Change In Brightness Temperature Research Articles

Detection of soil freezing from L-band passive microwave observations

We present a novel algorithm for detecting seasonal soil freezing processes using L-band microwave radiometry. L-band is the optimal choice of frequency for the monitoring of soil freezing, due to the inherent high contrast of the microwave signature between the frozen and thawed states of the soil medium. Dual-polarized observations of L-band brightness temperature at a range of observation angles were collected from a tower-based instrument, and evaluated against ancillary information on soil and snow properties over four winter seasons. During the first three winter periods the measurement site was located over mineral soil on a forest clearing, for the fourth winter the instrument was moved to a wetland site. Both sites are located in Sodankylä, Northern Finland. The test sites represent two environments typical for the northern boreal forest zone. The data were applied to derive an empirical relation between the onset and progress of soil freezing and the observed passive L-band signature. A retrieval algorithm was developed using the observations at the forest opening site. The algorithm exploits the perceived change in brightness temperature and the change in the relative difference between the signatures at horizontal and vertical polarization. With the collected experimental dataset, these features were linked optimally to the progress of soil freezing by choice of observation angle, polarization and temporal averaging. The wetland site observations provided the first opportunity for demonstrating the developed algorithm over a different soil type, giving a first estimate of the algorithm performance over larger heterogeneous targets. The future objective is to adapt the algorithm to L-band satellite observations. The present study is highly relevant for the development of freeze–thaw algorithms from current and future L-band satellite missions such as SMOS and SMAP.

Meteosat Derived Planetary Temperature Trend 1982–2006

24 year of Meteosat hourly thermal infrared data have been used to study planetary surface temperature change. Thermal infrared radiation in the 10.5–12.5mm spectral window is not affected by CO2 and only slightly by atmospheric water vapor. Satellite thermal infrared data have been converted to brightness temperatures as prescribed by Eumetsat. Hourly brightness temperature images were then composed to corresponding noon and midnight temperature data fields. The resulting data fields were cloud filtered using 10, 20 and 30 day maximum temperature substitution. Filtered data were subsequently averaged for two 10 yearly periods: 1986–1995 and 1996–2005. Finally the change in brightness temperature was determined by subtraction. In addition nine locations were selected and data series were extracted and studied for the period 1982–2006. Our observations point to a decrease in planetary temperature over almost the entire hemisphere, most likely due to an increase of cloudiness. Two small areas are found where a considerable temperature increase has occurred. They are explained in terms of major human interventions in the hydrological balance at the earth surface.

Microwave Properties of Ice-Phase Hydrometeors for Radar and Radiometers: Sensitivity to Model Assumptions

AbstractA simplified framework is presented for assessing the qualitative sensitivities of computed microwave properties, satellite brightness temperatures, and radar reflectivities to assumptions concerning the physical properties of ice-phase hydrometeors. Properties considered included the shape parameter μ of a gamma size distribution and the melted-equivalent mass median diameter D0, the particle density, the dielectric mixing formula, and the choice of complex index of refraction for ice. These properties are examined for selected radiometer frequencies of 18.7, 36.5, 89.0, and 150.0 GHz and radar frequencies at 2.8, 13.4, 35.6, and 94.0 GHz—consistent with existing and planned remote sensing instruments. Passive and active microwave observables of ice particles are found to be extremely sensitive to the D0 of the size distribution. Similar large sensitivities are found for variations in the ice volume fraction whenever the geometric mass median diameter exceeds approximately ⅛th of the wavelength. At 94 GHz the two-way path-integrated attenuation is potentially large for dense/compact particles. The distribution parameter μ has a comparatively weak effect on any observable: less than 1–2 K in brightness temperature and a maximum of 2.7 dB (S band only) in the effective radar reflectivity. Reversal of the roles of ice and air in the Maxwell Garnett dielectric mixing formula leads to a substantial change in both microwave brightness temperature (~10 K) and radar reflectivity (approximately 2 dB across all frequencies). The choice of the complex index of refraction of ice can produce a 3%–4% change in the brightness temperature depression.

