ISSN:0143-1161    

DOI:10.1080/01431168908903992

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Radiative transfer models for 37 GHz brightness temperatures are developed under simplifying assumptions and the implications of the models are illustrated and discussed using the Nimbus-7 scanning multichannel microwave radiometer (SMMR) observations of the Earth's surface from January 1979 to December 1985. The difference of vertically and horizontally polarized brightness temperatures (ΔT) decreases with increasing vegetation density (that is, as the canopy water content per unit ground area increases) and this effect of vegetation density on ΔTis illustrated by the time series of ΔTover areas with varied vegetation density and phenology. Furthermore, the ΔTdecreases as the surface roughness increases, and thus mountainous areas within sparsely vegetated areas (like the Sahara) have the appearance of more densely vegetated areas. Also, the ΔTvalues increase when the soil gets wet or flooded, and this effect is illustrated by correlating the temporal variations of ΔTwith the river height for an area within the Amazon Basin. As examples of a more quantitative analysis of the ΔTdata, empirical relations are developed between the integrated-average of the 84 months of ΔTdata and the annual rainfall, primary productivity and actual evaporation. An analytical relation between primary productivity and ΔTis also established from physically based models under simplifying assumptions. Colour-coded maps of primary productivity and actual evaporation based upon the SMMR data are displayed and discussed in relation to other observations. It is concluded that to realize the full potential of the ΔTdata it would be necessary to calibrate and validate this data against ground observations and develop more realistic radiative transfer models in order to gain a better physical understanding.

ISSN:0143-1161    

DOI:10.1080/01431168908903993

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data for a one-year period were compared with contemporaneous, registered data sets of Scanning Multichannel Microwave Radiometer (SMMR) microwave polarization difference temperature (MPDT) for the study of vegetation phenology. Sample areas were selected for the major land-cover types of Africa and South America. Comparison of the data showed the MPDT response to differ considerably from the seasonal pattern exhibited by the NDVI. The MPDT showed a general decrease with increasing vegetation density, whereas the NDVI showed a positive relationship with the amount of green leaf material. In contrast to the NDVI, the MPDT showed a generally poor response to seasonal variations in green leaf material. Only those vegetation types exhibiting a substantial seasonal variation in the areal extent of vegetated cover showed strong seasonality in the MPDT data, e.g. annual grasslands. Several cover types with large proportions of trees and shrubs displayed seasonal variation in the NDVI but no equivalent change in MPDT. The previously stated relationship between leaf water content and MPDT is brought into question. Emphasis must now be placed on determining the relationship between MPDT and vegetation structure at 37 GHz, in particular through field measurement programmes.

ISSN:0143-1161    

DOI:10.1080/01431168908903994

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Images using reflected visible and near-infrared data and images using passive microwave data were compared in terms of their usefulness for characterizing land-cover types in South America and Africa. The former images are of the normalized difference vegetation index (NDVI) subsampled to approximately 15-20 km resolution in the NOAA global vegetation index product. The latter images of the microwave polarization difference temperature (MPDT) are derived from the difference between horizontally and vertically polarized radiation in the 37 GHz band. Results of maximum-likelihood classifications applied to multi-temporal data sets indicate that, overall, the NDVI data sets are substantially better than the MPDT data sets for land-cover characterization. However, the greater sensitivity of the MPDT data in semi-arid areas results in their superior performance for some classes in these areas. The combined use of MPDT and NDVI data sets show clear synergistic benefits in using the two data sets. However, the evidence suggests that for most cover types, increasing the temporal frequency of the NDVI images is more advantageous than incorporating MPDT data sets.

ISSN:0143-1161    

DOI:10.1080/01431168908903995

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Monthly 37 GHz microwave polarization difference temperatures (MPDT) derived from the Nimbus-7 scanning multichannel microwave radiometer (SMMR) for southern Africa from 1979 to 1985 are compared with rainfall and Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data. MPDT rose sharply during a drought episode which occurred within the period included in the data. The rise was seen not only in the growing season, but also in the dry season MPDT when no actively photosynthetic, water-containing leaves are present. The results suggest that scattering of the emitted microwave radiation by dead and living vegetation is a more important factor than has previously been recognized. The sensitivity of MPDT to small quantities of dry vegetation encourages the hope that standing dead vegetation and plant litter may be remotely sensed. In the absence of vegetation, rough terrain reduced the MPDT whereas a damp surface increased it.

ISSN:0143-1161    

DOI:10.1080/01431168908903996

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Coincident Scanning Microwave Multi-channel Radiometer 37 GHz and Advanced Very High Resolution Radiometer normalized difference vegetation index satellite data have been compared from drought-affected regions of sub-Saharan Africa and northeastern Brazil for the time period of 1980–1985. Although the two satellite data types can be highly correlated, differences between them were found for the Sahel zone in 1985 and for northeastern Brazil from 1984–1985. These findings suggest that scattering or surface roughness contributions may be greater than previously assumed for the 37 GHz microwave data. A programme of field measurements should be undertaken to increase our understanding of natural vegetation at 37 GHz and higher microwave frequencies.

ISSN:0143-1161    

DOI:10.1080/01431168908903997

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The microwave polarization difference temperature (MPDT) prepared from 37 GHz Scanning Multifrequency Microwave Radiometer (SMMR) data from the Nimbus-7 satellite allows a unique vision of hydrological features, especially of humid areas. From the series of monthly images from 1979 to 1985 the major hydrological features of four major river basins of South America are briefly examined. These include rivers of the Amazon, La Plata, Orinoco and Sao Francisco basins, wetlands in the La Plata and Amazon basins and floods in the La Plata basin. Many hydrological features appear much as they do on conventional maps. An interesting perspective can be derived from the ways in which they differ from conventional maps.

ISSN:0143-1161    

DOI:10.1080/01431168908903998

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Comparison between the microwave polarized difference temperature (MPDT) derived from 37 GHz band data and the normalized difference vegetation index (NDVI) derived from near-infrared and red bands, from several empirical investigations are summarized. These indicate the complementary character of the two measures in environmental monitoring. Overall the NDVI is more sensitive to green leaf activity, whereas the MPDT appears also to be related to other elements of the above-ground biomass. Monitoring of hydrological phenomena is carried out much more effectively by the MPDT. Further work is needed to explain spectral and temporal variation in MPDT both through modelling and field experiments.

ISSN:0143-1161    

DOI:10.1080/01431168908903999

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Multispectral data from the Advanced Very High Resolution Radiometer (AVHRR) were digitally processed and merged with Scanning Multichannel Microwave Radiometer (SMMR) imagery. Five channels of AVHRR data, four channels of SMMR brightness temperatures and SMMR-derived ice concentration and ice type were co-registered to a polar stereographic grid. The merged data sets are currently being used in combination with meteorological information for integrated studies of clouds and sea ice.

ISSN:0143-1161    

DOI:10.1080/01431168908904000

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor