Terms Of Brightness Temperature Research Articles

Thermal emission in the successive orders of scattering (SOS) radiative transfer approach

The Successive Orders of Scattering (SOS) approach [1] is one of the well known methods for solving the Radiative Transfer (RT) problem. Its efficiency in terms of speed and accuracy of computation was already demonstrated for scattering and absorbing atmospheres in Solar spectrum. Although there are no principle limitations to account for the emission processes, the application of the SOS method for atmospheres with thermal emission is not widely used yet. In this paper we present a SOS-based RT approach accounting for the full source function, which enables its application from the UV (UltraViolet) to the TIR (Thermal InfraRed) parts of the electromagnetic spectrum. The atmospheric vertical discretization in this extended SOS scheme is a key point in order to properly retain the scattering and emission processes. An analysis of different methodologies to perform this vertical discretization is presented. The numerical implementation has been included in GRASP (Generalized retrieval of Atmosphere and Surface Properties) RT code [2]. In comparison with the widely used code DISORT (DIScrete-ORdinatemethod for Radiative Transfer) [3], the developed SOS scheme achieves a mean accuracy of radiance calculation of −0.005 K (−0.003%) expressed in terms of brightness temperature. Under the same configuration and vertically inhomogeneous atmospheric conditions, GRASP SOS RT is approximately eight times faster than DISORT. The analysis of the sensitivity of GRASP TIR SOS scheme to the number of layers and the effect of polarization are also investigated in the paper.

Exploiting Tropospheric Measurements From Sun-Tracking Radiometer for Radiopropagation Models at Centimeter and Millimeter Wave

The objective of this paper is to characterize the atmospheric radiometeorological parameters (optical thickness and brightness temperature) at centimeter and millimeter waves. To this aim, we have exploited two radiative transfer models (RTMs): a three-dimensional model (3D-RTM), driven by numerical weather forecasts, and a stochastic one-dimensional (1-D) model (1D-RTM), fed by a synthetic clouds dataset. We have compared the radiative transfer simulations with measurements from two ground-based microwave radiometers: a profiler and a Sun-tracking radiometer. The peculiarity of the latter is the capacity of simultaneously measuring atmospheric optical thickness and brightness temperature in all-weather conditions and at variable elevation angles. The comparisons between simulations and measurements provide satisfactory results that assess the reliability of the 3D-RTM with some biases, in terms of brightness temperature, that should be investigated. 3D-RTM turns out to be able to successfully reproduce correlations between brightness temperature and optical thickness and correlations among the different frequency channels. This confirms the potential of the combined use of weather forecast models and physically-based RTMs. On the other hand, 1D-RTM reveals to be able to reproduce frequency-channel correlation trends, but additional climatological set-up are needed to make the model exploitable for the computation of statistics of atmospheric optical thickness. These comparisons also highlight some possible calibration errors in the Sun-tracking radiometer that must be fixed. Finally, we have exploited measurements from the Sun-tracking radiometer to develop a model to retrieve the probability of atmospheric optical thickness conditioned to a given elevation angle.

Intercomparison Between VIIRS and CrIS by Taking Into Account the CrIS Subpixel Cloudiness and Viewing Geometry

AbstractThe Cross‐track Infrared Sounder (CrIS) onboard the Suomi National Polar‐orbiting Partnership (SNPP) has high radiometric accuracy, which can be used to intercompare and understand the radiometric performance of the thermal emissive bands of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the same platform. Previous studies usually selected uniform scenes to avoid potential uncertainties due to geolocation errors. This study focuses on all scenes (including nonuniform scenes), as well as CrIS subpixel cloudiness, local zenith angle and surface type, and their impacts on VIIRS/CrIS intercomparisons. A geolocation error correction scheme is applied to mitigate the geolocation mismatch between CrIS and VIIRS. Intercomparison results of three days from 19 to 21 September 2016, a quarterly warm up/cool down calibration period, show overall good agreement in terms of radiance biases (less than 0.2 K in terms of brightness temperature) between VIIRS and CrIS, for scene temperatures between 220 and 300 K; however, larger biases are evident outside of this range. The three factors—CrIS subpixel cloudiness, local zenith angle, and surface type—show weak impacts on VIIRS radiance biases. Both clear‐sky and overcast conditions have slightly different radiance biases from other partially cloudy conditions because of more contributions from colder or warmer scenes. The time series show the impact of warm up/cool down and the effectiveness of the correction algorithm for reprocessing VIIRS M15 and M16 data sets. The methods and tools can be applied to monitor and validate other infrared imager/sounder duos onboard the same platform.

On the Correlation Between GNSS-R Reflectivity and L-Band Microwave Radiometry

© 2008-2012 IEEE. This work compares microwave radiometry and global navigation satellite systems-reflectometry (GNSS-R) observations using data gathered from airborne flights conducted for three different soil moisture conditions. Two different regions are analyzed, a crops region and a grassland region. For the crops region, the correlation with the I/2 (first Stokes parameter divided by two) was between 0.74 and 0.8 for large incidence angle reflectivity data (30°-50°), while it was between 0.51 and 0.61 for the grassland region and the same incidence angle conditions. For the crops region, the correlation with the I/2 was between 0.64 and 0.69 for lower incidence angle reflectivity data ( > 30°), while it was between 0.41 and 0.6 for the grassland region. This indicates that for large incidence angles the coherent scattering mechanism is dominant, while the lower incidence angles are more affected by incoherent scattering. Also a relationship between the reflectivity and the polarization index (PI) is observed. The PI has been used to remove surface roughness effects, but due to its dependence on the incidence angle only the large incidence angle observations were useful. The difference in ground resolution between microwave radiometry and GNSS-R and their strong correlation suggests that they might be combined to improve the spatial resolution of microwave radiometry measurements in terms of brightness temperature and consequently soil moisture retrievals.

First evaluation of the simultaneous SMOS and ELBARA-II observations in the Mediterranean region

The SMOS (Soil Moisture and Ocean Salinity) mission was launched on November 2, 2009. Over the land surfaces, simultaneous retrievals of surface soil moisture (SM) and vegetation characteristics made from the multi-angular and dual polarization SMOS observations are now available from Level-2 (L2) products delivered by the European Space Agency (ESA). Therefore, first analyses evaluating the SMOS observations in terms of Brightness Temperatures (TB) and L2 products (SM and vegetation optical depth TAU) can be carried out over several calibration/validation (cal/val) sites selected by ESA over all continents. This study is based on SMOS observations and in situ measurements carried out in 2010 over one of the main SMOS cal/val sites in Europe: the VAS (Valencia Anchor Station) site in the region of Utiel–Requena, close to Valencia, Spain. The main vegetation types in the region are vineyards, orchards and natural Mediterranean vegetation. The SMOS observations were analyzed in conjunction with those carried out by the L-band ELBARA-II radiometer over a vineyard which is considered as representative of the main land use of the VAS site. Time series of TB and retrievals of SM and TAU based on both the SMOS (L2 products) and the ELBARA-II observations were compared and evaluated against in situ measurements. A good agreement was found between the time variations in TB and in the retrieved SM values computed over the site from the SMOS and ELBARA-II observations (the determination coefficient R2 was >0.88 for the TB values and R2>0.64 for the retrieved SM values). However, it was found that the SMOS L2 SM products underestimated the SM values retrieved from ELBARA-II by ~0.2m3/m3. It is likely this offset can be partially explained by differences between the observed scenes: while the ELBARA-II footprint includes a single vineyard, the heterogeneous SMOS footprint includes not only a large number of vineyards but also a natural Mediterranean vegetation with persistent leaves overlaying rocky soils. The time variations in TAU retrieved from the ELBARA-II observations were found to be closely related to those of the NDVI (Normalized Difference Vegetation Index) vegetation index obtained from MODIS (Moderate Resolution Imaging Spectroradiometer) (R2=0.61) and revealed the vegetation cycle over the year. Conversely, the time variations in the SMOS Level-2 TAU product did not reveal any trends in relation to the vegetation development over the site.