Reconstructing snowmelt runoff in the Yukon River basin using the SWEHydro model and AMSR‐E observations

AbstractSnowmelt timing and snow water equivalent (SWE) from the Advanced Microwave Scanning Radiometer for EOS (AMSR‐E) are used as inputs to the SWEHydro model to simulate spring snowmelt runoff in high‐latitude, snow‐dominated drainages. AMSR‐E data from 2003 to 2010 are used to determine the timing of melt onset and snow saturation on the basis of changes in brightness temperature (Tb) and diurnal amplitude variations (DAV). Pre‐melt SWE data are combined with terrain information and melt rate estimates to calculate runoff. After melt onset, there is a ‘melt transition period’ with daytime melt and nocturnal refreeze. The melt transition is characterized by high Tb oscillations (high DAV). At the end of high DAV, the snowpack is melting at a higher rate. The model uses four parameters: snowmelt rate during and after melt transition (defined by Tb and DAV thresholds) and flow timing during and after melt transition. The model effectively simulates spring freshet, peak timing and magnitude, and volume (between days 50 and 180) in basins lacking sufficient meteorological measurements for conventional models. We compare the model response in the Pelly and Stewart Rivers, tributaries to the Yukon River, to evaluate model parameters in broadly similar basins under varying conditions. Simulated freshet timing is strongly related to snowmelt timing, and the modeled hydrograph is most sensitive to the flow timing parameter. This observationally based model has potential as a module for quantifying spring snowmelt runoff and timing in physically based models. Copyright © 2012 John Wiley & Sons, Ltd.

A contextual fire detection algorithm based on observation geometry for HJ-1B-IRS

MODIS V4 fire detection algorithm has problems of fire omissions at large scan geometries.In order to solve this problem,the brightness temperature changes of fire pixels,which consist of fire and background surface at different scan geometries,were analyzed.Active potential fire detecting thresholds were developed according to scan geometries.A fire detection algorithm for the HJ-1B-IRS based on MODIS V4 algorithm was presented.Using HJ-1B-CCD data,we validated the fire detection results of HJ-1B-IRS with the fire detection method based on smoke plumes.In comparison to MODIS fire products,HJ-1B-IRS fire product shows advantage on fire location and small fire detection.

Towards Core Body Temperature Measurement via Close Proximity Radiometric Sensing

Presented is a noncontact model and radiometric sensor developed to facilitate core body temperature extraction. The system has been designed as a close-proximity sensor to detect thermal emissions radiated from deep inside the human body. The radiometer uses a cavity-backed slot antenna (CBSA) designed to account for performance degradation which occurs in the near field of the human body. Tissue-simulating materials with electrical properties similar to the human body have been identified, and layered configurations of these phantoms are used to mimic the electromagnetic characteristics of a human stomach volume: hence, defines the core model. Positioned approximately 7 mm from the core model, the sensor tracks the change in brightness temperature as the subsurface physical temperature is varied. A mathematical noncontact model (NCM) is subsequently used to correlate the observed brightness temperature to the subsurface temperature, accounting for artifacts induced by the sensor's remote positioning from the specimen. The accuracy of the NCM is validated through an analysis of the measurement data extracted from the test bed before and after applying the model. The results illustrate that the measurement is highly sensitive to the antenna impedance match and atmospheric temperature. The measurement is less sensitive to the physical temperature of the instrument and emissivity of the specimen. The results of this study demonstrate that radiometric sensors are capable of close proximity, subsurface extraction of biological data given that certain parameters are closely monitored.

Characteristics of Seismic Thermal Radiation of the Japan MS9.0 and Myanmar MS7.2 Earthquake

AbstractUsing the satellite thermal infrared brightness temperature data from Chinese geostationary meteorological satellite, we got their relative power spectra and analyzed this information in the wide range and full‐time. The results show that there are significant changes in brightness temperature before the Japan Ms9.0 and Myanmar Ms7.2 earthquake. The thermal‐infrared anomalies appeared about six months before the earthquakes. The change amplitude is more than 10 times than the normal relative amplitude. There are significant differences between the two earthquakes in the size of the abnormal regions and the time curves. Such differences may be associated with geological and climatic conditions.

On the Use of GNSS-R Data to Correct L-Band Brightness Temperatures for Sea-State Effects: Results of the ALBATROSS Field Experiments

Sea surface salinity is a key oceanographic parameter that can be measured by means of L-band microwave radiometry. The measured brightness temperatures over the ocean are influenced by the sea state, which can entirely mask the salinity signature. Sea-state corrections parameterized in terms of wind speed and/or significant wave height have proven not to be fully satisfactory. In 2003, it was proposed to use reflectometry using navigation opportunity signals [Global Navigation Satellite System Reflectometer (GNSS-R)] for sea-state determination and correction of the measured L-band brightness temperature changes associated to the sea state. The novelty of the approach relies in the measurement of the whole Delay-Doppler Map that captures the scattering of the GNSS signals in the whole glistening zone. In this framework, the “Advanced L-BAnd emissiviTy and Reflectivity Observations of the Sea Surface” (ALBATROSS) field experiments were undertaken in 2008 and 2009, collecting an extensive data set of collocated radiometric and reflectometric measurements over the Atlantic Ocean, as well as oceanographic and meteorological data. In this paper, the experimental results and conclusions of the ALBATROSS 2009 field experiment are compiled and presented, showing the great potential of this technique to perform the necessary corrections in future salinity missions. Empirical relationships are derived among measured brightness temperature variations due to the sea-state effect and direct GNSS-R observables, and the sea surface correlation time at L1 band, a key parameter for GNSS-R data processing since it determines the maximum coherent integration time, was experimentally determined.

Impact of Antenna Pattern on Measurement of the Third Stokes Parameter From Space at L-Band

The third Stokes parameter will be observed from space for the first time at L-band by the Soil Moisture and Ocean Salinity and Aquarius/SAC-D satellites. The correlation between polarizations, which is the source of the third Stokes parameter, is of interest at L-band to measure Faraday rotation and also to indicate novel features of the surface. However, spurious signals (false indication of correlation) can occur in the third Stokes parameter. For example, this happens when the radiometer crosses boundaries associated with a large change in brightness temperature, such as land-water boundaries. In this paper, calculations with the Aquarius radiometer antennas will be used to show that these spurious signals are due to the cross-polarization coupling and large beamwidth associated with realistic L-band antennas in space.

Investigating the Influence of Carbon Dioxide and the Stratosphere on the Long-Term Tropospheric Temperature Monitoring from HIRS

Abstract Contrary to a midtropospheric warming trend detected from Microwave Sounding Unit (MSU) measurements, High-Resolution Infrared Radiation Sounder (HIRS) temperature (15 μm) channels, sensitive to the thermal emission from the troposphere, produce distinct cooling trends for the period 1980–99. This apparent discrepancy in the tropospheric temperature trend is investigated through radiative transfer simulations using Geophysical Fluid Dynamics Laboratory climate model output and the profiles of the standard model atmospheres. Radiative simulations with time-invariant carbon dioxide concentration throughout the entire analysis period produce trends that are qualitatively similar to that obtained from the MSU observations, implying that the observed cooling trends of the HIRS temperature channels are attributable to increased carbon dioxide concentration over the 20-yr period. Additional simulations with the observed time-varying concentration of carbon dioxide confirm this basic result. Whereas temperature fluctuations dominate variability on monthly to interannual time scales, carbon dioxide changes dominate the decadal trends in both the observations and model simulations. Further simulations examined the sensitivity of the brightness temperature change with respect to the changes in tropospheric and stratospheric temperature. These calculations indicate that the influences of stratospheric temperature on the measured radiances are greater for the HIRS temperature channels relative to the MSU midtropospheric channel. These results highlight the contributions of time-varying carbon dioxide concentrations and stratospheric temperature to the HIRS 15-μm (temperature channel) radiance record and underscore the importance of accurately accounting for these changes when using HIRS measurements for long-term monitoring.

