H. B. Howell

Radiometric calibration of IR Fourier transform spectrometers: solution to a problem with the High-Resolution Interferometer Sounder

A calibrated Fourier transform spectrometer, known as the High-Resolution Interferometer Sounder (HIS), has been flown on the NASA U-2 research aircraft to measure the infrared emission spectrum of the earth. The primary use-atmospheric temperature and humidity sounding-requires high radiometric precision and accuracy (of the order of 0.1 and 1 degrees C, respectively). To meet these requirements, the HIS instrument performs inflight radiometric calibration, using observations of hot and cold blackbody reference sources as the basis for two-point calibrations at each wavenumber. Initially, laboratory tests revealed a calibration problem with brightness temperature errors as large as 15 degrees C between 600 and 900 cm(-1). The symptom of the problem, which occurred in one of the three spectral bands of HIS, was a source-dependent phase response. Minor changes to the calibration equations completely eliminated the anomalous errors. The new analysis properly accounts for the situation in which the phase response for radiance from the instrument itself differs from that for radiance from an external source. The mechanism responsible for the dual phase response of the HIS instrument is identified as emission from the interferometer beam splitter.

Atmospheric Emitted Radiance Interferometer. Part I: Instrument Design

Abstract A ground-based Fourier transform spectrometer has been developed to measure the atmospheric downwelling infrared radiance spectrum at the earths surface with high absolute accuracy. The Atmospheric Emitted Radiance Interferometer (AERI) instrument was designed and fabricated by the University of Wisconsin Space Science and Engineering Center (UW-SSEC) for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program. This paper emphasizes the key features of the UW-SSEC instrument design that contribute to meeting the AERI instrument requirements for the ARM Program. These features include a highly accurate radiometric calibration system, an instrument controller that provides continuous and autonomous operation, an extensive data acquisition system for monitoring calibration temperatures and instrument health, and a real-time data processing system. In particular, focus is placed on design issues crucial to meeting the ARM requirements for radiometric calibration, spectral cali...

Near-Continuous Profiling of Temperature, Moisture, and Atmospheric Stability Using the Atmospheric Emitted Radiance Interferometer (AERI)

Abstract The Department of Energy Atmospheric Radiation Measurement Program (ARM) has funded the development and installation of five ground-based atmospheric emitted radiance interferometer (AERI) systems at the Southern Great Plains (SGP) site. The purpose of this paper is to provide an overview of the AERI instrument, improvement of the AERI temperature and moisture retrieval technique, new profiling utility, and validation of high-temporal-resolution AERI-derived stability indices important for convective nowcasting. AERI systems have been built at the University of Wisconsin—Madison, Madison, Wisconsin, and deployed in the Oklahoma–Kansas area collocated with National Oceanic and Atmospheric Administration 404-MHz wind profilers at Lamont, Vici, Purcell, and Morris, Oklahoma, and Hillsboro, Kansas. The AERI systems produce absolutely calibrated atmospheric infrared emitted radiances at one-wavenumber resolution from 3 to 20 μm at less than 10-min temporal resolution. The instruments are robust, are a...

Observations of the Infrared Radiative Properties of the Ocean—Implications for the Measurement of Sea Surface Temperature via Satellite Remote Sensing

Abstract The Atmospheric Emitted Radiance Interferometer (AERI) was used to measure the infrared radiative properties and the temperature of the Gulf of Mexico during a 5-day oceanographic cruise in January 1995. The ocean skin temperature was measured with an accuracy believed to be better than 0.1°C. The surface reflectivity/emissivity was determined as a function of view angle and sea state. The radiative properties are in good theoretical consistency with in situ measurements of ocean bulk temperature and the meteorological observations made from the oceanographic vessel. The AERI and in situ measurements provide a strong basis for accurate global specifications of sea surface temperature and ocean heat flux from satellites and ships.

Downwelling spectral radiance observations at the SHEBA ice station: Water vapor continuum measurements from 17 to 26μm

Earth loses energy to space in the form of longwave (or infrared) radiation. Much of this energy is radiated through the transparent portion of the water vapor rotational band from 17 to 33 μm (300 to 600 cm−1). Very few measurements have been made in this spectral region to characterize how water vapor absorbs and emits longwave radiation. An Atmospheric Emitted Radiance Interferometer (AERI) with extended longwave spectral coverage has been deployed at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station 300 miles north of the Alaskan coast to measure downwelling radiances at wavelengths of 3 to 26 μm (380 to 3000 cm−1). The spectral and radiometric performance of the instrument, installation at the ice station, and initial observations are shown. Comparisons to line-by-line radiative transfer calculations for selected clear-sky cases are presented, and air-broadened water vapor continuum absorption coefficients are determined in the wing of the pure rotational band from 17 to 26 μm (380 to 600 cm−1). Comparisons of the coefficients with the widely used Clough Kneizys Davies (CKD) water vapor continuum model suggest empirical modifications to this model are necessary. Comparisons to laboratory measurements of Burch et al. [1974] made at room temperature suggests little or no temperature dependence of the continuum from 400 to 550 cm−1. Implications of these modifications on top-of-atmosphere and surface fluxes, as well as atmospheric cooling rates, are discussed.

