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first deployment of a ground-based instrument to retrieve
ATMOSPHERIC OPTICAL PARAMETERS AND SURFACE BRDF DURING THE
HAPEX-SAHEL EXPERIMENT, NIGER 1992
A.K. WILSON
British National Space Centre,
Remote Sensing Applications Development Unit,
Monks Wood, Abbots Ripton, Huntingdon
Cambridgeshire. PE 17 2LS (UK)
ABSTRACT:
This paper reports on the first field deployment of a ground-based instrument: The Auto-Tracking Land and
Atmosphere Sensor (ATLAS) in support of surface radiation budget estimation during the second HydrologicalAtmospheric
HydrologicalAtmospheric Pilot Experiment (HAPEX-Sahel). A part of the experiment required validation data for joint
retrievals of surface albedo and atmospheric parameters from multi-angle remotely sensed data. These were
based on co-ordinated ground, aircraft and satellite measurements that were carried out in Niger, W. Africa
during the Intensive Observing Period GOP) in August and September 1992. The design criteria for a single
instrument capable of retrieving a range of atmospheric optical parameters and acquiring multispectral
directional surface reflectance from the local cover types is outlined and examples of the datasets presented. The
current status of database construction, radiometric and spectral calibration, and preliminary modelling is
discussed with emphasis on the potential for retrieval of atmospheric optical parameters. Further enhancements
to the instrument are being made in the light of this initial field deployment and future research plans are
¡resented.
KEYWORDS: HAPEX-Sahel, Surface Radiation Budget, Atmospheric Optical Parameters, Surface BRDF
1. INTRODUCTION
HAPEX-Sahel is one of a series of international experiments under the WCRP-IGBP Joint Working Group on
Land Surface Experiments and is the second in the Hydrological Atmospheric Pilot Experiment (HAPEX)
series on large scale energy, water and carbon balance. The experiment took place during the summer of 1992
in the Sahelian region of Niger, W. Africa (Goutourbe et al, 1993a and 1993b). The overall objective of the
experiment is to improve the parameterization of semi-arid land surface processes in General Circulation Models
(GCMs) by a combination of low intensity long term monitoring of a large GCM scale area and intensive
observations at a key period of the year at a few representative locations. An Intensive Observation Period GOP)
took place between 17 August and 10 October 1992 with three 10 km by 10 km super-sites situated within the
100km by 100km study area. The entire programme consisted of work on ground water recharge, unsaturated
zone soil moisture, vegetation water use, micrometeorology, atmospheric boundary layer sounding and both
aircraft and satellite remote sensing. One particular research project carried out at the southern (UK) super-site,
Estimation of the Surface Radiation Budget using Multi-Look Remotely Sensed Data" is based on the retrieval
of surface albedo for representative surface cover-types and the use of remotely sensed airborne and satellite
data to provide aggregation of local measurements to sub-site, super-site and GCM scales. To validate the joint
retrieval of the surface albedo and the atmospheric loading from the remotely sensed data, both the directional
reflectance properties of the characteristic cover-types and the atmospheric optical parameters, defining the
irradiance field incident at the surface, are required. 1
1 INSTRUMENTATION AND METHODS
2.1 The Auto-Tracking Land and Atmosphere Sensor (ATLAS)
An instrument being developed by the Remote Sensing Applications Development Unit (RSADU) in support of
research on retrieval of atmospheric optical parameters and based on the only instantaneous recording