Simulations on the influence of lunar surface temperature profiles on CE-1 lunar microwave sounder brightness temperature

Surface temperature profile is an important parameter in lunar microwave remote sensing. Based on the analysis of physical properties of the lunar samples brought back by the Apollo and Luna missions, we modeled temporal and spatial variation of lunar surface temperature with the heat conduction equation, and produced temperature distribution in top 6.0 m of lunar regolith of the whole Moon surface. Our simulation results show that the profile of lunar surface temperature varies mainly within the top 20 cm, except at the lunar polar regions where the changes can reach to about 1.0 m depth. The temperature is stable beyond that depth. The variations of lunar surface temperature lead to main changes in brightness temperature (TB) at different channels of the lunar microwave sounder (CELMS) on Chang’E-1 (CE-1). The results of this paper show that the temperature profile influenced CELMS TB, which provides strong validation on the CELMS data, and lays a solid basis for future interpretation and utilization of the CELMS data.

Wenchuan earthquake: Brightness temperature changes from satellite infrared information

By using an alternative processing method for satellite infrared remote sensing data and adopting the China Geostationary Meteorological Satellite (FY-2C) infrared remote sensing brightness temperature data, we studied the great 2008 Wenchuan Ms8.0 earthquake. Results indicate the obvious characteristic period and amplitude, and distribution of thermal infrared anomalies before the Wenchuan earthquake. And we discussed the mechanism of the observed pre-earthquake thermal anomalies. The characteristics of observed thermal infrared anomalies are easy to be recognized and applied, and could provide a criterion for thermal anomalies of earthquakes.

Identification of snow cover regimes through spatial and temporal clustering of satellite microwave brightness temperatures

Areas of similar ecology are often delineated based on homogenous topography, temperature, and land cover. Once delineated, these zones become the basis for a wide variety of scientific research and management activities. For instance, in Canada, ecozones are commonly utilized ecological management units delineated using geographic, topographic, and climatic information aided by spring and summer vegetation conditions. Snow cover has an influence on local and regional hydrological conditions and climate, as well as on animal habitats. As such, we posit that inclusion of winter conditions, incorporating spatial- and temporal-variation in snow cover is an additional element for consideration when delineating areas with homogenous conditions. In our analysis we use satellite passive microwave brightness temperatures from 19 years of Special Sensor Microwave/Imager (SSM/I) measurements to produce a daily time-series on snow cover, and demonstrate how these data can be used to delineate areas of similar winter conditions. We use splines and curve fitting to generalize the dense time-series (of over 6900 days) to a set of metrics, and select three for use in cluster-based generalization of snow cover regimes: annual maximum difference between 37 and 19 GHz SSM/I measurements (with differences in magnitudes indicative of snow accumulation), variation of 37–19 GHz brightness temperatures (indicative of snow cover variability), and variation in the rate of brightness temperature change during the snow melt season (indicative of seasonal change). Our results indicate that these metrics produce spatial units that are unique, and not captured by conventional ecological management units, while also producing spatial units that cohere to those generated from summer conditions. Spatial units that are found to have spatial cohesion between summer and winter data sources are located in regions where the amount of snow tends to be low, and snow cover variability minimal. We propose that snow cover regimes may be used to augment typical vegetation-based ecological zonations or to provide insights on hydrology and animal habitat conditions. Inclusion of winter conditions is especially important when areal delineations are used to monitor impacts of climate change, and as a baseline for monitoring changes in snow cover amount, extent, and/or distribution.

MAXIMUM BRIGHTNESS TEMPERATURE OF AN INCOHERENT SYNCHROTRON SOURCE : INVERSE COMPTON LIMIT—A MISNOMER

We show that an upper limit of ~ 10^{12} K on the peak brightness temperature for an incoherent synchrotron radio source, commonly referred to in the literature as an inverse Compton limit, may not really be due to inverse Compton effects. We show that a somewhat tighter limit T_{eq} ~ 10^{11} is actually obtained for the condition of equipartition of energy between radiating particles and magnetic fields which happens to be a configuration of minimum energy for a self-absorbed synchrotron radio source. An order of magnitude change in brightness temperature from T_{eq} in either direction would require departures from equipartition of about eight orders of magnitude, implying a change in total energy of the system up to ~ 10^{4} times the equipartition value. Constraints of such extreme energy variations imply that brightness temperatures may not depart much from T_{eq}. This is supported by the fact that at the spectral turnover, brightness temperatures much lower than ~ 10^{11} K are also not seen in VLBI observations. Higher brightness temperatures in particular, would require in the source not only many orders of magnitude higher additional energy for the relativistic particles but also many order of magnitude weaker magnetic fields. Diamagnetic effects do not allow such extreme conditions, keeping the brightness temperatures close to the equipartition value, which is well below the limit where inverse Compton effects become important.