The simulated sea ice thermal microwave emission at window and sounding frequencies

Although the sea ice microwave emissivity is well defined in terms of brightness temperature and the effective temperature it is difficult to derive in practice and its link to physical processes in the snow and sea ice cover is not understood in detail. Future applications of assessing the sea ice microwave emission potentially include atmospheric sounding over sea ice for channels peaking near the surface. Here the microwave emission processes from sea ice are simulated using the combination of a one-dimensional thermodynamic snow and sea ice model and a microwave emission model. The emission model is a sea ice version of the Microwave Emission Model for Layered Snow-packs (MEMLS). It is demonstrated that the simulated seasonal variability of the emissivity, the spectral gradient and the polarization are comparable to satellite measurements. The spectral gradient ratio defined as the difference over the sum of the 18 and 36 GHz at vertical polarization is related to the emissivity at the atmospheric temperature sounding channels at around 50 GHz. Further the brightness temperature at neighbouring channels is highly correlated. However, the effective temperature at lower frequencies (18, 23, 36 and 50 GHz) is poorly correlated with the surface or air temperature.

The simulated sea ice thermal microwave emission at window and sounding frequencies

Although the sea ice microwave emissivity is well defined in terms of brightness temperature and the effective temperature it is difficult to derive in practice and its link to physical processes in the snow and sea ice cover is not understood in detail. Future applications of assessing the sea ice microwave emission potentially include atmospheric sounding over sea ice for channels peaking near the surface. Here the microwave emission processes from sea ice are simulated using the combination of a one-dimensional thermodynamic snow and sea ice model and a microwave emission model. The emission model is a sea ice version of the Microwave Emission Model for Layered Snow-packs (MEMLS). It is demonstrated that the simulated seasonal variability of the emissivity, the spectral gradient and the polarization are comparable to satellite measurements. The spectral gradient ratio defined as the difference over the sum of the 18 and 36 GHz at vertical polarization is related to the emissivity at the atmospheric temperature sounding channels at around 50 GHz. Further the brightness temperature at neighbouring channels is highly correlated. However, the effective temperature at lower frequencies (18, 23, 36 and 50 GHz) is poorly correlated with the surface or air temperature.

Approximations of the Planck Function for Models and Measurements Into the Submillimeter Range

A brightness temperature is defined as a linear function of the Planck radiance, with the linear coefficients optimized to minimize the difference between the brightness temperature and the physical temperatures of atmospheric and terrestrial emitters. Radiative transfer (RT) calculations can be accelerated by formulating the integration in terms of this brightness temperature while producing output in terms of radiance or brightness temperature. Approximation errors are < 0.012 K for RT model applications up to 400 GHz, for any upward, downward, or limb-view geometry, which is about an order of magnitude smaller than for the common brightness temperature derived from a second-order expansion of the Planck function. When products of an RT model that uses this optimized Planck approximation are compared with measurements and the measured radiance is high (equivalent brightness temperature is >170 K), it can be advantageous to apply a complementary approximation to the measurements to benefit from error compensation between the model and the measurements. Alternatively, error compensation can be obtained if the calibration and RT equations use consistent brightness temperature approximations.

Solar Irradiance and Effective Brightness Temperature for SWIR Channels of AVHRR/NOAA and GOES Imagers

Abstract Satellite observations in the shortwave infrared (SWIR) part of spectrum between 3.5 and 4.0 μm deliver critically important information for many applications. The satellite signal in this spectral band consists of solar-reflected radiation and thermal radiation emitted by surface, clouds, and atmosphere. Accurate retrievals require precise knowledge of solar irradiance values within a channel's bandwidth. The magnitudes of solar irradiance for shortwave infrared channels (3.7–3.9 μm) for the Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration-7 (NOAA-7) to NOAA-18 satellites and the Geostationary Operational Environmental Satellite-8 (GOES-8) to GOES-12 are considered in this paper. Four recent solar reference spectra [those of Kurucz, Gueymard, the American Society for Testing and Materials (ASTM), and Wehrli] are analyzed to determine uncertainties in the knowledge of solar irradiance values for SWIR channels of the listed sensors. Because thermal radiation is frequently converted to effective blackbody temperature for analysis, computations, and calibration purposes, it is proposed here to express band-limited solar irradiance values in terms of brightness temperature as well. It is shown that band-limited solar irradiance for AVHRR radiometers expressed in terms of blackbody equivalent brightness temperature correspond to the range 355–360 K, and vary around 345 K for the SWIR channels of the GOES imagers. The values of band-limited solar irradiance and brightness temperatures are provided for various reference solar spectra. The relative differences in band-limited solar irradiance computed for the considered reference solar spectra are between 0% and 2.5%. Differences expressed in terms of brightness temperatures may reach 0.8 K. The results for the ASTM and the Kurucz reference spectra agree within 0.1% relative difference. Parameters of linear fits relating effective brightness temperatures and spectral radiance equivalent temperatures are also determined for all sensors. They are required for precise radiance–temperature and temperature–radiance conversion through Planck's functions in the case of the finite spectral response of real sensors.

