Thermal Infrared Wavelengths Research Articles

Wide‐Field Imaging at Mid‐Infrared Wavelengths: Reconstruction of Chopped and Nodded Data

ABSTRACTGround‐based astronomical observations at thermal infrared wavelengths (λ≥2.4 μm) face the problem of extracting the weak astronomical signal from the large and rapidly variable background flux. The observing strategy most commonly used, the “chopping and nodding” differential technique, provides reliable representations of the target uniquely in the case of compact sources, while extended and complex sources can be easily distorted by their negative counterparts. A restoration method, designed to remove the negative values and to provide reliable representations of extended sources, has been proposed by us in two previous papers and validated on simulated images. In this paper we apply our algorithm to real images taken at the UKIRT telescope with the MPIA camera MAX. We show that the algorithm successfully removes the distortions due to the negative counterparts and, in addition, provides noise reduction. In several cases an enlargement of the field is obtained, in the sense that the restored larger image provides reliable information on the source structure outside the central field of view. The restorations may be affected by artifacts, whose origin can be predicted theoretically. We suggest and demonstrate computational and observational procedures for their reduction. Once combined with the proper observing strategies, our inversion method can provide a viable solution to the problem of deep imaging of extended sources with large ground‐based telescopes.

Modeling of Scattering and Absorption by Nonspherical Cirrus Ice Particles at Thermal Infrared Wavelengths

This paper examines a number of commonly used methods for the calculation of the scattering and absorption properties of nonspherical ice crystals at thermal infrared wavelengths. It is found that, for randomly oriented nonspherical particles, Mie theory using equivalent ice spheres tends to overestimate the absorption efficiency while the anomalous diffraction theory (ADT) and the geometric optics method (GOM) tend to underestimate it. The absorption efficiency is not sensitive to the particle shape when the size parameter is large. Herein a composite scheme is used that is valid for nonspherical particles with a wide range of size parameters. This scheme is a composite of Mie theory, GOM, and ADT to fit the single-scattering properties of hexagonal particles derived from the GOM for large size parameters and the finite-difference time domain technique for small size parameters. Applying this composite technique, errors in the broadband emissivity of cirrus clouds associated with conventional approaches are examined. It is shown that, when the projected area is preserved, Mie results overestimate the emissivity of cirrus clouds while, when the volume is preserved, Mie results underestimate the emissivity. Mie theory yields the best results when both projected area and volume are preserved (the relative errors are less than 10%). It is also shown that the ADT underestimates cirrus cloud emissivity. In some cases, the relative errors can be as large as 30%. The errors in the GOM are also significant and are largely a result of nonspherical particles with size parameters smaller than 40.

Potential vegetation indices for determining global forest cover

The potential of National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data acquired in middle (MIR) and thermal infrared (TIR) wavelengths for deriving global forest cover information was examined. As an exploratory step, these wavelengths were related to percentage cover of temperate coniferous forests in the Cascade Range of Oregon, U.S.A. These wavelengths individually were not strongly related to percentage forest cover. However, their inclusion within vegetation indices served to strengthen relations between remotely-sensed data and forest cover. One such index was the complex division index (C3/(C1*C2*C4*C5)), which was also seen to separate among the four forest successional stages present at this site. These findings have implications for the use of remotely-sensed data acquired in MIR and TIR wavelengths for deriving global forest cover information.

Thermal infrared lightcurves of the impact of comet Shoemaker‐Levy 9 fragment R

The impact of fragment R was observed at thermal infrared wavelengths of 7.85, 10.3 and 12.2 µm from the NASA/Infrared Telescope Facility on July 21 UT, using the MIRAC2 mid‐infrared array camera. Thermal emission at the three wavelengths was sampled sequentially using a 1.8% circular variable filter, with an average time interval of 17 seconds between observations at different wavelengths. Continuous imaging of Jupiter in this mode began at 5:08 UT and extended to 5:55 UT. We present calibrated lightcurves for the three wavelengths. Clear evidence for enhanced emission from the impact region first appears at 5:41 UT, with the peak in emission at all three wavelengths occurring ∼3.5 minutes later. The information content of the data is presented in terms of plots of the product of emissivity times angular size versus source temperature for each wavelength. Assuming that the peak in the lightcurves is due to rotation of the hot impact site into view from Earth, we estimate a diameter of ∼1900 km for the source emitting area at 5:44:30 UT and estimate a lower limit on the source temperature at this time of ∼1350 K. This lower limit drops to 800 K if the diameter of the emitting region was actually a factor of two larger.

