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COMPUTATION AND USE OF THE REFLECTIVITY AT 3.75 [\M
FROM AVHRR THERMAL CHANNELS.
J.C. ROGER and E. F. VERMOTE
NASA Goddard Space Flight Center - Code 923
Greenbelt, MD 20771, USA
ABSTRACT:
Global study of land surface properties uses AVHRR channels 1 and 2 but channel 3 may be of interest too but
its use requires preprocessing. It consists of both a reflective part and an emissive part, the former can be derived
from T 3 , T 4 and T 5 . Moreover, since the water vapor affects channel 3, its content is retrieved from the channel
4 and 5 using the Split Window Technique. A formula of reflective part retrieval at 3.75 (im is first tested in the
case of sunglint observations where the emissivities of channels 4 and 5 can be set to the unity. In a second part
of the formula is adapted and validated to land surface using the FIFE-87 data set. The last section is devoted to
some preliminary applications of the reflectance at 3.75 |im to the studies of surface properties retrieval, aerosol
retrieval over land and desertic aerosol retrieval.
KEY WORDS: Reflectivity at 3.75 (xm, Water vapor retrieval, Thermal channels, AVHRR
1 - INTRODUCTION
For the last 25 years, data of the Earth-Atmosphere system has been continuously acquired from the Advanced
Very High Resolution Radiometers (AVHRR) on-board NOAA polar orbiting satellites. Since the AVHRR
carries visible, infrared and thermal bands, and since a complete coverage of the Earth is available twice daily
with 2 resolutions (1 and 4 km), and from two platforms, the data represent a potentially valuable source of
information. This allows for the first time a possible monitoring of the ocean, terrestrial and atmospheric
processes at local and global scales.
Over ocean extensive uses have been done using channels 4 and 5 for the determination of the Sea Surface
Temperature [Prabhakara et al., 1974; Me Millin, 1975; for the first ones], or channel 1 for aerosol optical
thickness retrieval [Rao et al., 1989]. Over land, the principal use of AVHRR has been to combine the channels
1 and 2 to derive the Normalized Difference Vegetation Index (NDVI) [Tucker et al, 1981; Gatlin et al., 1983;
Justice et al., 1985] and the quantitative estimate of biomass production [Daughtry et al., 1983; Tucker et al,
1983 and 1985]. Channel 3 has been used to assess deforestation and biomass burning [Malingreau et al., 1989;
Setzer, 1991]. In some recent papers, understanding of the signal measured by channels 4 and 5 over land has
been demonstrated [Becker and Li, 1990; Kerr et al., 1992; Ottld and Vidal-Madjar, 1992; Seguin et al., 1992].
The Challenge in using channel 3 is the decoupling between thermal emission and reflective properties of the
target. It is tempting to try to combine the channels 4, 5 and 3 to derive quantitative estimate of the reflectance at
3.75pm. The use of the derived reflectance is important for AVHRR aerosol retrieval method over land, as
shown by Vermote et al. (1993), but could be also interesting for sunglint correction over ocean, studies of
bidirectionnal reflectances or desertics aerosol outbreaks.
In this paper, we try to validate a formula of reflectance at 3.75 pm based on the channels 3, 4 and 5, NDVI
observations and MODTRAN simulations. Because channel 3 is sensitive to water vapor, the problem of water
vapor retrieval, both over land and sea, is also examined using one year of SSM/I water vapor retrieval, AVHRR
data and sunphotometer measurements. In a second part, we validate the formula in the case of sunglints
observations where the emissivities of channels 4 and 5 can be set to 1.00. In a third part, the reflectance over
land is retrieved and compared to indirect measurements performed during FIFE-87 experiment. In this case, the
emissivities of the channels 4 and 5 are no longer equal to 1.00 but can be estimated using NDVI. The latest part
presents applications of the reflectance formula for studies of surface properties retrieval, aerosol retrieval over
land and desertic aerosol studies.