Retrieval of semitransparent ice cloud optical thickness from atmospheric infrared sounder (AIRS) measurements

An approach is developed to infer the optical thickness of semitransparent ice clouds (when optical thickness is less than 5) from Atmospheric Infrared Sounder (AIRS) high spectral resolution radiances. A fast cloud radiance model is developed and coupled with an AIRS clear-sky radiative transfer model for simulating AIRS radiances when ice clouds are present. Compared with more accurate calculations based on the discrete ordinates radiative transfer model, the accuracy of the fast cloud radiance model is within 0.5 K (root mean square) in terms of brightness temperature (BT) and runs three orders of magnitude faster. We investigate the sensitivity of AIRS spectral BTs and brightness temperature difference (BTD) values between pairs of wavenumbers to the cloud optical thickness. The spectral BTs for the atmospheric window channels within the region 1070-1135 cm/sup -1/ are sensitive to the ice cloud optical thickness, as is the BTD between 900.562 cm/sup -1/ (located in an atmospheric window) and 1558.692 cm/sup -1/ (located in a strong water vapor absorption band). Similarly, the BTD between a moderate absorption channel (1587.495 cm/sup -1/) and the strong water absorption channel (1558.692 cm/sup -1/) is sensitive to ice cloud optical thickness. Neither of the aforementioned BTDs is sensitive to the effective particle size. Thus, the optical thickness of semitransparent ice clouds can be retrieved reliably. We have developed a spectrum-based approach and a BTD-based method to retrieve the optical thickness of semitransparent ice clouds. The present retrieval methods are applied to a granule of AIRS data. The ice cloud optical thicknesses derived from the AIRS measurements are compared with those retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) 1.38and 0.645-/spl mu/m bands. The optical thicknesses inferred from the MODIS measurements are collocated and degraded to the AIRS spatial resolution. Results from the MODIS and AIRS retrievals are in reasonable agreement over a wide range of optical thicknesses.

The spectral signature of mixed-phase clouds composed of non-spherical ice crystals and spherical liquid droplets in the terrestrial window region

An outstanding problem facing the cloud modeling and remote sensing community is to improve satellite-derived cloud microphysical and macrophysical properties when a single cloud layer exists within a temperature range for which a combination of water and ice particles may be present. This is typically known as a “mixed-phase” cloud condition, and is prevalent when the cloud-top temperature lies between −40°C and 0°C. In this paper we report on a sensitivity study of the spectral signature of mixed-phase clouds in the infrared terrestrial window region (8– 13 μm ). Mixed clouds are assumed to be a vertically uniform cloud layer composed of a mixture of pristine hexagonal ice crystals and spherical water droplets. Unlike the conventional approach that derives the bulk scattering properties of mixed-phase clouds by a linear weighting of the contributions of ice and water components, the bulk single-scattering properties of mixed-phase clouds are formulated on the basis of fundamental physics. With the aid of a line-by-line radiative transfer model and a discrete ordinates radiative transfer (DISORT) computational program, we investigate the high-resolution spectral signature, expressed in terms of brightness temperature, of mixed-phase clouds with various effective sizes, ice fraction ratios, and optical thicknesses. Small particles are found to have a significant impact on the infrared spectral signature of mixed-phase clouds when the size discrepancy between the ice and water particles is large. Furthermore, the simulation results show that the infrared radiative spectrum associated with cirrus clouds can be quite different from their counterparts for cirrus clouds even if a small amount of water droplets exist in the mixed-phase cloud layer.

