Infrared Interferometers and Microwave Radiometers

1. Infrared Interferometers and Microwave Radiometers Dr. David D. Turner Space Science and Engineering Center University of Wisconsin - Madison dturner@ssec.wisc.edu AOS 340 28 April 2007

2. 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

3. Electromagnetic Spectrum More Opaque More Transparent Microwave Observations Infrared Observations

4. 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

5. Example: Upwelling IR Spectrum IASI covers most of this spectral region

6. Example: Upwelling vs. Downwelling

7. Primary Absorption Bands Water Vapor Water Vapor Clouds Aerosols CO2 O3 Trace gases (CFC, CH4, etc) absorb in various regions

8. 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

9. AERI Interferometer Assembly Front End Assembly IR Detector Dewar with Cooler Cold Finger Blackbodies Scene Mirror Assembly Forced Air Inlet Rain Sensor Sun Sensor ABB Stirling Cooler Compressor HBB Bomem Interferometer Front-end Closeout (thermal) Optics Bench Shock Mounts (4) Interferometer / AERI Electronics Interface Box Knuteson et al., JTECH, 2004

10. Calibration Targets (Blackbodies) are Key to Accurate Radiances Emissivity > 0.999

11. Apply a FFT to the interferogram to yield the spectrum 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)

12. Example: Raw AERI Spectra

13. Calibration of AERI Spectra

14. 20 Wavelength [µm] 3.3 318.5 Brightness Temperature [K] 317.5 500 Wavenumber [cm-1] 3000 Calibration Verification: 3-Body Test ARM Mobile Facility Black Forest, Germany 15 July 2007

15. Clear Sky Spectra 25 µm 15 µm 10 µm 7.1 µm

16. 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

17. Microwave Spectrum

18. Microwave Radiometers: Various Shapes, Sizes, and Capabilities

19. Basic Components RPG HATPRO Radiometer

20. Microwave Radiometer Blackbody

21. Sensitivity Water Vapor and Liquid Water

22. 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”

23. 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

24. 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

25. Clear Sky Spectra 25 µm 15 µm 10 µm 7.1 µm

26. AERI / LBLRTM Results Turner et al., JTECH, 2003

27. Using the AERI / LBLRTM Results to Look into the “Diurnal Issue” Turner et al., JTECH, 2003

28. 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?