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NASA NTRS1982

Passive microwave remote sensing of the earth from space - A review

A brief discussion of the historical development of microwave radiometry from space is provided, followed by a review of radiometer system concepts as applied to spacecraft platforms. One of the earliest meteorological applications of spaceborne radiometry was in global atmospheric temperature sounding. Surface-sensing applications can be divided into three main categories, including ocean, ice, and land. A radiometer system description is presented, and spacecraft considerations are discussed. Spaceborne radiometric sensors are examined, taking into account the Nimbus-5 Microwave Spectrometer, the Electrically Scanning Microwave Radiometer, the Skylab microwave radiometer/scatterometer and altimeter instrument, S-194 on Skylab, the Scanning Microwave Spectrometer on Nimbus 6, the Scanning Multichannel Microwave Radiometer on Seasat and Nimbus 7, Soviet sensors, and an Indian microwave radiometer.

Njoku, E. G.↗
NASA NTRS1982

Passive microwave measurements of temperature and salinity in coastal zones

Experimental methods and results from the maritime remote sensing (MARSEN) experiments using dual frequency microwave radiometer detecting systems on board aircraft are described. The radiometers were operated at 1.43 and 2.65 GHz and flown above U.S. Atlantic coastal areas, Chesapeake Bay, around Puerto Rico, and over the German Bight. The advanced switched radiometers used were configured to be independent of gain variations and errors originating from front-end losses and determined the absolute brightness temperatures to within a few tenths Kelvin. Corrections to the observed brightness temperature of the ocean are analytically defined, including accounts made for roughness, the cosmic background radiation, and the solar radio source. The coastal flight data for salinity gradients and surface temperatures were compared with sea truth measured from ships and found to be accurate to within 1 C and 1 pph.

Blume, H.-J. C.↗
NASA NTRS1982

Development of an early warning system of crop moisture conditions using passive microwave

Emissivities were calculated from the Nimbus 5 electrically scanning microwave radiometer (ESMR) over 25 km grid cells for the southern Great Plains includin the western two-thirds of Kansas and Oklahoma and northwest Texas. These emissivities, normalized for seasonal temperature changes, were in excellent agreement with theory and measurements made from aircraft and truck sensors at the 1.55 cm wavelength of ESMR. These emissivities were related to crop moisture conditions of the winter wheat in the major wheat producing counties of the three states. High correlations were noted between emissitivity and an antecedent precipitation index (API) used to infer soil moisture for periods when the soils were essentially bare. The emissivities from ESMR were related through API and actual crop condition reports to progress of fall planting, adequacy of crop moisture for stand establishment, and periods of excessive moisture that necessitated replanting. Periods of prolonged frozen soil in the winter were observable at several grid points. The average emissivities of the canopy/soil surface during the maximum canopy development times in the spring showed a good agreement with moisture stress inferred from rainfall and yield data.

Mcfarland, M. J.↗
NASA NTRS1981

A survey of passive microwave and hybrid remote sensing of precipitation

It is noted that for practical realization of new information on precipitation during this decade, satellite observing systems must be coupled into surface-based observations and computer models of weather systems as they develop. Methods to combine the satellite/surface-based/model capabilities are discussed and several precipitation estimation pilot studies are proposed and outlined.

Weinman, J. A.↗
NASA NTRS1981

On the measurement of precipitation frequencies by passive microwave radiometery

Over most of the microwave spectrum, raindrops both absorb and scatter radiation producing large changes in brightness temperatures relative to clear or cloudy conditions. Since the structure of rain varies substantially for different rain rates and climatological backgrounds, the raindrop size distribution, the rain layer thickness and the ice clouds above the rain layer are all important inputs to the model computations. The subsequent modeling involves applying the Mie theory to derive the absorption and scattering effects and the radiative transfer calculation is based upon a variational iterative approach which takes account of the multiple scattering effect of the rain layer. Results over both ocean and land backgrounds are demonstrated. It is also demonstrated that by using discrimination tests of the radiometric data, the rain/no rain decision can be made and the rainfall rate can be retrieved from a statistical inversion technique.

Kidder, S. Q.↗
NASA NTRS1981

Effects of vegetation on passive microwave estimates of soil moisture

Data collected over plots using a microwave radiometer with a wavelength of 21 cm were used to evaluate a theoretical model of the effect of vegetative cover on the emission of soils. The theoretical model was used as a basis of a simplified model which predicts the emissivity of a soil-vegetation complex with a full canopy cover. This model, when transformed to predict soil moisture, requires measurements of the microwave brightness temperature, surface temperature and vegetation biomass. Since surface temperature can be determined using thermal infrared measurements and vegetation biomass could be estimated with visible and near infrared measurements, the simplified model could be applied using only remotely sensed data.

