Change In Brightness Temperature Research Articles

Microwave Observations and Modeling of a Lunar Eclipse

Microwave observations of the Moon during the eclipse of November 28-29, 1993, are reported. Observations were made at 1.327 mm wavelength with a 12-m-aperture radio telescope. Points observed were at the lunar equator and 42° north and south lunar latitude, all near the sub-Earth meridian of lunar longitude. Brightness temperature ( T B) was observed to decrease a maximum of 25% below the pre-eclipse value of T B. Comparisons are made between our observations and previous data sets collected under similar circumstances. A thermal model has been constructed with which we calculate eclipse cooling curves that agree with our observations. From this model inferences are made about the porosity of the lunar regolith necessary to produce the observed changes in brightness temperature throughout the eclipse. The effect of rocks resting on top of the regolith on theoretical eclipse cooling curves is examined. The data are well fit with a regolith-only model that assumes a density of 1.4 g cm -3 (porosity of 59%) for the top 1 cm of regolith. Comparison of lunar high latitude with equatorial observations suggests that regolith density increases with depth within this top 1 cm. Inclusion of rocks in the model increases the amplitude of the eclipse cooling curve for a fixed regolith density, so that a regolith + rocks model fitting the data must have a lower regolith density than a regolith-only model. The regolith model described here to explain observed eclipse cooling curves is shown to be consistent with observed diurnal cooling curves reported by Low and Davidson (1965, Astrophys. J. 142, 1278-1282).

Comparison of Nimbus 7 scanning multichannel microwave radiometer radiance and derived sea ice concentrations with Landsat imagery for the north water area of Baffin Bay

Landsat imagery for the northern Baffin Bay (May to June, 1981) is used to compare ice concentration estimates derived from Nimbus 7 scanning multichannel microwave radiometer (SMMR) data. Periods of premelt, onset of melt and melt are identified based on changes in brightness temperature and on gridded air temperature data from the European Center for Mid‐Range Weather Forecasting. Differences between multispectral SMMR‐ and Landsat‐derived total ice concentrations are within 3.5% for premelt and onset of melt periods. During melt, differences increase to about 10%. Differences for individual SMMR footprints are greater and contribute to a mean correlation coefficient of 0.53 between SMMR‐ and Landsat‐derived ice concentrations. The SMMR data yield slightly lower estimates of total ice concentration, and for individual SMMR footprints they tend to yield lower estimates at high concentrations and higher estimates at low concentrations relative to the Landsat interpretation. Although statistically different, the mean concentrations derived using the Landsat and SMMR data were separated by less than 10% in virtually all cases. Possible reasons for these discrepancies are discussed. The brightness temperatures for compact pack ice compared with fast ice are on the average 10 K higher at vertical polarization and 15 K higher at horizontal polarization for 18 and 37 GHz during the premelt period. Classification of ice types such as first‐year ice, young ice, and nilas by means of polarization ratios seems feasible for large homogeneous areas of a single ice type.

The Dependence of Sea-Surface Microwave Emission on Wind Speed, Frequency, Incidence Angle, and Polarzation over the Frequency Range from 1 to 40 GHz

The dependence of sea-surface microwave emissive prop erties on wind speed, frequency, incidence angle, and polarization arn discussed in detail, in terms of wind-speed sensitivity of the brightnes temperature (defined as a change in brightness temperature with a un, change in wind speed) to the last three observational conditions, ovet the frequency range from 1 to 40 GHz, comparing our results at 6.7 and 18.6 GHz with many investigators' results at various levels from surface to satellite. This wind-speed sensitivity shows marked incidence angle and polarization dependencies. In vertical polarization, a wind-induced increase in the brightness temperature decreases with incidence angle. Then, the vertically polarized brightness temperature of the roughened sea surface is equal to that of the calm surface at an incidence angle between 50 ° and 600, and is higher below, and lower above, this angle; in other words, the above sensitivity is higher and takes a positive value at lower incidence angle and it is lower and takes negative value at higher incidence angle, with a wind-speed-insensitive angle existing between 500 and 600. A wind-induced change in the horizontally polarized brightness temperature shows no marked incidence angle dependence, but a uniform increase over the incidence angle. Both polarizations do not essentially change their characteristics in the above dependencies but gain wind-speed sensitivity with frequency; however, the horizontal polarization is much more sensitive to wind speed than the vertical one over the entire incidence and frequency ranges.

