Seasat Scanning Multichannel Microwave Radiometer Research Articles

Linear retrieval and global measurement of precipitable water from the SEASAT SMMR data

The Seasat Scanning Multichannel Microwave Radiometer (SMMR) measurements in the 18.0, 21.0 and 37.0 GHz channels, both horizontal and vertical polarizations, are primarily used for precipitable water, cloud liquid water content and rainfall rate determination. Linear regressions using a leaps and bounds procedure are used for the retrieval of precipitable water. The radiation simulated for all the ten SMMR channels with varied global environmental parameters were used for subset selection for water vapour retrieval. Only subsets with channels having uniform grid size (18, 21 and 37 GHz) were used for the analysis. A total of eight subsets using two to five frequencies of the SMMR are examined to determine their potential in the retrieval of atmospheric water vapour content. Our analysis indicates that the information concerning the 18 and 21 GHz channels are optimum for the water vapour retrieval. An attempt to use all the SMMR channels simultaneously gives no significant improvement. A comparison with the radiosonde observations gave an rms accuracy of 0.4 g/cm2. The rms accuracy of retrieved precipitable water using different subsets was within 10 percent.

Linear retrieval, validation and mapping of the sea surface temperature over western mid-latitude North Pacific using Seasat SMMR data

Abstract The Scanning Multichannel Microwave Radiometer (SMMR) aboard the Seasat satellite measured emitted radiation in both horizontal and vertical polarizations at microwave frequencies of 6.6, 1069, 18.0, 21.0 and 37.0 GHz. Retrieval algorithms, for sea surface temperature (SST) determination, from subsets of one to three SMMR channels, are obtained by a two-step statistical technique. The technique first selects the best subsets of a given size defined by an R2 criterion (coefficient of determination), of a given size by the application of an efficient leaps and bounds technique on a statistical data base. It then performs a regression analysis on the selected subsets. The statistical data base employed a large (600) set of seasonally and geographically diverse atmospheric and surface parameters for radiative transfer calculations. The results of the study of one to three-channel subset retrieval algorithms indicate the possibility of using 6.6 V, 6.6 H and 18 V channels for SST determinatidn from Se...

Measurement of Global Oceanic Winds from Seasat-SMMR and Its Companson with Seasat-SASS and ALT Deinved Winds

Wind speed has been retrieved using different channel combinations of Seasat Scanning Multichannel Microwave Radiometer (SMMR) data. The sets of channel combinations were derived by applying a leaps and bounds procedure to a statistical data base of brightness temperature and geophysical parameters. Different best subsets of sizes two to five SMMR channels were obtained, based on R2, the coefficient of determination criterion. Wind speeds derived using the best two channel subsets (10.6 H and 18.0 V) were compared with Joint Air-Sea Interaction Experiment)(JASIN) in situ observations of winds and an RMS difference of 1.5 m/s was found. The retrieval accuracy of wind speed from different subsets of two to five channels agreed to within about 10 percent. Global maps of oceanic winds have been generated and compared with maps obtained from Seasat scatterometer and altimeter derived winds. The three sensors depictedthe general features of wind distribution in both the northern and southern hemispheres and co form with the prevailing ideas, but differed quantitatively over certain latitude belts requiring further research.

Atmospheric Water Distribution in a Midlatitude Cyclone Observed by the Seasat Scanning Multichannel Microwave Radiometer

Patterns in the horizontal distribution of integrated water vapor, integrated liquid water and rainfall rate derived from the Seasat Scanning Multichannel Microwave Radiometer (SMMR) during a September 10-12, 1978 North Pacific cyclone are studied. These patterns are compared with surface analyses, ship reports, radiosonde data, and GOES-West infrared satellite imagery. The SMMR data give a unique view of the large mesoscale structure of a midlatitude cyclone. The water vapor distribution is found to have characteristic patterns related to the location of the surface fronts throughout the development of the cyclone. An example is given to illustrate that SMMR data could significantly improve frontal analysis over data-sparse oceanic regions. The distribution of integrated liquid water agrees qualitatively well with corresponding cloud patterns in satellite imagery and appears to provide a means to distinguish where liquid water clouds exist under a cirrus shield. Ship reports of rainfall intensity agree qualitatively very well with SMMR-derived rainrates. Areas of mesoscale rainfall, on the order of 50 km x 50 km or greater are detected using SMMR derived rainrates.