Arctic sea ice concentrations from special sensor microwave imager and advanced very high resolution radiometer satellite data

Nearly coincident data from the special sensor microwave imager (SSM/I) and the advanced very high resolution radiometer (AVHRR) are used to compute and compare Arctic sea ice concentrations for different regions and times of the year. To help determine overall accuracies and to highlight sources of differences between passive microwave, optical wavelength, and thermal wavelength data, ice concentrations are estimated using two operational SSM/I ice concentration algorithms and with visible‐ and thermal‐infrared wavelength AVHRR data. All algorithms capture the seasonal patterns of ice growth and melt. The ranges of differences fall within the general levels of uncertainty expected for each method and are similar to previous accuracy estimates. The estimated ice concentrations are all highly correlated, with uniform biases, although differences between individual pairs of observations can be large. On average, the NASA Team algorithm yielded 5% higher ice concentrations than the Bootstrap algorithm, while during nonmelt periods the two SSM/I algorithms agree to within 0.5%. These seasonal differences are consistent with the ways that the 19‐GHz and 37‐GHz microwave channels are used in the algorithms. When compared to the AVHRR‐derived ice concentrations, the Team‐algorithm results are more similar on average in terms of correlation and mean differences. However, the Team algorithm underestimates concentrations relative to the AVHRR output by 6% during cold months and overestimates by 3% during summer. Little seasonal difference exists between the Bootstrap and AVHRR results, with a mean difference of about 5%. Although the mean differences are less between the SSM/I‐derived concentrations and concentrations estimated using AVHRR channel 1, the correlations appear substantially better between the SSM/I data and concentrations derived from AVHRR channel 4, particularly for the Team algorithm output.

Light scattering by nonspherical particles: Remote sensing and climatic implications

Calculations of the scattering and adsorption properties of ice crystals and aerosols, which are usually nonspherical, require specific methodologies. There is no unique theoretical solution for the scattering by nonspherical particles. Practically, all the numerical solutions for the scattering of nonspherical particles, including the exact wave equation approach, integral equation method, and discrete-dipole approximation, are applicable only to size parameters less than about 20. Thus, these methods are useful for the study of radiation problems involving nonspherical aerosols and small ice crystals in the thermal infrared wavelengths. The geometric optics approximation has been used to evaluate the scattering, absorption and polarization properties of hexagonal ice crystals whose sizes are much larger than the incident wavelength. This approximation is generally valid for hexagonal ice crystals with size parameters larger than about 30. From existing laboratory data and theoretical results, we illustrate that nonspherical particles absorb less and have a smaller asymmetry factor than the equal-projected area/volume spherical counterparts. In particular, we show that hexagonal ice crystals exhibit numerous halo and arc features that cannot be obtained from spherical particles; and that ice crystals scatter more light in the 60° to 140° scattering angle regions than the spherical counterparts. Satellite remote sensing of the optical depth and height of cirrus clouds using visible and IR channels must use appropriate phase functions for ice crystals. Use of an equivalent sphere model would lead to a significant overestimation and underestimation of the cirrus optical depth and height, respectively. Interpretation of the measurements for polarization reflected from sunlight involving cirrus clouds cannot be made without an appropriate ice crystal model. Large deviations exist for the polarization patterns between spheres and hexagonal ice crystals. Interpretation of lidar backscattering and depolarization signals must also utilize the scattering characteristics of hexagonal ice crystals. Equivalent spherical models substantially underestimate the broadband solar albedos of ice crystal clouds because of stronger forward scattering and larger absorption by spherical particles than hexagonal ice crystals. We illustrate that the net cloud radiative forcing at the top of the atmosphere involving most cirrus clouds is positive, implying that the IR greenhouse effect outweighs the solar albedo effect. If the radiative properties of equivalent spheres are used, a significant increase in cloud radiative forcing occurs. Using a one-dimensional cloud and climate model, we further demonstrate that there is sufficient model sensitivity, in terms of temperature increase, to the use of ice crystal models in radiation calculations.