The Timing of Initial Spring Melt in the Arctic from Nimbus-7 SMMR Data (Abstract)

The ablation of sea ice is an important feature in the global climate system. During the melt season in the Arctic, rapid changes occur in sea-ice surface conditions and areal extent of ice. These changes alter the albedo and vary the energy budgets. Understanding the spatial and temporal variations of melt is critical in the polar regions. This study investigates the spring onset of melt in the seasonal sea-ice zone of the Arctic Basin through the use of a melt signature derived by Anderson and others from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. The signature is recognized in the “gradient ratio” of the 18 and 37 GHz vertical brightness temperatures used to distinguish multi-year ice. A spuriously high fraction of multi-year ice appears rapidly during the initial melt of sea ice, when the snow-pack on the ice surface has started to melt. The brightness-temperature changes are a result of either enlarged snow crystals or incipient puddles forming at the snow/ice interface.The timing of these melt events varies geographically and with time. Within the Arctic Basin, the melt signatures are observed first in the Chukchi and Kara/Barents Seas. As the melt progresses, the location of the melt signature moves westward from the Chukchi Sea and eastward from the Kara/Barents Seas to the Laptev Sea region. The timing of the melt signal also varies with year. For example, the melt signature occurred first in the Chukchi Sea in 1979, while in 1980 the signature was first observed in the Kara Sea.There are also differences in the timing of melt for specific geographic locations between years. The melt signature varied almost 25 days in the Chukchi Sea region between 1979 and 1980. The other areas had changes in the 7–10 day range.The occurrence of these melt signatures can be used as an indicator of climate variability in the seasonal sea-ice zones of the Arctic. The timing of the microwave melt signature has also been examined in relation to melt observed on short-wave imagery. The melt events derived from the SMMR data are also related to the large-scale climate conditions.

The Timing of Initial Spring Melt in the Arctic from Nimbus-7 SMMR Data (Abstract)

The ablation of sea ice is an important feature in the global climate system. During the melt season in the Arctic, rapid changes occur in sea-ice surface conditions and areal extent of ice. These changes alter the albedo and vary the energy budgets. Understanding the spatial and temporal variations of melt is critical in the polar regions. This study investigates the spring onset of melt in the seasonal sea-ice zone of the Arctic Basin through the use of a melt signature derived by Anderson and others from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. The signature is recognized in the “gradient ratio” of the 18 and 37 GHz vertical brightness temperatures used to distinguish multi-year ice. A spuriously high fraction of multi-year ice appears rapidly during the initial melt of sea ice, when the snow-pack on the ice surface has started to melt. The brightness-temperature changes are a result of either enlarged snow crystals or incipient puddles forming at the snow/ice interface. The timing of these melt events varies geographically and with time. Within the Arctic Basin, the melt signatures are observed first in the Chukchi and Kara/Barents Seas. As the melt progresses, the location of the melt signature moves westward from the Chukchi Sea and eastward from the Kara/Barents Seas to the Laptev Sea region. The timing of the melt signal also varies with year. For example, the melt signature occurred first in the Chukchi Sea in 1979, while in 1980 the signature was first observed in the Kara Sea. There are also differences in the timing of melt for specific geographic locations between years. The melt signature varied almost 25 days in the Chukchi Sea region between 1979 and 1980. The other areas had changes in the 7–10 day range. The occurrence of these melt signatures can be used as an indicator of climate variability in the seasonal sea-ice zones of the Arctic. The timing of the microwave melt signature has also been examined in relation to melt observed on short-wave imagery. The melt events derived from the SMMR data are also related to the large-scale climate conditions.

Computerised plotting of bhaskara-I alternate mode brightness temperature data

A method has been developed to study the alternate mode BHASKARA SAMIR data with reference to a cartographic referencing system of India. The look angle dependence of the brightness temperature over sea region has been clearly depicted. The change in brightness temperature due to snow/ice in Himalayan region is also identified. A sharp variation of brightness temperature values has been observed at the land-sea crossings.

