example: neglect of temperature dependence of ozone cross-section in Brewer measurements, interference by PAN and other compounds with in-situ NO2 measurements, pump rate problems at high altitudes in ozone-sonde measurements, ...)

Validation Example

Example:

Validation of SCIAMACHY NO2 total columns with ground-based DOAS zenith-sky measurements

Results:• validation at several stations (latitudes)• validation of complete seasonal cycle• comparable measurement volume• good agreement

Problems:• ground-based measurements AM / PM twilight,

SCIAMACHY at 10:00 LT• zenith-sky measurements not sensitive to

tropospheric pollution• zenith-sky measurement is also remote

sensing measurement, not truly independent technique

LIDAR Measurements of tropospheric aerosols

Target Quantity: Tropospheric aerosol concentrations

Measurement Quantity: Backscatter ratio at 532 nm and time lag

Instrument type: LIDAR

Instrument: LITE on Space Shuttle, September 1994

LIDAR (LIght Detection And Ranging)

Idea: Use of an active system that emits light pulses and measures the intensity of the backscattered light (from air molecules, aerosols, thin clouds) as a function of time (optical Radar)

Instrument: • a strong laser with short pulses• possibly several wavelengths emitted• a large telescope to collect the weak signal

Measurement quantity:• time lag gives altitude information• signal intensity gives information on backscattering at given altitude and extinction

along the light path• measurements at different wavelengths provide information on absorbers and

aerosol types• polarisation measurements provide information on phase of scatterers

=> Very good vertical resolution can be achieved!

Lidar In-space Technology Experiment (LITE)

Instrument:• flashlamp-pumped Nd:YAG laser • 1064 nm, 532 nm, and 355 nm • 1-meter diameter lightweight telescope • PMT for 355 nm and 532 nm avalanche photodiode (APD) for 1064 nm

Mission Aims:• test and demonstrate lidar measurements from space• collect measurements on

cloudsaerosols (stratospheric & tropospheric)surface reflectance

Operation:• on Discovery in September 1994

as part of the STS-64 mission• 53 hours operation

http://www-lite.larc.nasa.gov/index.html

LITE: Example of Aerosol Measurements

• 5 minutes of LITE data over the Sahara• low maritime aerosol layer• high complex aerosol layer over Sahara• Atlas Mountains separate two regimes• clouds close to the ITCZ

Atlas mountains

Clouds (ITCZ)

complex aerosol layer

maritime aerosol layer

http://www-lite.larc.nasa.gov/index.html

UV absorption measurements of stratospheric O3

Target Quantity: Stratospheric Ozone columns

Measurement Quantity: Differential absorption of backscattered UV radiation

Instrument type: low resolution nadir viewing UV spectrometer

Instrument: TOMS (Total Ozone Mapping Spectrometer )

Total Ozone Mapping Spectrometer TOMS

Idea: • global measurements of ozone columns using differential measurements

in the UV• good spatial resolution through fast measurements• additional products (SO2, aerosols) by clever selection of wavelengths

• continuous measurements, long time series, high consistency, little changes in instrumentation => trends

The TOMS programme:Satellite Period Orbit

Nibus 7 Oct 78 – May 93 955 km

Meteor3 Aug 91 – Dec 94

Adeos Aug 96 – Jun 97 830 km

Earth Probe (EP) Jul 96 – Dec 97 500 km

Dec 97 – today 740 km

Wavelengths:

380.0 339.7 331.0 317.4 312.3 308.6 nmhttp://jwocky.gsfc.nasa.gov/

TOMS: Observation of the Ozone Hole

The Ozone Hole• forms in the Antarctic winter /

spring• formation of Polar Stratospheric

Clouds PSC in the extremely cold vortex

• heterogeneous activation of chlorine reservoirs on the cold PSC surfaces

• rapid ozone destruction by ClO and BrO as the sun rises

• end of ozone destruction after warming when chlorine is transformed back to its reservoirs HCl and ClONO2 and vortex air mixes with ozone rich air

http://jwocky.gsfc.nasa.gov/

UV/vis absorption measurements of tropospheric NO2

Target Quantity: Tropospheric Nitrogen Dioxide columns

Measurement Quantity: Differential absorption of backscattered radiation

Instrument type: medium resolution nadir viewing UV/vis spectrometer

Instrument: GOME (Global Ozone Monitoring Experiment) on ERS-2

Global Ozone Monitoring Experiment (GOME)

Idea: • simultaneous measurements from the UV to the near IR• good spectral resolution (0.2 – 0.4 nm)• use of DOAS to retrieve columns of several species (O3, NO2, OClO, BrO,

HCHO, SO2, H2O)• use of UV wavelengths to retrieve ozone profiles• global coverage

Launch: April 1995 on ERS-2 (sun synchronous)

GOME successor instruments:Instrument Satellite LaunchSCIAMACHY ENVISAT March 2002OMI EOS-Aura Spring 2004GOME-2 Metop-1 .. Metop-3 2006 – 2020

http://www.iup.physik.uni-bremen.de/gome/

GOME: tropospheric NO2 excess

• NOx plays a key role in the formation of photochemical ozone smog

• sources of NOx are both anthropogenic (combustion of fossil fuels, biomass burning) and natural (fires, soil emissions, lightning)

• NOx emissions are changing as result of

• changes in land use

• improvements in emission control

• economic development (e.g. China)

• GOME data provided the first global maps of tropospheric NO2

Data analysis:

1. cloud screening

2. DOAS retrieval of total slant columns

3. subtraction of clean Pacific sector to derive tropospheric slant columns

4. application of tropospheric airmass factor to compute tropospheric vertical column

UV/vis Measurements of Stratospheric Aerosols

Target Quantity: stratospheric aerosol concentrations

Measurement Quantity: backscattered radiation

Instrument type: solar occultation viewing UV/vis spectrometer

Instrument: SAGE-2 (Stratospheric Aerosol and Gas Experiment)

Stratospheric Aerosol and Gas Experiment (SAGE)

Measurement Geometry: solar occultation

Instrument: grating spectrometer with Si-detectors

Spectral coverage: 7 wavelengths between 385 – 1020 nm:

1020, 940, 600, 525, 453, 448 und 385 nm

Data analysis: onion peeling

Measurement targets: vertical profiles of O3, NO2, H2O and aerosol extinction (at 385, 453, 525 and 1020 nm)

Measurement range: stratosphere, at low stratospheric aerosol loading and outside the tropics also the upper troposphere

The SAGE programme:

SAM II 1978

SAGE I 1979-1981

SAGE II 1984 - today

SAGE III 2001 - today

280 – 1030 nm, 1-2 nm spectral resolutionCCD detector, lunar + solar occultation

http://www-sage3.larc.nasa.gov/

SAGE: Stratospheric Aerosols

• Stratospheric aerosols are dominated by volcanic input (H2SO4).

• Large eruptions inject ash and SO2 directly into the stratosphere.

• Transport towards poles within one year.

• Exponential decay over many years

1985: Nevado del Ruiz, Columbia

1990: Kelut, Indonesia

1991: Mt. Pinatubo

http://aerosols.larc.nasa.gov/optical_depth.html

Radio Occultation Measurements of Temperature Profiles

Target Quantity: temperature profiles

Measurement Quantity: excess phase of GPS signals

Instrument type: GPS occultation

Instrument: CHAMP (CHAllenging Minisatellite Payload)

