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MICROWAVE RADIOMETER OBSERVATIONS IN HAPEX-SAHEL
T.J. SCHMUGGE
USDA Hydrology Lab, Beltsville MD USA
Currently at INRA Unite de Bioclimatologie, Monfavet FRANCE
Y. KERR
LERTS, CNES, Toulouse, FRANCE
HAPEX-Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel) was an international
program focused on the soil-plant-atmosphere interactions in the West African sahelian zone.
Its objective is to improve parameterizations of the land atmosphere interactions at the GCM
grid scale. It was performed in a 1 ° x 1 ° area of west Niger over a 3-4 year period with an 8
week intensive observation period (IOP) from August to October 1992. The area selected
was from 2° to 3° east and 13° to 14° north. In this area there is a strong north south
gradient in annual precipitation with an increase of about 1 mm/km as you go from north to
south. (Goutorbe et al., 1994)
During the IOP there were 2 airborne microwave radiometer systems in operation. They
were the five frequency (5 to 90 GHz) PORTOS radiometer on the French ARAT aircraft and
the single frequency (1.42 GHz) multibeam PushBroom Microwave Radiometer (PBMR) on
the NASA C-130. Measurements of the microwave brightness temperatures (T B ) can be used
to infer parameters such as surface soil moisture, surface temperature, and vegetation biomass.
The lower frequencies are best suited for the soil moisture sensing, while the higher ones are
useful for surface temperature determinations; vegetation information can be obtained from
combinations of frequencies..
The PBMR has been in operation for about 10 years now and has been used in number of
earlier field experiments, e.g. FIFE (Wang et al., 1990) and the Monsoon 90 experiment which
was conducted in similar semi-arid condition in the southwest of the US (Schmugge et al.
1994). During the IOP of HAPEX-Sahel the PBMR obtained data from 9 flights over the
square. The PBMR operates at a wavelength of 21 cm (1.42 GHz) and has four horizontally
polarized beams which cover a swath of 1.2 times the aircraft altitude. Measurements of T B at
this wavelength give estimates of the moisture content in the surface 5-cm layer of the soil. In
HAPEX-Sahel two flight patterns were used. In one, four north-south transects were flown at
an altitude of 1500m (5000 feet) from 13° to 14° N to observe any moisture gradients over
the HAPEX square. At this altitude the resolution is -500m. The other pattern was at lower
altitudes, between 250 and 600m (-80 to 200m resolution), where a series of parallel lines
were flown over the east, west and south central sites to map the soil moisture variability. At
the 600m altitude complete coverage of the central sites was obtained, while at the lower
altitude there are gaps between the lines. The GPS (Global Positioning System) data from the
aircraft were used for georeferencing the PBMR data to create the T B images for these sites.
The PBMR data were oversampled to a 50 m grid in the images. The images for the East and
West Central site cover a 30 by 13 km area. There were several rains during the period of
observation so that significant moisture variations were observed with low T B 's of 210 to
220K for the wettest conditions and high values of 280 to 290K for the driest.
This was the first airborne campaign for the PORTOS instrument which had previously been
