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《Transactions of Atmospheric Sciences》 2018-01
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Soil moisture retrievals from FY-3B satellite microwave brightness and comparative analyses over China

WANG Guojie;XUE Feng;CHYI Dorina;TAN Yan;LOU Dan;LIU Yi;School of Geographical Sciences/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science & Technology;School of Atmospheric Sciences,Nanjing University of Information Science & Technology;International Center for Ecology Meteorology and Environment,School of Applied Meteorology,Nanjing University of Information Science & Technology;  
Soil moisture is a significant variable in environmental studies( e.g.,numerical weather prediction,modeling climate change, runoff prediction,drought monitoring,etc.).It is the most important terrestrial water resource for ecosystems.In addition, the soil moisture correlates terrestrial water and energy cycles, thus, this is an important predictor for hydro meteorological processes( e. g.,precipitation,drought,and heat waves).However,currently,soil moisture is not defined as a conventional component for meteorological monitoring.Therefore, several studies were carried out which often didn't contain the soil moisture observations over the globe.In recent years, the soil moisture retrieval using satellite remote sensing technology has been widely developed.Microwave remote sensing of soil moisture has received great attentions and has shown appropriate prospects due to a number of unique features(e.g., its atmospheric transparency and sensitivity to surface soil moisture).The Fengyun-3B( FY-3B) satellite,which is managed by China Meteorological Administration( CMA),and FY-3B is the second satellite in the third Fengyun series,carrying a multi-frequency passive microwave radiometer on-board,the MicrowaveRadiation Imager( MWRI).Based on the brightness temperature observations of MWRI,we attempted to retrieve the land surface soil moisture for the period of July 2011 to July 2014 over China.This was done using a radiative transfer model as well as further estimations of the soil moisture's systematic errors.Next, the soil moisture retrievals are compared with in-situ soil moisture observations obtained from the CMA's agrometeorological monitoring network, in addition to the reanalyzed soil moisture data derived from ERA-Interim and National Centers for Environmental Prediction( NCEP) databases.The spatial distribution of FY-3B soil moisture appears increasing gradually from the northwest to the northeast and southeast of China, that agrees well with the in-situ observations as well as reanalysis soil moisture. The systematic errors of FY-3B retrievals are closely correlated with the vegetation density.Where it is densely become vegetated, the systematic errors are largest( e.g.,o ver the southwest of China),while the systematic errors are smaller where it is drier.In terms of the seasonal variations,FY-3B soil moisture agrees well with the in-situ agrometeorological observations.In most parts of the China territory, these two datasets confirm a positive correlation; it demonstrates that the FY-3B soil moisture captures the properties of temporal variations in China,g enerally representing that the soil moisture is relatively high in winter,while there is a decline in spring and summer seasons.However, soil moisture declining in Spring and Summer is not surprising the scholars due to the terrestrial water balance.In warm seasons, there are more precipitations than in cold seasons; however,evapo transpiration is largely increased in warm seasons due to the increasing the atmospheric demand of water vapor.Therefore, in warm seasons,water stored in soil becomes even less than that of in cold seasons.This agreement with in-situ observations demonstrates that the obtained FY-3B soil moisture is quite plausible in terms of temporal variations.However,FY-3B soil moisture shows a strong but negative correlation with ERA-Interim and NCEP reanalyzed soil moisture where soil is relatively wet, reaching -0. 8 ~-0. 5 in south of China as well as some parts of northeast of China,where the reanalyzed data are negatively correlated with in-situ observations as well.This negative correlation is mainly attributed to the differences in seasonal variations where the climate is relatively wet; in such regions, the reanalyzed soil moisture data shows a relatively dry winter and wet summer, in contract with that of the in-situ observations.Eventually, it can be concluded that the reanalyzed soil moisture data derived from ERA and the NCEP contain considerable uncertainties in wet climate regions; consequently,those data should be used with great caution.In terms of the temporal variations, the achieved FY-3B soil moisture observations are prior to the reanalyzed soil moisture data.
【Fund】: 国家重点研发计划项目(2017YFA0603701);; 国家自然科学基金资助项目(41561124014;91337108;41375099)
【CateGory Index】: S152.71
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