The SEASAT altimeter wet tropospheric range correction revisited

An expanded set of radiosonde observations was used to calculate the wet tropospheric range correction for the brightness temperature measurements of the SEASAT scanning multichannel microwave radiometer (SMMR). The accuracy of the conventional algorithm for wet tropospheric range correction was evaluated. On the basis of the expanded observational data set, the algorithm was found to have a bias of about 1.0 cm, and a standard deviation 2.8 cm. In order to improve the algorithm, the exact linear, quadratic and logarithmic relationships between brightness temperatures and range corrections were determined. Various combinations of measurement parameters were used to reduce the standard deviation between SEASAT SMMR and radiosonde observations to about 2.1 cm. The performance of various range correction formulas is compared in a table.

Tropical Pacific sea surface temperatures measured by SEASAT microwave radiometer and by ships

Sea surface temperatures (SST) obtained from the SEASAT scanning multichannel microwave radiometer (SMMR) are compared with in situ SST observations in the eastern and central tropical Pacific Ocean for late July 1978. Comparing pairs of SMMR and ship SST (injection temperatures), we find that the SMMR SSTs are biased 0.90C lower than ship SSTs, with a standard deviation (relative to the bias) of 1.70C. Since ship SSTs tend to concentrate along shipping lanes, while SMMR SST's are distributed nearly uniformly, the statistics based on data‐to‐data comparisons are spatially biased; in addition, there is ambiguity in comparing the spatially averaged (149 km×149 km) SMMR SST against the point‐observed ship SST. These ambiguities in data comparison can be reduced by comparing the SST fields derived from these data. The result shows that the SMMR‐derived SST field (TSMMR) is, on the average, lower than the ship‐derived SST field Tship by 0.80C with a standard deviation of 0.80C. Adjusted for 0.20C to 0.70C warm bias of ship SST [Saur, 1963; Tabata, 1978], this set of SMMR SST has a bias −0.6° to −0.1°C. The difference between rship and TSMMR shows a strong regional dependence, as found in comparing SST fields derived either from different data sets or from the same data set but with different methods [Barnett et al., 1981]. In our case, the standard deviation of the difference between these two fields is found to be inversely proportional to the spatial density of ship data. The implications are that the spatial variation of the SST field may be better derived from SMMR SST than from ship SST, which is handicapped by its nonuniform data distribution, and, after determining the biases of both SMMR and ship SST, these two data sets may complement each other for constructing SST fields.

Total precipitable water and rainfall determinations from the SEASAT scanning multichannel microwave radiometer

Three of the SEASAT scanning multichannel microwave radiometer's (SMMR's) five frequencies can be used to determine the total precipitable water in the atmosphere, the liquid water content, and the rainfall rate. In addition, two distinct algorithms were used to derive geophysical parameters from brightness temperature. Comparisons with surface observations from the Gulf of Alaska SEASAT Experiment and other selected sources were made for both algorithms. Generally, very good agreement was found between satellite and radiosonde observations of total precipitable water. An estimate of the precision of radiosonde‐derived total precipitable water amounts was made. Comparisons of precipitation occurrence as determined from the SMMR and surface observations also were made. In the tropics, very good agreement is found between satellite and surface observations. In the mid‐latitudes (Gulf of Alaska) it is found that small cells of precipitation are often missed by the SMMR but larger synoptic scale precipitation is detected.