Recent climatological trends in atmospheric aerosol backscatter derived from the Jet Propulsion Laboratory multiyear backscatter profile database

An update is provided on the Jet Propulsion Laboratory aerosol backscatter climatology database, with emphasis on the impact of the June 1991 eruption of Mt. Pinatubo. The data set is acquired at thermal infrared wavelengths with a range-gated coherent CO(2) lidar system, which has been in regular operation since 1984. A number of analyses have been carried out to assess long-term trends in the tropospheric and lower stratospheric aerosol backscatter, as observed from the lidar site at Pasadena, California.

Universal Multifractals: Theory and Observations for Rain and Clouds

The standard model of atmospheric motions divides the atmosphere into distinct two- and three-dimensional isotropic turbulent regimes separated by a dimensional transition, the “mesoscale gap.” It is argued that the “gap” is fictional and that the atmosphere is scaling but anisotropic at all scales. According to this alternative unified scaling model, the dynamics are governed by anisotropic (differentially stratified and rotating) cascade processes yielding highly variable multifractal fields. Just as Gaussian random variables are associated with (linear) sums of random variables, these (nonlinear) multiplicative processes are generically associated with (special) universal multifractals in which many of the details of the dynamics are irrelevant. Although an attempt is made to outline these arguments in a widely accessible form, they are not new to this paper; they provide its context and motivation. The principal purpose of this paper is to test these ideas empirically. This is done using Landsat, NOAA-9, and Meteosat cloud radiances at visible, near-infrared, and thermal infrared wavelengths with length scales spanning the range 166 m–4000 km, radar reflectivities of rain (in the horizontal, vertical, and time), and global daily rainfall accumulations. Spectral analysis, as well as the new double trace moment data-analysis technique, is applied. In each case, rather than the sharp dimensional transition predicted by the standard model, the scaling is found to be relatively well respected right through the mesoscale. The three fundamental universal multifractal exponents are then estimated and one can go on to outline how these exponents (with the help of appropriate space–time transformations) can be used to make dynamic multifractal models.

Technical note. A method for the correction of sea surface temperatures derived from satellite thermal infrared data in an area of sunglint

In calculating sea surface temperatures from thermal infrared data from satellites in an area of sunglint it is necessary to apply a correction to allow for reflected solar infrared radiation. A method for making this correction is proposed and it is based on using the well-known theory of Cox and Munk for thermal infrared wavelengths as well as for visible wavelengths. A simple formula enables the correction to be calculated on a pixel-by-pixel basis throughout the area of sunglint.

Combined use of visible, reflected infrared, and thermal infrared images for mapping Hawaiian lava flows : J Geophys Res V96, NB1, Jan 1991, P475–484

The weathering of Hawaiian basalts is accompanied by chemical and physical changes of the surfaces. These changes have been mapped using remote sensing data from the visible and reflected infrared and thermal infrared wavelength regions. They are related to the physical breakdown of surface chill coats, the development and erosion of silica coatings, the oxidation of mafic minerals, and the development of vegetation cover. These effects show systematic behavior with age and can be mapped using the image data and related to relative ages of pahoehoe and aa flows. The thermal data are sensitive to silica rind development and fine structure of the scene; the reflectance data show the degree of oxidation and differentiate vegetation from aa and cinders. Together, data from the two wavelength regions show more than either separately. The combined data potentially provide a powerful tool for mapping basalt flows in arid to semiarid volcanic environments.