Nimbus-6 earth radiation budget experiment.

This paper describes the Nimbus-6 earth radiation budget experiment including its prelaunch calibration and in-flight performance. A preliminary assessment of the data shows the ERB measurement of the solar constant to be 1392 W/m(2) which is 1.6% higher than the expected value of 1370 W/m(2). Both values are traceable to the cavity radiometer scale. There is a disagreement between the fixed wide-angle and scanning narrow-angle measurements of planetary outgoing longwave radiation flux. Since the scanning channels are calibrated in-flight and show good agreement with previous observations of the Nimbus-3 satellite, the discrepancy is believed to be due to erroneous wide-angle flux estimates. The erroneous estimates may be caused by the misinterpretation of the transfer function for the wide-angle-earth-flux sensing thermopile detectors when viewing the earth which, unlike the prelaunch calibration source, does not fill the field of view of the detector and is not an isotropic radiation source. A field of view factor for the wide-angle channels is determined using an in-flight calibration procedure using the night-time scanning channel longwave radiation flux measurements as the absolute standard. The planetary global albedoes, longwave radiation fluxes, and net radiation are about 30%, 240 W/m(2), and -4 W/m(2) for the months of July and August 1975, which is in good agreement with previous Nimbus-3 estimates.

GHIS—The GOES High-Resolution Interferometer Sounder

Abstract A high spectral resolution interferometer sounder (GHIS) has been designed for flight on future geostationary meteorological satellites. It incorporates the measurement principles of an aircraft prototype instrument, which has demonstrated the capability to observe the earth-emitted radiance spectrum with high accuracy. The aircraft results indicate that the theoretical expectation of 1°C temperature and 2°–3°C dewpoint retrieval accuracy will be achieved. The vertical resolution of the water vapor profile appears good enough to enable moisture tracking in numerous vertical layers thereby providing wind profile information as well as thermodynamic profiles of temperature and water vapor.

The First 18 Months of Planetary Radiation Budget Measurements from the Nimbus 6 ERB Experiment

Abstract The Nimbus 6 satellite Earth Radiation Budget (ERB) experiment has continuously monitored the solar radiation input and the reflected shortwave and emitted longwave radiation exitance from the earth-atmosphere system since July 1975. In this paper, the planetary radiation budget parameters observed during the first eighteen months in orbit (July 1975–December 1976) are presented. The results show that the annual mean planetary albedo and longwave radiation flux are 31% and 234 W m−2> (radiative equilibrium temperature of 254 K), respectively. The earth atmosphere system is observed to be in complete radiation balance over a one-year period to within the experimental error of observation. There is an annual cycle of the mean monthly planetary net radiation which is due predominantly to the annual cycle of incoming solar radiation caused by the time variation of earth-sun distance and the suns declination. Monthly variations in outgoing longwave radiation due to variation in global cloudiness and ...

Thermodynamic product retrieval methodology and validation for NAST-I

The National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed (NAST) consists of two passive collocated cross-track scanning instruments, an infrared interferometer (NAST-I) and a microwave radiometer (NAST-M), that fly onboard high-altitude aircraft such as the NASA ER-2 at an altitude near 20 km. NAST-I provides relatively high spectral resolution (0.25-cm(-1)) measurements in the 645-2700-cm(-1) spectral region with moderate spatial resolution (a linear resolution equal to 13% of the aircraft altitude at nadir) cross-track scanning. We report the methodology for retrieval of atmospheric temperature and composition profiles from NAST-I radiance spectra. The profiles were determined by use of a statistical eigenvector regression algorithm and improved, as needed, by use of a nonlinear physical retrieval algorithm. Several field campaigns conducted under varied meteorological conditions have provided the data needed to verify the accuracy of the spectral radiance, the retrieval algorithm, and the scanning capabilities of this instrumentation. Retrieval examples are presented to demonstrate the ability to reveal fine-scale horizontal features with relatively high vertical resolution.

The NPOESS Airborne Sounding Testbed Interferometer—Remotely Sensed Surface and Atmospheric Conditions during CLAMS

During the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS), the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder TestbedInterferometer (NAST-I), flying aboard the high-altitude Proteus aircraft, observed the spatial distribution of infrared radiance across the 650–2700 cm 1 (3.7–15.4 m) spectral region with a spectral resolution of 0.25 cm 1 . NAST-I scans cross track with a moderate spatial resolution (a linear ground resolution equal to 13% of the aircraft altitude at nadir). The broad spectral coverage and high spectral resolution of this instrument provides abundant information about the surface and three-dimensional state of the atmosphere. In this paper, the NAST-I measurements and geophysical product retrieval methodology employed for CLAMS are described. Example results of surface properties and atmospheric temperature, water vapor, ozone, and carbon monoxide distributions are provided. The CLAMS NAST-I geophysical dataset is available for use by the scientific community.