Evaluation of SEASAT SMMR wind speed measurements

The SEASAT scanning multichannel microwave radiometer (SMMR) is able to measure the wind speed at the ocean surface through the change in ocean surface microwave emissivity caused by the wind. In this paper we compare the SMMR‐derived wind speeds to the wind speeds derived from an active microwave scatterometer also aboard the SEASAT, the SEASAT A scanning scatterometer (SASS), Four orbits that passed over the severe storm that damaged the Queen Elizabeth II are examined in detail. These orbits and five others were used to investigate effects which degrade the SMMR wind retrievals. When the data are filtered for such effects, we find that the SMMR winds agree with the SASS winds with a scatter (1σ) of less than 2 m/s about a bias of 1 to 2 m/s.

Tropical and mid‐latitude north Pacific sea surface temperature variabililty from the SEASAT SMMR

Sea surface temperature (SST) data from the Scanning Multichannel Microwave Radiometer (SMMR) for the entire SEASAT mission (July 6 to October 10, 1978), was obtained from the SEASAT project office and has been analyzed for the mid‐latitude North Pacific (20°N–50°N, 145°E–130°W), and the eastern tropical Pacific (10°S–20°N, 95°W–145°W). Systematic cross‐scan SST biases were observed in the SMMR data, strongly dependent upon latitude, solar elevation, time of day, and month, The cross‐scan biases are ascribed to incomplete accounting for Faraday rotation of polarized microwave energy propagating upward through the ionosphere. A second systematic SST bias occurred between data of ascending and descending passes nearly 12 hours apart. The bias increased steadily through the mission as the SEASAT orbit moved from 6 A.M./P.M. to 12 A.M./P.M. local sun time coverage. The ascending/descending bias is ascribed to measurements being made at different times in the diurnal cycle of SST. Corrections for the two systematic effects were derived and applied to the SMMR data to produce maps of SST. In mid‐latitude, 10 and 30 day maps of SMMR SST were compared with similar maps produced from routine shipboard SST observations (mostly engine injection temperatures). The monthly SMMR and shipbased maps agreed at the 0.5–0.7°C rms level in July and September, but exhibited a 0.5°C rms scatter about a significant 1.0°C bias in August. The August bias may be a reflection of true midsummer vertical differences in temperature between the 1 cm depth of SMMR observations and the typically 5 m depth of ship observations. Monthly SST anomalies (deviation from climatology) for SMMR and ship exhibited similar patterns, but the SMMR maps appear superior because of their better spatial data coverage. In the eastern tropical Pacific, a sequence of 15 8‐day interval maps were constructed. The maps reproduce known climatological features, in particular the cold tongue of water along the equator. Week‐to‐week variations occur in the intensity of the cold tongue. In addition, wave‐like patterns occur along the meridional gradient just north of the equator, having ∼1000 km wavelength, and propagating west at about 42 km/day.

Sea surface temperature mapping with the SEASAT microwave radiometer

Sea surface temperature data from the SEASAT Scanning Multichannel Microwave Radiometer (SMMR) has been mapped for the period July 7 to August 6, 1978, in the western North Pacific, 20°N–50°N, 140°E–180°, and compared against similar maps constructed from all available ship observations. The temperature difference between the SMMR and ship maps has a standard deviation of 0.75°C, about a mean bias of 0.22°C, SMMR being warmer. The average standard deviation of all individual observations about the mapped values is 1.51°C and 2.10°C for SMMR and ship, respectively. Both the SMMR and ship maps exhibit the known major climatological features for the region. In addition, both data types yield similar maps of sea surface temperature anomaly (departure from climatology), with large‐scale (>500 km) anomaly features of 1°–3°C. Aside from having a lower noise level, the SMMR data is superior to the ship data for constructing large‐scale maps because it is more uniformly distributed in space and time.