Detecting and measuring new snow accumulation on ice sheets by satellite remote sensing

A new technique is described that detects when and where new snow falls on ice sheets and then determines the thickness of new accumulation. Measurements of vertically polarized passive emission at 85 GHz are filtered with the Hilbert–Huang Transform to identify periods where the surface snow has changed significantly. These are shown to be commonly the result of new snow by comparison with both field observations and in situ instrumentation. Temperature, atmospheric emission and clouds all affect the passive microwave signal but each is examined and shown not to prevent the identification of new snow events. The magnitude of the brightness temperature change is not strongly correlated with snowfall amount. To quantify the amount of new snow, the spatial extent and timing of new snowfalls are examined with ICESat/GLAS laser altimetry data. Crossover differences between altimetric profiles taken before, during, and after the snowfall event provide a measure of the thickness of new snow. Specific cases are presented where 11 and 13 cm of new snow were detected over large regions.

An experimental approach to correct the microwave radiometer wind speed affected by the change of the brightness temperature due to the relative wind direction

[1] We try to correct the change of the brightness temperature of the spaceborne microwave radiometer caused by the angle between the wind and the sensor azimuth (relative wind direction) and to reduce the effect on the wind speed by an experimental way. We make the preliminary approach by comparing the SSM/I brightness temperature collocated with the QuikSCAT wind vector cell. The change of brightness temperature in 37/19 GHz in the horizontal polarization caused by wind is defined as the shift from the relationship between the horizontal and the vertical polarizations without surface wind (PS37/PS19). We propose two hopeful approaches to correct the brightness temperatures. By comparing the observed PS37/PS19 with the value in the PS37/PS19 tables, the correction is done without knowing the information of the in situ vector wind. The relative wind direction effect on the retrieved wind speed is almost halved by this approach.

Toward the elimination of bias in satellite retrievals of sea surface temperature: 1. Theory, modeling and interalgorithm comparison

The along‐track scanning radiometer (ATSR), launched in July 1991 on ERS‐1, is an infrared radiometer designed to permit retrieval of skin sea surface temperature (SST) to the accuracy required for many climate research purposes. Using the prelaunch retrieval scheme, this accuracy (0.3 K) was achieved only when observations at 3.7 μm were available, i.e., SSTs derived from nighttime scenes before the failure of this channel in May 1992. Retrievals using only channels at 11 and 12 μm suffered significant biases. First, cold biases of up to 1.5 K arose from the radiative effects of the unanticipated presence of a significant loading of stratospheric aerosol following the eruption of Mount Pinatubo in June 1991. Second, cold biases of up to 0.4 K were associated with regions of high water vapor loading. We solve the first problem by choosing retrieval coefficients to be orthogonal to the modeled changes in brightness temperatures caused by variations in stratospheric aerosol optical depth. We attribute the second problem to deficiencies in radiative transfer modeling of water vapor continuum absorption and show that use of an updated parameterization reduces bias from wet atmospheres. Applying the new retrieval coefficients to ATSR data, we find good consistency between SSTs retrieved with and without the 3.7 μm channels, the global mean and standard deviation of differences between retrievals being of the order of 0.05 K and 0.25 K, respectively. We therefore anticipate that reprocessing ATSR data using our new retrieval scheme will result in a substantially improved record of ATSR SST, in that the following should be reduced to insignificant levels: (1) the artefactual trend (previously ∼0.25 K yr−1 in tropical regions) corresponding to the decaying load of post‐Pinatubo aerosol, (2) the discontinuity in SST retrievals (previously up to 0.7 K) associated with the failure of the 3.7 μm channel, and (3) cold biases (previously ∼0.4 K) in wet tropical regions. Thus this work represents a significant advance in terms of the quality of ATSR SSTs for climate research. The techniques are also applicable to both the ATSR‐2, flying on ERS‐2, and the advanced ATSR (planned for launch on the Envisat platform in 2000). However, we note that even with the improved physical modeling on which the new retrieval coefficients are based, we do not yet meet the stringent requirement of 0.1 K decade−1 stability in retrievals for climate change detection purposes.

