Research On The Correction Of Water Body Effect On Microwave Brightness In The Qinghai-Tibet Plateau

Passive microwave remote sensing is currently the main method for large-scale observation of surface soil moisture.It has the characteristics of wide spatial coverage,high temporal resolution,and high sensitivity to changes in surface soil moisture.However,due to its low spatial resolution,it is close to the coast or near the coast.In inland water bodies,the signals measured by passive microwave radiometers include radiation from land and water.The SMAP(Soil Moisture Active Passive)satellite mission aims to provide near-surface soil moisture measurement with a target accuracy of 0.04 m3/m3.Without careful consideration in the algorithm,the presence of a fixed water body within the field of view of about 40 km from the sensor will lower the observed brightness temperature,which will lead to an overestimation of the soil moisture retrieval results.Therefore,the water body in the microwave brightness temperature observation is corrected.The influence of,plays an important role in improving the accuracy of passive microwave inversion of soil moisture.The Qinghai-Tibet Plateau(QTP)is known as the"Asian Water Tower"and it is also an area with dense lakes in my country.With its unique topography,the QTP has become a sensitive area of global climate change.Obtaining large-scale and high-precision soil moisture information in the QTP is vital to the study of global climate change.Using microwave remote sensing data to retrieve the soil moisture in the QTP,the widely distributed water bodies will definitely affect the brightness temperature value observed by the sensor,making the soil moisture retrieval results have errors.Research on the correction of the influence of microwave bright temperature on the water body in the QTP is of practical significance for improving the accuracy of passive microwave inversion of soil moisture in the QTP.Based on the mixed pixel water body correction formula,this paper proposes a simulation-based water body correction method.By simulating the brightness temperature of the water body and calculating the water fraction of the water-containing pixel,the microwave brightness temperature water body influence correction is carried out.In this paper,96 lakes on the QTP with an area greater than80km~2are selected,and the effects of lake surface temperature,salinity,wind speed and other factors on the surface emissivity of the water body are considered.Based on the radiation transmission model,the surface emissivity of the water body is obtained and the lake brightness temperature simulation is performed.The results show that the simulated water body brightness temperature has obvious seasonal changes,which are mainly affected by its temperature.The horizontally polarized brightness temperature is more sensitive to wind speed changes.In addition,the lake brightness temperature is negatively correlated with the lake salinity.As the salinity increases,the brightness temperature decreases.Based on a 500-meter resolution data set of daily distribution of surface water on global inland land,this paper uses a moving window algorithm to calculate the fraction of water within the SMAP observation footprint.Based on the simulated water body brightness temperature and the calculated water fraction results,the SMAP microwave brightness temperature data of the QTP was corrected for water body impact.The results showed that the low brightness temperature area caused by the existence of the water body was eliminated after the correction;The boundaries of lakes on the QTP are also clearer.At the same time,the dependence of land brightness and temperature on moisture is eliminated after correction.In order to further verify the correction algorithm,this paper selects soil moisture and temperature monitoring network in the central QTP of Naqu,and uses a single-channel soil moisture retrieval algorithm.Validation results demonstrated that the simulation-based approach can effectively remove the water contamination from SMAP data with improved soil moisture retrievals.The proposed simulation-based water contamination correction method is an effective algorithm for improving SMAP TB and soil moisture retrievals near inland water bodies over the QTP.