Nimbus-7 Scanning Multichannel Microwave Radiometer Research Articles

Measuring the Global Distribution of Intense Convection over Land with Passive Microwave Radiometry

The global distribution of intense convective activity over land is shown to be measurable with satellite passive-microwave methods through a comparison of an empirical rain rate algorithm with a climatology of thunderstorm days for the months of June-August. With the 18 and 37 GHz channels of the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR), the strong volume scattering effects of precipitation can be measured. Even though a single frequency (37 GHz) is responsive to the scattering signature, two frequencies are needed to remove most of the effect that variations in thermometric temperatures and soil moisture have on the brightness temperatures. Because snow cover is also a volume scatterer of microwave energy at these microwavelengths, a discrimination procedure involving four of the SMMR channels is employed to separate the rain and snow classes, based upon their differences in average thermometric temperature.

Effect of vegetation on soil moisture sensing observed from orbiting microwave radiometers

The microwave radiometric measurements made by the Skylab 1.4 GHz radiometer and by the 6.6 GHz and 10.7 GHz channels of the Nimbus-7 Scanning Multichannel Microwave Radiometer were analyzed to study the large-area soil moisture variations of land surfaces. Two regions in Texas, one with sparse and the other with dense vegetation covers, were selected for the study. The results gave a confirmation of the vegetation effect observed by ground-level microwave radiometers. Based on the statistics of the satellite data, it was possible to estimate surface soil moisture in about five different levels from dry to wet conditions with a 1.4 GHz radiometer, provided that the biomass of the vegetation cover could be independently measured. At frequencies greater than about 6.6 GHz, the radiometric measurements showed little sensitivity to moisture variation for vegetation-covered soils. The effects of polarization in microwave emission were studied also.

Retrieval of snow water equivalent from Nimbus-7 SMMR data: Effect of land-cover categories and weather conditions

The effect of the four major surface types in Finland (forests, boglands, farmlands, and water (ice)) on the microwave brightness temperature of snow-covered areas is investigated. Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data are employed to derive the response of each surface type. An algorithm to retrieve the water equivalent of snow cover from Nimbus-7 SMMR data is tested in different dry snow conditions.

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.

Retrieval of Ocean Surface Parameters from the Scanning Multifrequency Microwave Radiometer(SMMR) on the Nimbus-7 Satellite

Sea-surface temperature retrievals have been tested on 2 months of Nimbus-7 scanning multichannel microwave radiometer data. Using the prelaunch versions of the instrument calibration and geophysical parameter retrieval algorithms the initial results were poor. Improved algorithms produced substantially better results. It appears that at least for the night-Southern Hemisphere portion of the Nimbus-7 orbit, a rms measurement accuracy of 1.45°C has been achieved. Similar tests with wind speed retrievals yield an accuracy of 2.7 m/s rms with no substantial differences between day and night measurements but limited by availability of surface observations to the Northern Hemisphere. Moreover, it appears that the retrieved wind speed is more nearly related to the square of the wind observed at the surface than to the wind itself.

Sea Surface Temperatures from Nimbus-7 SMMR Radiances

Global displays of sea surface temperatures (SSTs) from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) are obtained with spatial sampling intervals as small as 50 km rather than the 150 km spacing normally used for such retrievals. An example is illustrated using a composite global SMMR data set for January 1979, a preliminary version of the SST retrieval algorithm, and a sampling interval of 100 km. The results were found to be in qualitative agreement with in situ and climatic data, insofar as such comparisons were attempted. In addition to the expected climatic patterns, the global oceanic isotherms contain oscillations that are clearly not instrumental but geophysical in nature.

Nimbus-7 SMMR Polarization Responses to Snow Depth in the Mid-Western U.S.

Analysis of January and February 1979 Scanning Multichannel Microwave Radiometer (SMMR) data has revealed discrepancies in the responses of vertically and horizontally polarized brightness temperature (TB) data to snow in the state of Indiana. Ten satellite overpasses were analyzed between January 8 and February 21, 1979. In four cases, the horizontally polarized TBs correlated with snow depth significantly better than did the vertically polarized TBs. These four overpasses occurred either when the snowpack was building up or when it had metamorphosed and/or was melting. For example, on February 17, the coefficient of correlation, R, between vertically polarized TB and snow depth was R = -.46 while it was R = -.86 for the horizontally polarizedTB data. Just prior to February 17, air temperatures rose above 0°C in the study area causing melting of the snowpack. On February 17, air temperatures dropped well below 0°C and the snow and water in the snowpack froze causing a metamorphosis of the snowpack. It is concluded that horizontally polarized TB data may be more useful than vertically polarized data for analysis of snow in regions in which there are a range of snowpack conditions, or where a snowpack undergoes rapid metamorphosis. However, analysis of both polarizations may reveal information concerning the structure of a snowpack.

A Statistical Examination of Nimbus-7 SMMR Data and Remote Sensing of Sea Surface Temperature, Liquid Water Content in the Atmosphere and Surface Wind Speed

Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperature measurements over the global oceans have been examined with the help of statistical and empirical techniques. Such analyses show that zonal averages of brightness temperature measured by SMMR, over the oceans, on a large scale are primarily influenced by the water vapor in the atmosphere. Liquid water in the clouds and rain, which has a much smaller spatial and temporal scale, contributes substantially to the variability of the SMMR measurements within the latitudinal zones. The surface wind not only increases the surface emissivity but through its interactions with the atmosphere produces correlations, in the SMMR brightness temperature data, that have significant meteorological implications. It is found that a simple meteorological model can explain the general characteristics of the SMMR data. With the help of this model methods to infer over the global oceans, the surface temperature, liquid water content in the atmosphere, and surface wind speed are developed. Monthly mean estimates of the sea surface temperature and surface winds are compared with the ship measurements. Estimates of liquid water content in the atmosphere are consistent with earlier satellite measurements.

The Utilization of Nimbus-7 SMMR Measurements to Delineate Rainfall over Land

Approaches for developing an improved passive microwave technique to delineate rain from wet land surfaces are considered, and an investigation is conducted with the objective to study the application of these ideas with empirical data. The investigation includes a statistical analysis of Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) 37.0, 18.0, and 10.7 GHz data. The possibility was assessed to make use of the 18.0 and 10.7 GHz channels in order to reduce the ambiguities found in the 37.0 GHz radiometer data for differentiating rainfall areas from wet land surfaces. It was found, however, that none of the SMMR channels could differentiate rain from wet or dry land when surface temperatures were less than 15 C. It appears that an improved rainfall-over-land detection technique could be developed by two different methods, utilizing both satellite infrared and multifrequency dual polarized passive microwave data.

Selection of microwave bands for global snow detection

Microwave radiometers and scatterometers as all-weather and day-and-night sensors are excellent tools for the reliable, regular observation of dynamic features of the surface of the earth. The various hydrologic states, the development of the metamorphism, the water equivalent and other parameters of the snow cover cause a very distinct spectral behaviour of the microwave emission and backscattering. The results of a long-term ground-based investigation of the microwave radiation properties along with classical determinations of the snow parameters during five seasons on an alpine test area allow the definition of an optimum package of microwave passive and active sensors for the global monitoring of the snow cover. Additionally it has been demonstrated that the utilization of the Nimbus-7 Scanning Multi-channel Microwave Radiometer data permit the production of large scale maps of the extent of dry snow, the estimated water equivalent of dry snow and the areas changing between frozen and wet snow.

Remote Sensing of Precipitable Water over the Oceans from Nimbus 7 Microwave Measurements

Global maps of precipitable water over derived from scanning multichannel microwave radiometer (SMMR) data reveal salient features associated with ocean currents and the large scale general circulation in the atmosphere. Nimbus-7 SMMR brightness temperature measurements in the 21 and 18 GHz channels are used to sense the precipitable water in the atmospheric over oceans. The difference in the brightness temperature (T sub 21 -T sub 18), both in the horizontal and vertical polarization, is found to be essentially a function of the precipitable water in the atmosphere. An equation, based on the physical consideration of the radiative transfer in the microwave region, is developed to relate the precipitable water to (T sub 21 - T sub 18). It shows that the signal (T sub 21- T sub 18) does not suffer severely from the noise introduced by variations in the sea surface temperature, surface winds, and liquid water content in non rain clouds. The rms deviation between the estimated precipitable water from SMMR data and that given by the closely coincident ship radiosondes is about 0.25 g/ sq cm