Combined use of visible, reflected infrared, and thermal infrared images for mapping Hawaiian lava flows

The weathering of Hawaiian basalts is accompanied by chemical and physical changes of the surfaces. These changes have been mapped using remote sensing data from the visible and reflected infrared and thermal infrared wavelength regions. They are related to the physical breakdown of surface chill coats, the development and erosion of silica coatings, the oxidation of mafic minerals, and the development of vegetation cover. These effects show systematic behavior with age and can be mapped using the image data and related to relative ages of pahoehoe and aa flows. The thermal data are sensitive to silica rind development and fine structure of the scene; the reflectance data show the degree of oxidation and differentiate vegetation from aa and cinders. Together, data from the two wavelength regions show more than either separately. The combined data potentially provide a powerful tool for mapping basalt flows in arid to semiarid volcanic environments.

Altitude and seasonal characteristics of aerosol backscatter at thermal infrared wavelengths using lidar observations from coastal California

A calibrated CO2 lidar has been used to measure boundary layer aerosol backscatter and vertical profiles of tropospheric and lower stratospheric aerosol backscatter over a 1984–1987 time period from a site in Pasadena, California. The lidar data have been taken at two wavelengths, 9.25 and 10.6 μm. Data are presented which show altitude, seasonal, and trend characteristics of backscatter for various air mass histories. Results of trajectory studies indicate the influences of convective activity and other factors on the backscatter profiles.

Satellite Analyses of Antarctic Katabatic Wind Behavior

Abstract Prominent warm signatures of strong, negatively buoyant, katabatic airstreams are present at thermal infrared wavelengths as a result of intense vertical mixing and drift-snow transport within stable boundary layers. These tracers are used to illustrate several aspects of the behavior of katabatic winds in the Ross Sea sector of the Antarctic. The satellite features are compared with surface-based observations whenever possible. Converging surface-wind signatures upslope from Terra Nova Bay are shown to closely follow the observed time-averaged streamlines of drainage airflow. The satellite-observed core of the katabatic airstream descends to sea level via a direct route, but complex three-dimensional trajectories are manifested in marginal regions. Katabatic winds propagating horizontally for hundreds of kilometers over the southwestern Ross Sea do not exhibit the expected influence of the Coriolis force. Katabatic signatures are shown to be climatological features over the Ross Ice Shelf which ...

Advanced sensors fly on Landsat 6

New systems to sense ocean color and temperature and to acquire data in thermal infrared wavelengths will form part of a 6000‐pound (∼2700‐kg) payload on Landsat 6, due to be launched in early 1991. The new remote sensing satellite will carry the Sea Wide Field Sensor (Sea‐WiFS) and a thermal infrared (TIR) capability, in addition to the Enhanced Thematic Mapper now used on Landsats 4 and 5.The color and temperature data from Sea‐WiFS will provide information on currents, weather, and the global distribution of phytoplankton and fish that feed on them. TIR applications include measurement and evaluation of vegetation patterns, groundwater resources, snow cover, and crop yield.

High-Resolution Fourier Transform Spectroscopy with a VLT

The major advantages of the FTS technique are (1) multiplexing, (2) throughput, (3) instrumental profile, (4) stability of frequency calibration, and (5) spectrophotometry accuracy. The multiplex advantage is realized only if one is detector noise limited for the signal within an individual spectral-resolution element. At optical and thermal infrared wavelengths, this is only the case at high spectral resolution (≥ 50000) for modern detectors. By the time the VLT is operating one expects this to also be the case in the 1- to 2.5-micron region. At resolutions ≥ 50000 there are severe problems matching dispersive spectrographs to the VLT aperture, whereas existing FTS instruments already have adequate through-put to match to fields of a few arcsec with a VLT. When the other advantages are considered, the FTS is the instrument of choice for high-resolution (≥ 50000) spectroscopy of absorption features with a VLT. Foreseeable astrophysical applications include observations of interstellar and circumstellar features and of fully resolved profiles of photospheric and planetary lines.

