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USE OF SATELLITE MICROWAVE RADIOMETRY OVER SEMI-ARID AREAS
J.-C. Calvet, J.-P. Jullien, J. Noilhan and J.-P. Goutorbe
Météo-France/CNRM/GMME
42, Av. Coriolis, 31 057 Toulouse Cedex (France)
ABSTRACT
The 19.35 and 37 GHz SSM/I brightness temperatures (TBs) are analysed over central Spain during the june
1991 EFEDA experiment. A simple thresholding on the average TBs displays regional structures which are
consistent with a NOAA/AVHRR-NDVI classification. Atmospheric corrections are performed using the French
weather forecast model outputs. Surface temperature at SSM/I pixel size is obtained from METEQSAT infrared
(IR) TBs. The surface emissivity corresponding to the METEOSAT IR window is estimated from the
NOAA/AVHRR-derived surface temperature (channels 3,4 and 5). The problem with retrieving geophysical
parameters from SSM/I (i.e. canopy structure, surface temperature and near-surface soil moisture) is then
addressed by applying methods which were finalized from a field experiment.
KEYWORDS: passive microwaves, SSM/I, semi-arid areas, surface temperature, soil moisture.
1 - INTRODUCTION
Accurate description of land surface-atmosphere interactions is required for meteorological mesoscale modeling.
Satellite microwave radiometry is likely to gain access to the surface energy and water budget-driven variables
(temperature and water content) at the adequate scale (Schmugge, 1983; Noilhan and Calvet, 1993). However,
temperature and moisture retrieval needs a realistic model of the surface microwave emission. Moreover, one
must check that available channels include enough information to describe the considered surface. Additional
information can possibly be obtained from other wavelengths (from visible to infra-red). Calvet et al. (1994)
describe a case study of geophysical variables retrieval from SMMR data over the Amazon forest.
In order to test such methods over semi-arid areas, the analysis of SSM/I, AVHRR and METEOSAT
data was performed in the framework of the EFEDA campaign (European Field Experiment in a Desertificationthreatened
Desertificationthreatened Area), central Spain, june 1991 (Bolle et al. 1993). In this study we particularly focussed on the
Tomelloso EFEDA central-site which is a rather large, homogeneous vineyard. The other EFEDA central-sites
(Belmonte and Barrax) as well as 11 homogeneously vegetated targets are only briefly examined. Preliminary
results are presented through the three following sections. The second section summarizes the ground and
satellite data basic processing. In the third section, methods for extracting surface variables at high frequencies
are discussed according to PORTOS-93 experiment (Chanzy et al. 1994) preliminary results. The application of
such methods to satellite data analysis over the EFEDA specified targets is presented in the last section.
2 - DATA
2.1. Surface data
In this study we particularly focussed on the Tomelloso (39 <> 10 N, 3°01 W) vineyard site. Field measurements of
soil and vine IR temperature (seperately) were performed. They are available on a 10-minutes basis (de Bruin
1993). The IR effective emissivities (for both surfaces) are determined from near-surface (10 mm) soil
temperature measured by Météo-France/CNRM. Furthermore, Oliver and Sene (1992) presented a detailed
description of the plant growth and soil moisture. The only rains (of june) occured on june 1 and 2. The surface
soil layer dried up in a few days.
2.2. Description of the atmosphere
For the 14 prescribed targets, forecasted air temperature, humidity and pressure profiles were extracted from the
outputs of the Météo-France model. PERIDOT, at 0600 and 1800 UTC. At Tomelloso, surface observations of
the cloud layer is also available. Lastly, ECMWF analysis of rainfalls were used to identify the dry periods over
Spain.