Radiometric Technique for Measuring Changes in Lung Water (Short Papers)

In this paper, we describe a new approach to continuous, noninvasive monitoring of changes in lung-water content based on radiometry. It is shown theoretically, as based on a planar model of the lung, that a one-percent change in lung-water content corresponds to a 0.260-K change in brightness temperature, which is within the detection sensitively of available radiometers. Practicably, taking into account reflections at the tissue interfaces and attenuation in the chest wall, it is estimated that it is possible to detect a three to four-percent change in lung-water content. Initial experimental measurements at 1 GHz also have shown promising results. Some of the basic problems associated with the adaptability of the radiometric technique for clinical use have been identified and are discussed along with suggested solutions.

Variation of the Microwave Brightness Temperature of Sea Surfaces Covered with Mineral And Monomolecular Oil Films

Airborne microwave radiometer measurements over mineral and monomolecular oil films and adjacent clean sea surfaces are reported. An artificial crude-oil spill experiment in the New York Bight area showed a brightness temperature increase of the sea surface at 1.43 GHz as expected from a multilayered system with different dielectric constants. However, a monomolecular surface-film experiment with oleyl alcohol conducted in the North Sea during MARSEN in 1979 showed a strong brightness temperature depression at 1.43 GHz and no change in brightness temperature at 2.65 GHz. It is postulated that the monomolecular layer, because of its physical and chemical properties, polarized the underlying water molecules so strongly that the emissivity decreased from 0.31 to 0.016. It is estimated that the effective dielectric constant changed from 90 to 5.2 × 104. Because these phenomena occurred at 1.43 GHz it may be concluded that this frequency is very close to the center of a new anomalous dispersion region resulting from a restructuring of the water layer below the surface film.

Thermal microwave emission from a scattering layer

The microwave radiometric temperature of heterogeneous materials is altered by internal scattering. Radiative transfer theory and a Rayleigh scattering model are used to obtain the microwave temperature and its polarization as a function of view angle for emission from a nonuniformly thick layer containing scatterers. The scatter-induced change in brightness temperature is generally negative and may be many tens of degrees. There are situations, however, such as the emission from ice over freshwater, where the scatterers may increase the brightness temperature by as much as 30°C. Lunar microwave temperatures may be decreased from a few degrees to more than 50°C depending upon wavelength and upon the specific lunar target; the brightness temperature as a function of wavelength of a lava flow may be decreased from 1°C to more than 30°C by near-surface vesicles; snow may appear from a few degrees to more than 60°C darker as it ages; and air bubbles in ice over freshwater may increase the brightness temperature by as much as 30°C or decrease it by as much as 50°C depending upon the thickness of the ice and the size and concentration of the bubbles.

Effect of temperature, angle of observation, salinity, and thin ice on the microwave emission of water

Numerical computations of the polarized emissivity of smooth water for various nadir angles of observation and different water temperatures, in the 0.1- to 10-cm region of the microwave spectrum, indicate that the polarization of the emitted microwave radiation of water is negligible for small angles of observation, increases with the increase of the angle of observation, and has a maximum that shifts toward larger angles for longer wavelengths. The changes of polarized emissivity with temperature are especially strong in the 0.2- to 2-cm region and for large nadir angles of observation; however, the changes become negligible for wavelengths greater than 3 cm. Numerical computations of polarized emission of water with various salinities indicate a 4°K change of brightness temperature for an equivalent salinity change from 6.7 to 32.3 parts per thousand in the wavelength region of 10 cm. The effects are smaller at shorter wavelengths; the polarized emission also decreases with increasing angles of observation. The polarized emissivity of thin ice on top of water calculated for various angles of observation indicates a considerable change in the emissivity of water when the ice thickness is of the same order of magnitude as the wavelength of the microwave radiation. When the ice thickness is small in comparison to the wavelength, a systematic change of the polarized emissivity is observed, which suggests the possibility of determining ice thickness by airborne passive microwave radiometry.