A modified model for specular sea surface emissivity at microwave frequencies

Modifications to the L.A. Klein et al. model (1997) for specular ocean emissivity have recently been suggested by W.J. Ellison et al. (1996) in order to improve the performance at high microwave frequencies. The work presented in the present article tests both the original and modified models using a set of satellite and ground-based observations that is designed to eliminate as much as possible the dependence of the test on parameters other than the surface emission itself. Clear sky, low humidity, and low wind conditions were used exclusively to reduce the dependence of the test on atmospheric and wind-roughened sea models. Radiosonde observations (RaObs) coincident with TOPEX satellite overpasses were used to reduce errors due to inexact knowledge of the atmosphere. Their tests confirm the superior performance of the Ellison model at higher frequencies. In an effort to remove the residual bias between the models and the observations, they also suggest a parameterized modification to both models that "best fits" the models to the data. In this case, the modified Ellison model maintains its superior performance at high frequencies, suggesting that it has an inherently more accurate frequency dependence. The root mean-squared (RMS) error in the modified Ellison emissivity model, over the range of 18-40 GHz, is found to be 0.0037, which translates into a model error of approximately 1 K in terms of brightness temperatures.

Thermal-infrared field radiometer for vicarious cross-calibration: characterization and comparisons with other field instruments

A four-band (8.2 to 9.2, 10.5 to 11.5, 11.5 to 12.5, and 8 to 14 mm), prototype, thermal-IR radiometer, model CE 312 (CE 312 is the company model number. In previous papers, the CE 312 was called the CLIMAT (conveyable low-noise IR radiometer for measurements of at- mosphere and ground-surface targets)), with a built-in radiance refer- ence is been fabricated by CIMEL Electronique (Paris, France) for use as a field instrument. The instrument is briefly described, laboratory char- acterization is detailed, and its field measurements are compared with those from three other radiometers. The CE 312's main characteristics are linearity of better than 0.8%, field of view of 9.5 deg; noise-equivalent temperature difference of 0.06 to 0.2 K (depending on the band) for brightness temperatures of 0 to 75°C; SNR greater than 1100 for the broadband and greater than 400 for the other bands for brightness tem- peratures between 10 and 80°C; and repeatability of the measured radi- ance smaller than 0.35% after four field campaigns, corresponding to 0.2 K in terms of brightness temperature. Field measurements were con- ducted over different periods during 1996 at Jornada Experimental Range, New Mexico, Lunar Lake and Railroad Valley, Nevada, and Lake Tahoe, California. The CE 312 compares quite favorably with the other instruments: the brightness temperature at two different sites compared to within 0.3 K with two instruments. These measurements show that the CE 312 thermal-IR radiometer is very stable for ambient temperatures varying between 15 and 60°C and that the availability of several filters in the thermal-IR region can help tremendously to improve the accuracy of the radiance determination. © 1999 Society of Photo-Optical Instrumentation Engi- neers. (S0091-3286(99)01902-9)

Effect of Surface Finish and Wall Thickness on the Operating Temperature of Graphite Radio-Tube Anodes

The effect of rough and smooth surfaces and variable wall thicknesses on the operating temperature of radio-tube graphite anodes has been determined. Simultaneous measurements of internal and external temperatures were made with two optical pyrometers on rough and polished hollow graphite cylinders with variable wall thicknesses. Because of the ideal black-body conditions inside the graphite tubes, true internal temperatures were obtained. The optical pyrometer sighted on the outside of the cylinder measured the radiation at a wave length of 6530 angstrom units in terms of brightness temperature. From these data, calculations were made showing the true temperature differences and differences in emissivity and in radiation of the various graphite cylinders. Observations made show that varying the wall thickness of these graphite cylinders from 0.07 to 0.10 inch gave a difference of not more than 20 degrees centigrade in operating temperature. Polishing the surface of the graphite reduced the emissivity by 8 per cent, and increased the surface temperature by 25 degrees centigrade over that obtained for the rough-surface cylinder at 1000 degrees centigrade. Radiation from these surfaces indicates that a roughened anode surface at 1000 degrees centigrade has a radiation capacity increased over that of the polished anode by 1.1 watts per square centimeter.