Microwave vegetation optical depth and inverse modelling of soil emissivity using Nimbus/SMMR satellite observations期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Microwave vegetation optical depth and inverse modelling of soil emissivity using Nimbus/SMMR satellite observations

Authors:	A A van de Griend M Owe

Institution:	(1) Present address: Institute of Earth Sciences, Vrije Universiteit, de Boelelaan 1085, NL-1081 HV Amsterdam, The Netherlands;(2) Present address: Hydrological Sciences Branch, NASA/GSFC, Code 974, 20771 Greenbelt, MD, USA

Abstract:	Summary A radiative transfer model has been used to determine the large scale effective 6.6 GHz and 37 GHz optical depths of the vegetation cover. Knowledge of the vegetation optical depth is important for satellite-based large scale soil moisture monitoring using microwave radiometry. The study is based on actual observed large scale surface soil moisture data and observed dual polarization 6.6 and 37 GHz Nimbus/SMMR brightness temperatures over a 3-year period. The derived optical depths have been compared with microwave polarization differences and polarization ratios in both frequencies and with Normalized Difference Vegetation Index (NDVI) values from NOAA/AVHRR. A synergistic approach to derive surface soil emissivity from satellite observed brightness temperatures by inverse modelling is described. This approach improves the relationship between satellite derived surface emissivity and large scale top soil moisture fromR ²=0.45 (no correction for vegetation) toR ²=0.72 (after correction for vegetation). This study also confirms the relationship between the microwave-based MPDI and NDVI earlier described and explained in the literature.List of Symbols f frequency Hz] - f _i(p) fractional absorption at polarizationp - h surface roughness - h h cos² - H horizontal polarization - n _i complex index of refraction - p polarization (H orV) - R _s microwave surface reflectivity - T _B(p) brightness temperature at polarizationp - T ^* normalized brightness temperature - T polarization difference (T _v-T _H) - T _s temperature of soil surface - T _c temperature of canopy - T _max daily maximum air temperature - T _min daily minimum air temperature - V vertical polarization - soil moisture distribution factor; also used for the constant to partition the influence of bound and free water components to the dielectric constant of the mixture - empirical complex constant related to soil texture - microwave transmissivity of vegetation (=e ^–) - ^* effective transmissivity of vegetation (assuming =0) - microwave emissivity - _s emissivity of smooth soil surface - _rs emissivity of rough soil surface - _vs emissivity of vegetated surface - soil moisture content (% vol.) - K dielectric constant F·m^–1] - K _fw dielectric constant of free water F·m^–1] - K _ss dielectric constant of soil solids F·m^–1] - K _m dielectric constant of mixture F·m^–1] - K _o permittivity of free space 8.854·10^–12 F·m^–1] - $\kappa _{w^x }$ high frequency limit ofK _wf F·m^–1] - wavelength m] - incidence angle degrees from nadir] - polarization ratio (T _H/T _V) - b soil bulk density gr·cm^–3] - s soil particle density gr·cm^–3] - R surface reflectivity in red portion of spectrum - NIR surface reflectivity in near infrared portion of spectrum - _eff effective conductivity of soil extract mS·cm^–1] - vegetation optical depth - _6.6 vegetation optical depth at 6.6 GHz - ₃₇ vegetation optical depth at 37 GHz - ^* effective vegetation optical depth (assuming =0) - single scattering albedo of vegetation With 12 Figures

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