Comparison of microwave radiative transfer calculations obtained with three different approximations of hydrometeor shape

The sensitivity of microwave radiative transfer through the atmosphere with respect to three different raindrop geometries (spheres, oblate spheroids and oblate raindrop shapes realistically approximated by a series of Chebyshev polynomials) is investigated with a one-dimensional numerical model. All rotational symmetric particles are aligned with their axis of rotation along the vertical. Single-scattering calculations are performed with an extended boundary condition T-matrix code provided by Mishchenko and the Discretized Mie Formalism. In contrast to the former code the latter allows to model the scattering by the Chebyshev-shaped raindrops. The change of brightness temperature when using one of the oblate particle shapes instead of the widely used spherical assumption reaches up to −5 K for 85 GHz at upward directions. The calculated microwave signatures are most sensitive to the shape of liquid water drops at frequencies around 20 GHz, while the results at 37 and 85 GHz show larger impact by ice particles. Polarization differences exhibit large changes when using oblate particle shapes instead of spheres, leading to changes of +6 K at upward and −15 K at downward directions. The slight variation of geometry when using Chebyshev shapes gives additional changes of −2 and +4 K at the same directions of emerging radiation. Calculations with an ice particle layer above the rain layer prove that the scattering signature of the underlying rain layer will not be screened by the ice particle scattering. This result shows the importance of correct treatment of raindrop shape even in the presence of large ice particles.

Spatial Patterns of Climate Variability in Upper-Tropospheric Water Vapor Radiances from Satellite Data and Climate Model Simulations

Abstract The use of multivariate fingerprints and spatial pattern correlation in the detection and attribution of climate change has concentrated on radiosonde temperature fields. However, the large body of radiance data from satellite-borne instruments includes contiguous datasets of up to 17 yr in length and in future years will present the most well-calibrated and large-scale data archive available for climate change studies. Here the authors give an example of the spatial correlation technique used to analyze satellite radiance data. They examine yearly mean brightness temperatures from High Resolution Infrared Spectrometer (HIRS) channel 12, sensitive to upper-tropospheric water vapor and temperature. Atmospheric profiles from a climate change run of the Hadley Centre GCM (HADCM2) are used to simulate the pattern of brightness temperature change for comparison to the satellite data. Investigation shows that strong regional brightness temperature changes are predicted in the Tropics and are dominated ...

SSM/I Brightness Temperature Corrections for Incidence Angle Variations

The incidence angles of the SSM/I radiometers on the DMSP satellites vary from satellite to satellite and exhibit variations of up to 1.5° during one orbit. The effects of these variations on the measured brightness temperatures are investigated on the basis of simulated and measured data for oceanic arm. A deviation of 1° from the nominal incidence angle of 53.0° causes brightness temperature changes of up to 2 K depending on surface and atmospheric conditions. Errors of retrieved geophysical parameters on the order of 5%–10% result when the incidence angle variation is not taken into account. This is a common property of most published statistical algorithms. For total precipitable water and cloud liquid water content the error increases with increasing parameter value. For wind speed the error is largest for low wind speed and decreases with increasing wind speed. Due to the slowly varying latitudinal dependence of the incidence angle, these errors do not cancel out when monthly means are computed. A correction method is developed on the basis of simulated data and tested successfully with measured data. Observed brightness temperature differences between DMSP F10 and F11 are reduced when using corrected data. If diurnal variations of geophysical parameters are investigated, the incidence angle correction is mandatory to obtain useful results, especially for DMSP F10.