Research On Multiband Microwave Radiative Properties Of Forest Litter

Water cycle factors are crucial center belt which it acts as link between six circles includes geosphere,atmosphere,hydrosphere,cryosphere,biosphere and anthroposphere.In the terrestrial water cycle,the key factors are soil moisture and snow parameters.So it is very important to obtain the factors precisely.Passive microwave remote sensing is the main way to obtain macro scale water cycle elements.However,owing to the complex structure of forest systems,the low inversion accuracy of surface parameters does not meet the needs of the water cycle model.The layer of litter covering forest soils attenuates microwave radiation coming from soil.In satellite remote sensing data this reduces the sensitivity of brightness temperature to land surface parameters(e.g.,soil moisture,snow depth,and snow water equivalent),resulting in poorer inversion accuracy.To quantify the effects of microwave radiative properties of litter at different band frequencies,especially on transmissivity,a novel approach was developed for modelling radiative transfer(RT)through litter.This approach is based on a zero-order RT model that accounts for scattering effects(the ?-? model).Controlled and theoretically derived experiments were conducted to obtain brightness temperatures at several band frequencies(1.4 GHz,18.7 GHz,and 36.5 GHz)as affected by the thickness and moisture content of the litter.The effects of measurement errors on transmissivity were then evaluated.This novel method,which is not only based on sound theory but can also prevent calibration errors,can be used to obtain parameters such as the extinction coefficient and transmissivity.In order to study transmissivity of litter in frozen state,the transmissivity difference between frozen and unfrozen litter layer is analyzed by Controlled experiments.The emissivity of litter at L band is also analyzed based on radiative transfer theory without scattering.The results of this study provide new insights into the microwave RT theory of forest systems,allowing for more appropriate brightness temperatures corrections for satellites data,and providing a guide for controlled experiments.The main conclusions are as follows:(1)In any frequency,the dielectric constant increased with moisture content.At L band,the real part is much larger than the imaginary part.And real part changes obviously,which llustrates dielectric loss is small.At K band,the trend of real and imaginary part is similar.Moreover,the real part is larger than the imaginary.At Ka band,the imaginary part is sensitive to moisture because of larger dielectric loss.Coherent radiation at L band causes large error of imaginary part because of relationship between wavelength and thickness of litter,namely the quarter.The difference of dielectric constant in frozen and unfrozen state is obvious.This is because dielectric properties of water and ice are different.So that transmissivities in the states are not coincident,the maximum difference 0.36.(2)The coefficients for the Ka and K bands increased sharply at moisture contents of approximately 14% and 20%,respectively,reflecting the significant increase in dielectric loss at relatively high moisture contents.However,for moisture contents exceeding 34.7% and 53.4% at the Ka and K band,respectively,the coefficients were no longer considered accurate owing to the dominance of water radiation.A sharp decline in transmissivity occurred at a moisture content of 14% for the Ka band and at 20% for the K band.In addition,the transmissivities were relatively high at low moisture contents but declined abruptly with increases in humidity and thickness.Under natural conditions,the moisture content of litter usually varies between 10% and 50%,while thickness varies between 5 and 10 cm;therefore,the results confirm that the effects of litter cannot be ignored.Transmissivities decreased linearly with increasing water content at L band.For very dry litter,the transmissivity was high,but relatively insensitive to thickness;sensitivity to thickness increased with increasing water content(The lack of sensitivity to thickness for dry conditions is important).The linear relationship established between transmissivity and water content in this study provided a tool for correcting brightness temperatures from satellite data.(3)The emissivity and weight moisture content have the same trend,especially in condition of daily observations,which owns to poor heterogeneity of litter.Emissivities various from 0.1 to 0.4.Even drier litter,the emissivity is 0.1,so that its radiation can not be ignored.