Determinations by Seasat of atmospheric water and synoptic fronts

The Seasat Scanning Multichannel Microwave Radiometer (SMMR)1,2 determined the total atmospheric water vapour, qt, over 600-km wide swaths with a resolution of 54 km. Using radiosonde data from the Joint Air–Sea Interaction experiment, JASIN3, we show here that the SMMR qt distributions can be used to detect the position of atmospheric fronts in the lower troposphere. Unlike visible and IR radiometry these SMMR determinations are not hampered by extensive cirrus or by lack of frontal cloud. Advantages of the SMMR over previous passive microwave instruments on satellites such as Nimbus 5 and 6 (refs 4–9) are: the use of more channels, allowing better discrimination between the effects of liquid water, water vapour and sea state; and improved spatial resolution. The SMMR performance has been evaluated at several workshops10–14. Our results15 show that the SMMR qt determinations have similar accuracy to in situ radiosonde measurements.

Microwave Radiometric Measurements of Sea Surface Temperature from the Seasat Satellite: First Results

Initial results from the Seasat scanning multichannel microwave radiometer indicate that the sea surface temperature can be measured with a root-mean-square sensitivity of 1.2 degrees C or better. The first microwave map of sea surface temperature for the entire Pacific has been produced.

The Seasat scanning multichannel microwave radiometer (SMMR): Radiometric calibration algorithm development and performance

Radiometric calibration algorithms for processing Seasat scanning multichannel microwave radiometer (SMMR) flight data are presented. An equation is derived which accounts for losses and reradiation in the microwave components and nonlinearities in the radiometer. The application of pre-launch calibration data to this derivation is described. A performance evaluation of the post-launch operation of the algorithm is presented, indicating temperature resolution of 0.63 K at 6.6 GHz and 1.39 K at 37 GHz.

The Seasat scanning multichannel microwave radiometer (SMMR): Antenna pattern corrections - Development and implementation

The antenna temperatures measured by the Seasat scanning multichannel microwave radiometer (SMMR) differ from the true brightness temperatures of the observed scene due to antenna pattern effects, principally from antenna sidelobe contributions and cross-polarization coupling. To provide accurate brightness temperatures convenient for geophysical parameter retrievals the antenna temperatures are processed through a series of stages, collectively known as the antenna pattern correction (APC) algorithm. A description of the development and implementation of the APC algorithm is given, along with an error analysis of the resulting brightness temperatures.

The Seasat scanning multichannel microwave radiometer (SMMR): Instrument description and performance

The scanning multichannel microwave radiometer (SMMR) is an imaging 5-frequency radiometer flown on the Seasat and Nimbus-7 earth satellites launched in 1978. It measures dual-polarized microwave radiances from the earth's atmosphere and surface, primarily for the purpose of deriving global and nearly all-weather measurements of sea surface temperature, wind speed, and atmospheric liquid water and water vapor. This paper describes the SMMR instrument and its calibration, antenna pattern measurements, and data processing procedures. Analysis of early data from the Seasat SMMR shows that the expected engineering performance in flight was achieved, and the measurement of sea surface temperature and wind speed with accuracies of 1.5 K and 2 m/s, respectively, may be achievable once the geophysical data processing algorithms and analysis have been completed.

Seasat Scanning Multichannel Microwave Radiometer: Results of the Gulf of Alaska Workshop

The scanning multichannel microwave radiometer results for the Gulf of Alaska Seasat Experiment Workshop are quite encouraging, especially in view of the immaturity of the data-processing algorithms. For open ocean, rain-free cells of highest-quality surface truth wind determinations exhibit standard deviations of 3 meters per second about a bias of 1.5 meters per second. The sea-surface temperature shows a standard deviation of approximately 1.5 degrees C about a bias of 3 degrees to 5 degrees C under a variety of changing meteorological conditions.

Seasat scanning multichannel microwave radiometer: results of the Gulf of Alaska workshop : Lipes, R. G. et al., 1979. Science, 204(4400): 1415–1417

Seasat scanning multichannel microwave radiometer: results of the Gulf of Alaska workshop : Lipes, R. G. et al., 1979. Science, 204(4400): 1415–1417