Martin G. Mlynczak

First light from the Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument

[1]xa0We present first light spectra that were measured by the newly-developed Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument during a high-altitude balloon flight from Ft. Sumner, NM on 7 June 2005. FIRST is a Fourier Transform Spectrometer designed to measure accurately the far-infrared (15 to 100 μm; 650 to 100 wavenumbers, cm−1) emission spectrum of the Earth and its atmosphere. The flight data successfully demonstrated the FIRST instruments ability to observe the entire energetically significant infrared emission spectrum (50 to 2000 cm−1) at high spectral and spatial resolution on a single focal plane in an instrument with one broad spectral bandpass beamsplitter. Comparisons with radiative transfer calculations demonstrate that FIRST accurately observes the very fine spectral structure in the far-infrared. Comparisons also show excellent agreement between the atmospheric window radiance measured by FIRST and by instruments on the NASA Aqua satellite that overflew the FIRST flight. FIRST opens a new window on the spectrum that can be used for studying atmospheric radiation and climate, cirrus clouds, and water vapor in the upper troposphere.

Spectral signature of ice clouds in the far-infrared region: Single-scattering calculations and radiative sensitivity study

(extinction efficiency, absorption efficiency, and the asymmetry factor of the scattering phase function) are calculated for small particles using circular cylinders and for large crystals using hexagonal columns. The scattering properties are computed for particle sizes over a size range from 1 to 10,000 mm in maximum dimension from a combination of the T-matrix method, the Lorenz-Mie theory, and an improved geometric optics method. Bulk scattering properties are derived subsequently for 30 particle size distributions, with effective particle sizes ranging from 15 to 150 mm, obtained from various field campaigns for midlatitude and tropical cirrus clouds. Furthermore, a parameterization of the bulk scattering properties is developed. The radiative properties of ice clouds and the clear-sky optical thickness computed from the line-by-line method are input to a radiative transfer model to simulate the upwelling spectral radiance in the far-IR spectral region at the research aircraft height (20 km). On the basis of the simulations, we investigate the sensitivity of far-IR spectra to ice cloud optical thickness and effective particle size. The brightness temperature difference (BTD) between 250 and 559.5 cm � 1 is shown to be sensitive to optical thickness for optically thin clouds (visible optical thickness t 8), the BTD between 250 and 410.2 cm � 1 is shown to be sensitive to the effective particle size up to a limit of 100 mm. INDEX TERMS: 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 0649 Electromagnetics: Optics; KEYWORDS: far-infrared, cirrus cloud, ice crystal

Far-infrared: a frontier in remote sensing of Earth's climate and energy balance

The radiative balance of the troposphere, and hence climate, is influenced strongly by radiative cooling associated with emission of infrared radiation by water vapor, particularly at far-infrared (far-IR) wavelengths greater than 15 micrometers and extending out beyond 50micrometers . Water vapor absorption and emission is principally due to the pure rotation band, which includes both line and continuum absorption. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are well-recognized as major uncertainties in understanding and predicting future climate. Up to half of the outgoing longwave radiation (OLR) from the Earth occurs beyond 15.4 micrometers (650 cm-1_ depending on atmospheric and surface conditions. Cirrus clouds also modulate the outgoing longwave radiation in the far-IR. However, despite this fundamental importance, far-IR emission (spectra of band- integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 micrometers . In this talk we will review the role of the far-IR radiation in climate and will discuss the scientific and technical requirements for far-IR measurements of the Earths atmosphere.

Far-infrared spectroscopy of the troposphere (FIRST): sensor development and performance drivers

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principle absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR, and up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the mid-infrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA is now developing the technology required to make regular far-IR measurements of the Earth’s atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) is being developed for NASA’s Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with radiometric calibration techniques in the spectral range from 10 to 100 μm (1000 to 100 cm-1). FIRST will incorporate a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. The FIRST performance goal is a NEΔT of 0.2 K from 10 to 100 μm.

Far-IR Measurements at Cerro Toco, Chile: FIRST, REFIR, and AERI

In mid-2009, the Radiative Heating in the Underexplored Bands Campaign II (RHUBC-II) was conducted from Cerro Toco, Chile, a high, dry, remote mountain plateau, 23°S , 67.8°W at 5.4km, in the Atacama Desert of Northern Chile. From this site, dominant IR water vapor absorption bands and continuum, saturated when viewed from the surface at lower altitudes, or in less dry locales, were investigated in detail, elucidating infrared (IR) absorption and emission in the atmosphere. Three Fourier Transform InfraRed (FTIR) instruments were at the site, the Far-Infrared Spectroscopy of the Troposphere (FIRST), the Radiation Explorer in the Far Infrared (REFIR), and the Atmospheric Emitted Radiance Interferometer (AERI). In a side-by-side comparison, these measured atmospheric downwelling radiation, with overlapping spectral coverage from 5 to 100μm (2000 to 100cm-1), and instrument spectral resolutions from 0.5 to 0.643cm-1, unapodized. In addition to the FTIR and other ground-based IR and microwave instrumentation, pressure/temperature/relative humidity measuring sondes, for atmospheric profiles to 18km, were launched from the site several times a day. The derived water vapor profiles, determined at times matching the FTIR measurement times, were used to model atmospheric radiative transfer. Comparison of instrument data, all at the same spectral resolution, and model calculations, are presented along with a technique for determining adjustments to line-by-line calculation continuum models. This was a major objective of the campaign.

Far-infrared Spectroscopy of the Troposphere (FIRST): sensor calibration performance

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principal absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR. Up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the midinfrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA has now developed the sensor required to make regular far-IR measurements of the Earths atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) was developed for NASAs Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. The FIRST payload will also be proposed for science flights in support of validation of the various experiments on the Earth Observing System (EOS). We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with its radiometric calibration in the spectral range from 10 to 100 µm (1000 to 100 cm-1). FIRST incorporates a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. We also discuss the actual performance of the FIRST instrument relative to its performance goal of a NE(delta)T of 0.2 K from 10 to 100 μm.

Absolute radiance re-calibration of FIRST

The FIRST (Far-InfraRed Spectroscopy of the Troposphere) instrument is a 10 to 100 micron spectrometer with 0.64 micron resolution designed to measure the complete mid and far-infrared radiance of the Earths Atmosphere. FIRST has been successfully used to obtain high-quality atmospheric radiance data from the ground and from a high-altitude balloon. A Fourier transform interferometer is used to provide the spectral resolution and two on-board blackbodies are used for calibration. This paper discusses the recent re-calibration of FIRST at Space Dynamics Laboratory for absolute radiance accuracy. The calibration used the LWRICS (Long Wave Infrared Calibration Source) blackbody, which NIST testing shows to be accurate to the ~100 mK level in brightness temperature. There are several challenges to calibrating FIRST, including the large dynamic range, out of phase light, and drift in the interferogram phase. The accuracy goal for FIRST was 0.2 K over most of the 10 to 100 micron range, and results show FIRST meets this goal for a range of target temperatures.