Drought detection and quantification by reflectance and thermal responses

Drought can be detected by its effect on plant canopy development as indicated by measurements of biomass production, leaf area index (LAI), and ground cover in the reflective wavelengths (0.4–2.5 μm) and by canopy minus air temperature in the thermal infrared (8–14 μm) wavelengths. Both measures are quantitative, but quantification in terms of drought's economic impact requires that relationships between canopy ‘appearance’ as revealed by the measurements and effect on salable plant parts be established. The measurements themselves are becoming routine and their quantification in terms of drought's economic impact on grain sorghum and wheat grain yields have been demonstrated, but are not yet routine. Seven considerations in a system for drought quantification are discussed, and results of semi-operational applications to drought assessment that incorporate most of the seven considerations are described. Future drought assessment systems will likely combine physiologically based crop growth and yield models with soil characteristics, weather data, and Earth satellite spectral scanner observations.

Effect of viewing angle on the infrared brightness temperature of snow

Remote sensing of snowpack temperatures from satellites requires knowledge of the spectral emissivity of snow. A model for spectral emissivity is combined with the Planck function to calculate brightness temperature of snow in thermal infrared wavelengths for a range of grain sizes and viewing angles. Emissivity variations caused by density, grain shape, liquid water, and grain size are apparently unimportant, but emissivity varies with viewing angle to produce differences between thermodynamic temperature and brightness temperature as large as 3 K at wavelengths 12 to 14 μm, within the major atmospheric infrared window. This difference is also verified by experimental measurements. An equation to convert brightness temperatures to thermodynamic temperatures is presented, and this is also combined with a dual‐wavelength atmospheric correction method. The spectral emissivity model is also used to calculate an ‘all‐wave’ emissivity of snow: 0.985–0.990 for all grain sizes.

Aircraft measurement of sea surface temperature during the West Coast Experiment

During the West Coast Experiment in March 1977, a test was conducted to ascertain the effectiveness of using remote sensing techniques to estimate sea surface temperature (SST) from infrared (IR) emissions of the sea surface. Aircraft flights were made over three buoys moored in southern California coastal waters, and data was collected of sea surface emissions at thermal IR wavelengths ( 7.95-13.5 \mu m). SST obtained from the remote sensing measurements were compared with in situ SST measured with thermistors mounted on the buoys. The remotely determined SST were from 1.4-2.9\deg C lower than the in situ measurements. Several factors are discussed that could account for the differences.

Possible climatic and biological impact of nearby supernovae

OZONE plays a very important part in atmospheric radiative transfer. The absorption of the solar ultraviolet radiation by ozone is the dominant heating mechanism in the stratosphere. At thermal infrared wavelengths the main ozone contribution comes from the 9.6 µm band. Clark et al.1 were unclear, however, whether the supernovae removal of ozone from the atmosphere of the Earth would heat or cool the surface of the planet. There is evidence, reported here, which enables a more precise statement to be made of the effect upon the global Earth. The reduction in the concentration of ozone will cool the stratosphere, troposphere and surface layers of the Earth.

Separability of Agricultural Cover Types by Remote Sensing in the Visible and Infrared Wavelength Regions

The purpose of this study was to determine the statistical separability of multispectral measurements from agricultural cover types: com, soybeans, green forage (hay and pasture), and forest, in one to 12 spectral channels. Multispectral scanner data in 12 spectral channels in the wavelength range 0.4-11.7 μm, acquired on July 16, 1971 for three flight lines, were analyzed by applying automatic pattern recognition techniques. The same analysis was performed for data acquired on August 12, 1971 over the same three flight lines to investigate the effect of time on the statistical separability of agriculture cover types. In subsets of one to six spectral channels, the combination of wavelength regions (where V, N, M, and T denote the visible, near-infrared, middle-infrared, and thermal-infrared wavelength regions, respectively): V, V M, V N M, V N M T, V V N M T, V V N M M T, respectively, were found to be the best choices for getting good overall statistical separability of the agricultural cover types for the data acquired on July 16 as well as August 12. An effort was made to explain these results on the basis of the spectral properties of agricultural cover types. The overall statistical separability of the agricultural cover types was found to be greater for the data of August 12 than the data of July 16 (Table II).