Retrieval of land surface temperature and emissivity from satellite data: Physics, theoretical limitations and current methods Prasanjit Dash1, Frank-M. Göttsche, Folke-S. Olesen and Herbert Fischer Institute for Meteorology and Climate Research Forschungszentrum Karlsruhe/Karlsruhe University Postfach 3640, D-76021 Karlsruhe, Germany ABSTRACT Remote sensing from satellites is the only means to obtain Land Surface Temperature (LST) and emissivity on a larger scale. LST has many applications, e.g., in radiation budget experiments and global warming, and desertification studies. Over the last decades, substantial amount of research was dedicated towards extracting LST and emissivity from surface- leaving radiance and de-coupling the two from each other. This paper provides the physical basis, discusses theoretical limitations, and gives an overview of the current methods for space-borne passive sensors operating in the infrared range, e.g., NOAA-AVHRR, Meteosat, ERS-ATSR, TERRA-MODIS, and TERRA-ASTER. Atmospheric effects on estimated LST are described and atmospheric-correction using a Radiative Transfer Model (RTM) is explained. The methods discussed are the single channel method, the Split Window Techniques (SWTs), and the multi-angle method.
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Correspondence: E-mail:
[email protected] Life-Member (ISRS); paper presented at 14th Annual Convention of ISRS, 21-22 Nov.2000, IIT Kanpur, Kanpur, India.
1.
INTRODUCTION
Land Surface Temperature (LST) is the temperature measured at surface level and can be regarded as the skin temperature of the ground. However, the ground of the Earth is far from being a skin or a homogenous surface: it is composed of different materials of varying geometry, which complicates the estimation of LST (Becker and Li 1995, Qin and Karnieli 1999). The LST of densely vegetated ground is the vegetation canopy surface temperature whereas for sparsely vegetated areas it is the average temperature of the vegetation canopy, the vegetation body, and the ground. Also, the surface is usually highly inhomogeneous at satellite spatial resolution. Hence, LST is defined as the average of the temperatures of the surface types in each pixel, weighted by their fractional cover (Kerr et al. 1992). Direct comparisons of LST estimated from radiance measurements with thermodynamic (thermometer) measurements are difficult, because thermometers are only in contact with a small area and, therefore, require a clear definition of ‘surface’. LST derived from emitted radiation represents the integrated effect of the whole target, and, thus does not need a clear definition of ‘surface’. However, practical complications for comparison with thermometric measurements arise due to the following limitations of satellite sensors: (a) limited spectral range (b) limited viewing angle (c) atmospheric attenuation (d) emission depends on emissivity, i.e., non-black body behaviour of natural surfaces, and (e) reflected (also due to non-black body behaviour) down-welling irradiance (Dash et al. 2001). A substantial amount of research was dedicated to overcome these limitations and, thus, estimate LST using space radiometry. This paper summarises the physics (section 2) and discusses the suitable frequency range of electromagnetic radiation, section 3 deals with radiative transfer, various practical methods for atmospheric correction are introduced in
section 4, which is followed by some conclusions and a discussion of the ongoing trends in this field of research. 2.
PHYSICAL BASIS OF LST AND EMISSIVITY ESTIMATION
LST can be estimated from the radiation emitted by any body via the inverse of Planck's law. This expresses the fact that the radiative energy emitted by any surface is directly related to its temperature. The signal received by sensors is electromagnetic radiation, which can be quantified; in satellite meteorology, the fundamental quantity is monochromatic radiance R (Kidder and Haar 1995), which is defined as radiant flux (W) per unit area (m2 ) per unit wavelength (µm-1 ) per unit solid angle (sr-1 ) [Wm-2 sr-1 µm-1 ]. R emitted by terrestrial surfaces (grey bodies: ε(λ)