CHAMP radio occultation

Principle:• GPS receiver observes

GPS satellite during occultation

• high accuracy time information provides excess phase

• this is related to the bending angle profile α

• which depends on refractive index n

• which is a function of p, T and humidity

http://www.copernicus.org/EGU/acp/acpd/4/7837/acpd-4-7837_p.pdf

+ good vertical resolution

+ large number of measurements

+ good sampling

- assumptions on 2 of the three variables necessary

- problems with critical layers

QBO Temperature Anomalies from CHAMP Radio Occultation

• downward propagation of temperature anomalies in the tropical stratosphere

• QBO (Quasi Biannual Oscillation) signal

• maximum amplitude of +/- 4.5 K

• impact on stratospheric ozone columns

http://www.copernicus.org/EGU/acp/acpd/4/7837/acpd-4-7837_p.pdf

Microwave Measurements of Wind Speed and Direction

Target Quantity: wind speed and direction

Measurement Quantity: reflected microwave intensity and polarisation

Instrument type: active microwave

Instrument: Synthetic Aperture Radar (SAR).

How to derive wind speed from Radar signals

Idea: Bragg-like resonance of cm-size ocean waves with Radar signals depends monotonically on surface wind speed

=> wind speed over oceans can be determined from scatterometer measurements if wind direction is known from model or other measurements

Relationship between radar backscatter and surface wind speed for C-band (5.3 Hz),vertical polarization at 45° off nadir angle.

Validation:

http://fermi.jhuapl.edu/sar/stormwatch/user_guide/bealguide_072_V3.pdf

Wind Speed from Radarsat SAR

Polar low of 05 Feb 1998 after application of wind algorithm,embedded in NOGAPS model wind field (arrows).

Polar low imaged by 430 km wide swath mode ofRadarsat SAR, before application of wind algorithm, 0602 GMT 05 Feb 1998.

http://fermi.jhuapl.edu/sar/stormwatch/user_guide/bealguide_072_V3.pdf

Passive Microwave Measurements of Sea Ice

Target Quantity: sea ice coverage and type

Measurement Quantity: reflected microwave intensity and polarisation

Instrument type: passive microwave radiometer

Instrument: AMSR-E (Advanced Microwave Scanning Radiometer - EOS )

12 channels and 6 frequencies ranging from 6.9 to 89.0 GHz

two polarisations

Sea Ice Maps from AMSR-E

Basic principle:• strong contrast in thermal

microwave emission between ice and open ocean

• assumption of linear relationship between brightness and ice cover

• parameters: sea ice concentration,surface ice temperature,snow depth on ice

• ice type by frequency dependence of emission

http://www.seaice.de/

IR Measurements of Sea Surface Temperature

Target Quantity: sea surface temperature

Measurement Quantity: emitted IR radiation

Instrument type: nadir broad band IR measurements

Instrument: AVHRR (Advanced Very High Resolution Radiometers)

Reminder: El Niño – La Niña

• reversal of Walker circulation• change of direction of Trade Winds• change of ocean upwelling• displacement of convection areas• link to Southern Oscillation

(difference of surface pressure between Tahiti and Darwin)

Sea Surface Anomaly during El Nino Event

• Sensor: AVHRR• Technique: broad band IR

measurements• Quantity: sea surface

temperature

•Sensor: TOPEX

•Technique: radar altimeter

•Quantity: height

Summary

• Remote Sensing of atmospheric and surface parameters from space relies on analysis of electromagnetic radiation emitted / scattered / reflected by the atmosphere and surface

• The target quantities interact with the radiation through absorption, emission, scattering, reflection or by indirectly changing the optical properties

• Remote Sensing measurements provide a large number of parameters for atmospheric physics and chemistry on a global scale and often over long time periods

• Remote Sensing measurements are indirect measurements and need thorough and continuous validation

• Spatial and temporal resolution of the measurements are limited and not always appropriate for detailed case studies

• Technological improvements and progress in data algorithms will further improve the usefulness of satellite measurements in the future

• Remote Sensing will always be only one of many data sources needed to understand the Earth System

Andreas Richter Institute of Environmental Physics University of Bremen tel. ++49 421 218 4474

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