Infrared Interferometers and Microwave Radiometers

Dr. David D. TurnerSpace Science and Engineering Center

University of Wisconsin - Madison

dturner@ssec.wisc.edu

AOS 34028 April 2007

Outline

• Infrared interferometers– Applications– Basic components– Calibration

• Microwave radiometers– Applications– Basic components– Calibration

• Example using both Microwave Radiometers and Infrared Interferometers to investigate accuracy of radiosonde moisture observations

Electromagnetic Spectrum

MicrowaveObservations

InfraredObservations

More Opaque

More Transparent

Infrared Interferometer Applications

• Clear sky radiative transfer– Spectroscopy (line strengths / widths)– Water vapor continuum – Other continua

• Atmospheric profiling (temperature and humidity)• Cloud properties

– Liquid, ice, and mixed-phase– Water path, particle size

• Aerosols• Trace gas (O3, CO, CH4, etc) retrievals• Sea surface temperature• Land surface emissivity• Satellite validation

Example: Upwelling IR Spectrum

IASI covers most of this spectral region

Example: Upwelling vs. Downwelling

Primary Absorption BandsWater Vapor

Water VaporO3CO2

Trace gases (CFC, CH4, etc) absorb in various regions

CloudsAerosols

Atmospheric Emitted Radiance Interferometer (AERI)

• Automated instrument measuring downwelling IR radiation from 3.3-19 µm at 0.5 cm-1 resolution

• Uses two well characterized blackbodies to achieve accuracy better than 1% of the ambient radiance

• Data used in a wide variety of research

• SSEC has built 13 AERIs for DOE and other universities

• Originally collected 3-min avg every 8 min, now 12-s avg every 30 s

AERI Interferometer AssemblyAERI Interferometer Assembly

BomemInterferometer

ABB

HBB

OpticsOpticsBenchBench

ShockShockMounts (4)Mounts (4)

Interferometer / AERIInterferometer / AERIElectronics Interface BoxElectronics Interface Box

IR DetectorIR DetectorDewar withDewar withCooler Cold FingerCooler Cold Finger

Stirling CoolerStirling CoolerCompressorCompressor

Front End AssemblyFront End AssemblyBlackbodiesBlackbodiesScene Mirror AssemblyScene Mirror AssemblyForced Air InletForced Air InletRain SensorRain SensorSun SensorSun Sensor

Front-endFront-endCloseoutCloseout(thermal)(thermal)

Knuteson et al., JTECH, 2004

Emissivity > 0.999

Calibration Targets (Blackbodies) are Key to Accurate Radiances

How an Interferometer Works

Move one mirror slowly back-and-forth to create an interference pattern (interferogram) at the detector

Record the inteferogram as a function of time (or mirror position)

Apply a FFT to the interferogram to yield the spectrum

Example: Raw AERI Spectra

Calibration of AERI Spectra

Calibration Verification: 3-Body Test

317.5

318.5

500 3000

Bri

gh

tnes

s T

emp

erat

ure

[K

]

Wavenumber [cm-1]

Wavelength [µm]20 3.3

ARM Mobile FacilityBlack Forest, Germany

15 July 2007

Clear Sky Spectra25 µm 7.1 µm10 µm15 µm

Microwave Radiometer Applications

• Clear sky radiative transfer– Spectroscopy (line widths)

• Precipitable water vapor (PWV)• “Calibrating” radiosonde moisture observations• Atmospheric profiling (temperature and humidity)• Cloud properties

– Liquid water path

• Satellite validation

Microwave Spectrum

Microwave Radiometers: Various Shapes, Sizes, and Capabilities

Basic Components

RPG HATPRO Radiometer

Microwave Radiometer Blackbody

Sensitivity Water Vapor and Liquid Water

Using AERI and MWR Data

• AERI and Microwave radiometer (MWR) offer complimentary ways to characterize the atmosphere

• To “compare” the two radiometers, we need to use detailed radiative transfer (RT) models and profiles of the atmospheric state (i.e., profiles of temperature, water vapor, ozone, etc)

• Of course, the atmospheric state measurements have to be “good”

Dual Sonde Launch ExamplesVaisala RS-80H

1996 WVIOP

1997 WVIOP

Calibration differences between radiosondes appear to act as height-independent scale factors in the lower troposphere!

Revercomb et al., BAMS, 2003

Radiance Closure Exercise

• Objective is to get agreement between observed radiance and computed radiance (within uncertainties)

• Three critical components:– Radiance observations– Model physics and spectroscopy– Input data for model

• In short, we are:– Using radiosonde profiles to drive the RT models– Using the MWR observations to provide a better estimate of

precipitable water vapor, and using this to ‘correct’ the radiosonde observation (single scale factor)

– Comparing the RT model output using the 2 different sets of input (regular sonde and MWR-scaled) with the AERI

Clear Sky Spectra25 µm 7.1 µm10 µm15 µm

AERI / LBLRTM Results

Turner et al., JTECH, 2003

Using the AERI / LBLRTM Results to Look into the “Diurnal Issue”

Turner et al., JTECH, 2003

Final Words

• I know I’ve presented a TON of material today• Range of applications that can be addressed with

spectrally resolved infrared data and microwave radiometer data

• The basic idea how:– An infrared interferometer works– Microwave radiometers work– Basic calibration concept

• The important diurnal bias in Vaisala radiosonde RH observations, revealed by ARM microwave radiometer and AERI observations

• Thank you for your attention. Any questions?