Jackson, T. J.↗
NASA NTRS1982

Passive microwave detection of river-plume fronts in the German Bight

The NASA P-3 aircraft with the L- and S-band radiometer system on board participated in the MARSEN experiments carried out between August 30 and September 23, 1979. Measurements of surface temperature and salinity were concentrated on freshwater outflows of the Rivers Weser and Elbe in the German Bight. Three missions were carried out, on September 19, 22, and 23. The values of salinity are plotted as a function of geographic position, and contour maps of the salinity distribution are generated from the value plots. It is noted that on September 22, during a noon flood-tide, two river-plume salinity fronts were detected when a body of water with higher salinity existed in front of the Weser delta near Bremerhaven generating with the river outflow of lower saline waters a front of delta-S equals 5 per thousand. Another pocket of lower saline waters was found in front of the Elbe delta at Cuxhaven, which in turn set up a strong front of delta-S equals 6 per thousand. The morning low tide of September 23 did not exhibit this extreme condition.

Blume, H.-J. C.↗
NASA NTRS1983

Passive microwave sensing of soil moisture content - The effects of soil bulk density and surface roughness

Microwave radiometric measurements over bare fields of different surface roughness were made at frequencies of 1.4 GHz, 5 GHz, and 10.7 GHz to study the frequency dependence, as well as the possible time variation, of surface roughness. An increase in surface roughness was found to increase the brightness temperature of soils and reduce the slope of regression between brightness temperature and soil moisture content. The frequency dependence of the surface roughness effect was relatively weak when compared with that of the vegetation effect. Radiometric time-series observations over a given field indicate that field surface roughness might gradually diminish with time, especially after a rainfall or irrigation. The variation of surface roughness increases the uncertainty of remote soil moisture estimates by microwave radiometry. Three years of radiometric measurements over a test site revealed a possible inconsistency in the soil bulk density determination, which is an important factor in the interpretation of radiometric data.

Wang, J. R.↗
NASA NTRS1983

Antartic sea ice, 1973 - 1976: Satellite passive-microwave observations

Data from the Electrically Scanning Microwave Radiometer (ESMR) on the Nimbus 5 satellite are used to determine the extent and distribution of Antarctic sea ice. The characteristics of the southern ocean, the mathematical formulas used to obtain quantitative sea ice concentrations, the general characteristics of the seasonal sea ice growth/decay cycle and regional differences, and the observed seasonal growth/decay cycle for individual years and interannual variations of the ice cover are discussed. The sea ice data from the ESMR are presented in the form of color-coded maps of the Antarctic and the southern oceans. The maps show brightness temperatures and concentrations of pack ice averaged for each month, 4-year monthly averages, and month-to-month changes. Graphs summarizing the results, such as areas of sea ice as a function of time in the various sectors of the southern ocean are included. The images demonstrate that satellite microwave data provide unique information on large-scale sea ice conditions for determining climatic conditions in polar regions and possible global climatic changes.

Zwally, H. J.↗
NASA NTRS1982

Passive microwave sensing of soil moisture content: Soil bulk density and surface roughness

Microwave radiometric measurements over bare fields of different surface roughnesses were made at the frequencies of 1.4 GHz, 5 GHz, and 10.7 GHz to study the frequency dependence as well as the possible time variation of surface roughness. The presence of surface roughness was found to increase the brightness temperature of soils and reduce the slope of regression between brightness temperature and soil moisture content. The frequency dependence of the surface roughness effect was relatively weak when compared with that of the vegetation effect. Radiometric time series observation over a given field indicated that field surface roughness might gradually diminish with time, especially after a rainfall or irrigation. This time variation of surface roughness served to enhance the uncertainty in remote soil moisture estimate by microwave radiometry. Three years of radiometric measurements over a test site revealed a possible inconsistency in the soil bulk density determination, which turned out to be an important factor in the interpretation of radiometric data.

Wang, J. R.↗
NASA NTRS1983

The effects of vegetation and soil hydraulic properties on passive microwave sensing of soil moisture: Data report for the 1982 fiels experiments

Field experiments to (1) study the biomass and geometrical structure properties of vegetation canopies to determine their impact on microwave emission data, and (2) to verify whether time series microwave data can be related to soil hydrologic properties for use in soil type classification. Truck mounted radiometers at 1.4 GHz and 5 GHz were used to obtain microwave brightness temperatures of bare vegetated test plots under different conditions of soil wetness, plant water content and canopy structure. Observations of soil moisture, soil temperature, vegetation biomass and other soil and canopy parameters were made concurrently with the microwave measurements. The experimental design and data collection procedures for both experiments are documented and the reduced data are presented in tabular form.

Oneill, P.↗
NASA NTRS1983

Arctic Sea ice by passive microwave observations from the Nimbus-5 Satellite

The results of a dynamic/thermodynamic numerical model of Arctic sea ice are compared with satellite images from the Nimbus 5 electrically scanning microwave radiometer. The model combines aspects of two previous sea ice models those of Parkinson and Washington and Ling, Rasmussen, and Campbell. A solid/fluid model basically follows the formulation of the Parkinson and Washington model with the addition of the constitutive equation and equation of state from the Ling model. The Parkinson and Washington model simulates the seasonal cycle of sea ice thicknesses and concentrations with a horizontal resolution of roughly 200 km and a timestep of 8 hours. The thermodynamics are calculated through energy balances at the interfaces between ice and air, water and ice, and water and air. The ice dynamics are calculated through a momentum equation balancing air stress, water stress, dynamic topography, and Coriolis force, with an adjustment for internal ice resistance.

