Surface-based Microwave and Millimeter wave Radiometric Remote Sensing of the Troposphere : a Tutorial

Surface-based radiometric sensing of tropospheric parameters has a long history of providing useful measurements of temperature, water vapor, and cloud liquid. In this tutorial, a general overview of physical fundamentals, measurement techniques, and retrieval methodology is given. Then several contemporary instruments are discussed and representative results are presented. Recent and promising developments include multi-frequency radiometers, scanning observations of clouds, and combined active-passive remote sensing. The primary applications of these new technologies are weather forecasting and climate, communications, geodesy and long-baseline interferometry, satellite data validation, air-sea interaction, and fundamental molecular physics. Introduction A more extensive review is given in [11] and some of the material in this tutorial has been extracted from this document. 16 IEEE Geoscience and Remote Sensing Society Newsletter • March 2005 EDUCATIONAL TUTORIAL Surface-based Microwave and Millimeter wave Radiometric Remote Sensing of the Troposphere: a Tutorial Ed R. Westwater Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA Environmental Technology Laboratory 325 Broadway MS R/E/ET1, Boulder, CO 80305 USA Tel: 303-497-6527 FAX: 303-497-3577 email: Ed.R.Westwater@noaa.gov http://www.etl.noaa.gov/~ewestwater Susanne Crewell Meteorologisches Institut, Universitaet Muenchen Theresienstr. 37 80333 Muenchen,Germany Tel: +49 (0) 89 / 2180-4210 FAX: +49 (0) 89 / 2805508 email: CREWELL@meteo.physik.uni-muenchen.de http://www.meteo.physik.uni-muenchen.de/ Christian Mätzler Institute of Applied Physics, University of Bern Sidlerstr. 5, CH-3012 Bern, Switzerland Tel:: +41 31 631 45 89 FAX: +41 31 631 37 65 email: matzler@iap.unibe.ch, http://www.iapmw.unibe.ch 2. General Physical Principles The basic ideas of radiative transfer and thermal emission are given in [12] and their application to microwave radiometric remote sensing is outlined in [13] . From the concept of an ideal black body and Kirchoff’s law, it is known that the emission from a black body depends only on its temperature and that the higher the temperature of the body, the more is its emission. The idea is made quantitative by calculating the spectral distribution of a blackbody emission from Planck’s law, which expresses the radiance BV(T) emitted from a blackbody at temperature T and frequency v as Bν(T) = 2hν 3 c2 1 (exp(hν/kT) − 1) , (1) where h = Planck’s constant, and k = Boltzman’s constant. The radiance expresses the emitted power per unit projected area per unit solid angle per unit frequency interval. The second consideration is to relate the emission from a real body, sometimes called a “grey” body, to that of a blackbody at the same temperature. If the fraction of incident energy from a certain direction absorbed by the grey body is A(v), then the amount emitted is A(v) BV(T). For a perfectly reflecting or transmitting body, A(v) is zero, and incident energy may be redirected or pass through the body without being absorbed. In the situation considered in this tutorial, namely upward-looking radiometers viewing a non-scattering medium, the equation that relates our primary observable, brightness temperature, Tb, to the atmospheric state is the radiative transfer equation (RTE) [13] Bν(Tb) = Bν(Tc ) exp(−τν)+ + ∫ ∞ 0 Bν(T(s))αν(s) exp(− s ∫ 0 αν(s′)d s′)d s, (2a) where s = path length in km, T(s) = Temperature (K) at the point s, Tc = Cosmic background brightness temperature of 2.75 K, Tv = opacity = total optical depth along the path s