AERI – Atmospheric Emitted Radiance Interferometer

Overview

The Atmospheric Emitted Radiance Interferometer, or AERI™, is a ruggedized automated sounding spectroradiometer system based on more than 20 years of field experience. Now in its 4^th generation, AERI instruments incorporate the newest generation of ABB’s FT-IR spectroradiometer (the field proven MR-300 series), University of Wisconsin blackbody cavity, latest COTS signal conditioning units, and new detector/Stirling cooler (ITAR-free) with extended lifetime.

It measures absolute down-welling atmospheric emitted infrared radiance with high accuracy. This remote sensing tool can work year-round during both day and night (except during precipitation), continuously providing temperature and humidity profiles unattended every 8 minutes.

AERI technology development through successive design generations is the result of 20 years of heritage at ABB

Gereral characteristics

• The AERI instrument generate radiance spectrum with high accuracy in the following spectral ranges:
  • 550 – 3000 cm⁻¹ (3.3 – 18.2 µm) for standard-range AERI
  • 425 – 3000 cm⁻¹ (3.3 – 23.5 µm) for extended-range version (useful for the dry Arctic air studies)

• It is based on a moderate resolution 2-port Michelson-type Fourier transform interferometer measuring downwelling radiation
  • Apodized spectral resolution of 1.0 cm⁻¹ (OPD = ±1 cm)
  • Better than 0.8 cm⁻¹, unapodized

• Channel wavenumber knowledge: better than 0.01 cm ⁻¹
  • Channel wavenumber stability: < 0.5 ppm

• The AERI produces calibrated absolute infrared spectral radiance of the sky directly above the instrument
   

Ruggedized design for outdoor application

• Input and calibration enclosure: −70° to +40°C
• Detector and control system: −30° to +40°C
• Marine version available (M-AERI)

• The AERI Standard measurement cycle includes a sky radiance measurement (3 min average) followed by measurements of the two internal blackbodies generating one accurately calibrated spectrum every 8 minutes


• Radiometric calibration absolute accuracy is tested with a controlled 3^rd blackbody at 45°C and an ice body at 0°C , both traceable to NIST standards.

• The knowledge of the AERI spectral calibration is better than 0.01 cm⁻¹ (3σ) in the range 500 – 3000 cm⁻¹
• Accuracy is suitable for long-term trending
• Accuracy is suitable as standard for other system validations, such as sounding satellites

• Blackbody characterization
• Temperature knowledge ±0.1°C (of absolute temperature)
• Emissivity knowledge < ±0.1%
• Temperature stability < 0.05°C over viewing period (≈120 s)

• Non-linearity knowledge < 0.1%
• Polarization knowledge < 0.1%

In the alternative Rapid sampling acquisition mode, the measurement rate can be increased to 125 calibrated sky spectra per hour, with reference blackbody measurements occurring only every 5 minutes

• Rapid sampling mode default measurement sequence:
  • 12 s observation time for each sky measurement
  • 24 s observation time for each BB calibration measurement
  • Measurement sequence = 8 sky measurements per BB

• This mode is especially useful for cloud study

The thermal emitted radiance from atmospheric measurements is primarily dependent on temperature and water vapor concentration, allowing profiles of the planetary boundary layer to be derived from the acquired calibrated spectra. The calculation is performed by sophisticated retrieval algorithms of the AERIPROF software.

Comparison of radiosondes with the nearest GOES retrievals and the final AERIPROF physical retrievals.

Atmospheric sounding performances

The sounding performance of the AERI was tested in a tempered climate zone. The following parameters are based on the AERI sounding performance test results:

• Vertical resolution 100 m at the surface; gradually increase to 250 m at 3 km altitude
• Limit of monitoring altitude 3 km or cloud ceiling
• 5% absolute water vapor accuracy
• 1 K temperature accuracy

The accuracy of profiles can be improved significantly by the synergetic effects in a multi-sensor network

Complement to radiosondes

• AERI networks can be deployed at automated weather stations that do not have balloon-launching capability
• Operation cost is advantageous compared with a series of radiosonde stations

• Placed at strategic locations, such as along the Gulf of Mexico, a network of 20 – 30 AERI systems could provide critical data to enhance accuracy and length of continent wide forecasts
• Due to their true absolute radiometric calibration and proven stability, AERI observations are particularly well suited for long-term observations
• Can be used for climate study and trend analyses

• Can be operated from fixed, mobile, or sea-based stations
• Mobile stations can be moved from site to site to study specific phenomena (e.g. El Niño, “Tornado Alley”)

AERI instrument family

AERI and M-AERI configurations as well as all know-how related to blackbody design, characterization and calibration techniques, instrument configuration, Field of View (FOV) matching techniques, Stirling cooler integration techniques are designed for unattended operations in hostile ambient conditions.

The AERI system is composed of two main sections:

• The front-end section, comprising the input and calibration module (including the blackbody sources and input scene selection mirror assembly).
• The back-end section, housing the FT-IR spectroradiometer, control electronics, and signal conditioning modules.

The first section is not stabilized in temperature and it is free running at exterior conditions. It is covered by an enclosure that protects against precipitation (rain or snow).

Physical characteristics

Like the terrestrial AEARI, M-AERIs proved to be robust, accurate, self-calibrating, seagoing FT-IR spectroradiometers to measure the emission spectra from the sea surface and marine atmosphere

• Addition of a second automated side view-port motorized hatch with precipitation sensor on the front-end enclosure
• Input scene selection module blackbody sources relocated to allow measurement at zenith, nadir, and slanted angles
• Front-end enclosure and scene selection module offer reversible installation configuration
• Use of marine-suitable parts to sustain exposition to marine corrosive salted air
  • Connectors, cables, TEC, seals, coatings, etc.