Campbell, W. J.↗
NASA NTRS1984

Passive microwave remote sensing of the atmosphere from satellites

Present and future satellite atmospheric sounding is discussed. Although present systems sound principally the temperature profile and the water vapor and precipitation abundances over ocean, future systems will measure other constituents such as O3, H2O2, and ClO, and will provide improved spatial resolution, retrieval accuracy, and altitude coverage for temperature and humidity. Present systems with only a few channels will be replaced by systems with 20 or more, and larger antennas with more sensitive radiometers will permit horizontal resolutions of 15 to 50 km or less, even from geosynchronous orbit. The success of imaging microwave spectrometers in penetrating clouds and in yielding uniquely accurate measurements ensure a growing role in operational and research systems for these remote sensing techniques.

Staelin, D. H.↗
NASA NTRS1984

Effects of corn stalk orientation and water content on passive microwave sensing of soil moisture

A field experiment was conducted utilizing artificial arrangements of plant components during the summer of 1982 to examine the effects of corn canopy structure and plant water content on microwave emission. Truck-mounted microwave radiometers at C (5 GHz) and L (1.4 GHz) band sensed vertically and horizontally polarized radiation concurrent with ground observations of soil moisture and vegetation parameters. Results indicate that the orientation of cut stalks and the distribution of their dielectric properties through the canopy layer can influence the microwave emission measured from a vegetation/soil scene. The magnitude of this effect varies with polarization and frequency and with the amount of water in the plant, disappearing at low levels of vegetation water content. Although many of the canopy structures and orientations studied in this experiment are somewhat artificial, they serve to improve understanding of microwave energy interactions within a vegetation canopy and to aid in the development of appropriate physically based vegetation models.

Oneill, P. E.↗
NASA NTRS1984

Satellite passive microwave rain rate measurement over croplands during spring, summer and fall

Rain-rate algorithms for spring, summer and fall that have been developed from comparisons between the brightness temperatures measured by the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) and rain rates derived from operational WSR-57 radars over land are described. Data were utilized from a total of 25 SMMR passes and 234 radars, resulting in about 12,000 observations of about 1600 sq/km areas. Multiple correlation coefficients of 0.63, 0.80 and 0.75 are achieved for the spring, summer and fall algorithms, respectively. Most of this information is in the form of multifrequency contrast in brightness temperature, which is interpreted as a measurement of the degree to which the land-emitted radiation is attenuated by the rain systems. The SMMR 37-GHz channel has more information on rain rate than any other channel. By combining the lower frequency channels with the 37-GHz observations, variations in land and precipitation thermometric temperatures can be removed, leaving rain attenuation as the major effect on brightness temperature. Polarization screening at 37 GHz is found to be sufficient to screen out cases of wet ground, which is only important when the ground is relatively vegetation free. Heavy rain cases are found to be significant part of the algorithms' success, because of the strong microwve signatures (low-brightness temperatures) that result from the presence of precipitation-sized ice in the upper portions of heavily precipitating storms. If IR data are combined with the summer microwave data, an improved (0.85) correlation with radar rain rates is achieved.

Spencer, R. W.↗
NASA NTRS1984

Passive microwave rain estimation over land and ocean at 37 GHz

The use of Nimbus-7 Scanning Multichannel Microwave Radiometer brightness-temperature data to estimate rain rates is discussed in a review of the author's recent experimental investigations. The performance of multiple-frequency spring, summer, and fall rain-rate algorithms for use over land; a single-frequency (37-GHz) algorithm for use over warm oceans; and 37-GHz brightness-temperature criteria for detecting severe thunderstorms is presented in graphs and tables and briefly characterized. The potential value of a microwave radiometer in GEO is indicated.

Spencer, R. W.↗
NASA NTRS1984

Observations of deep convection from an airborne high-frequency (92 and 183 GHz) passive microwave radiometer

Spencer et al. (1983) have reported that very low Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperatures at 37 GHz over land coincide with heavy thunderstorm rainfall, while Wilheit et al. (1982) used an aircraft-mounted radiometer operating at 92 and 183 GHz to observe convective precipitation associated with a tropical storm over the ocean. A scanning version of the instrument employed by Wilheit et al. is the Advanced Microwave Moisture Sounder (AMMS). The present paper has the objective to summarize the preliminary results of AMMS observations of convective raining clouds and to determine whether empirical relationships between rain rate and microwave brightness temperature, such as those developed for 37 GHz satellite data by Spencer et al., can be extended to higher microwave frequencies.

Hakkarinen, I. M.↗