The ability to correct for the influence of forest cover is crucial for retrieval of surface geophysical parameters such as snow cover and soil properties from microwave remote sensing.Existing correction approaches t...The ability to correct for the influence of forest cover is crucial for retrieval of surface geophysical parameters such as snow cover and soil properties from microwave remote sensing.Existing correction approaches to brightness temperatures for northern boreal forest regions consider forest transmissivity constant during wintertime.However,due to biophysical protection mechanisms,below freezing air temperatures freeze the water content of northern tree species only gradually.As a consequence,the permittivity of many northern tree species decreases with the decrease of air temperature under sub-zero temperature conditions.This results in a monotonic increase of the tree vegetation transmissivity,as the permittivity contrast to the surrounding air decreases.The influence of this tree temperature-transmissivity relationship on the performance of the frequency difference passive microwave snow retrieval algorithms has not been considered.Using ground-based observations and an analytical model simulation based on Mätzler’s approach(1994),the influence of the temperaturetransmissivity relationship on the snow retrieval algorithms,based on the spectral difference of two microwave channels,is characterized.A simple approximation approach is then developed to successfully characterize this influence(the RMSE between the analytical model simulation and the approximation approach estimation is below 0.3 K).The approximation is applied to spaceborne observations,and demonstrates the capacity to reduce the influence of the forest temperature-transmissivity relationship on passive microwave frequency difference brightness temperature.展开更多
Reliablemicrostructuremeasurement of snow is a requirement for microwave radiative transfer model validation.Snow specific surface area(SSA)can be measured using stereological methods,in which snow samples are cast in...Reliablemicrostructuremeasurement of snow is a requirement for microwave radiative transfer model validation.Snow specific surface area(SSA)can be measured using stereological methods,in which snow samples are cast in the field and photographed in the laboratory.Processing stereology photographs manually by counting intersections of test cycloids with air-ice boundaries reduces the problems in binary segmentation.This paper is a case study to evaluate the repeatability of the manually stereology interpretation by two independent research groups.We further assessed how uncertainty in snow SSA influences simulated brightness temperature(TB)driven by the Microwave Emission Model of Layered Snowpacks(MEMLS),and how stereology compares to Near Infrared(NIR)camera and hand lens.Data was obtained from two alpine snow profiles from Steamboat Springs,Colorado.Results showed that stereological SSA values measured by two groups are highly consistent,and the ground radiometer measured T_(B)at 19 and 37 GHz was successfully predicted(RMSE<3.8 K);simulations using NIR SSA and hand-lens geometric grain size(Dg)measurements have larger errors.This conclusion was not sensitive to uncertainty in the free parameters of TB modeling.展开更多
基金The research leading to these results has received fundings from Japan Aerospace Exploration Agency[contract number 20RT000300]the National Key Research and Development Program of China[grant number 2017YFE0111700]+1 种基金the NSERC Discovery Grant[grant number RGPIN-2017-04385]INTERACT(International Network for Terrestrial Research and Monitoring in the Arctic)under the European Union H2020 Grant Agreement[grant number 730938].
文摘The ability to correct for the influence of forest cover is crucial for retrieval of surface geophysical parameters such as snow cover and soil properties from microwave remote sensing.Existing correction approaches to brightness temperatures for northern boreal forest regions consider forest transmissivity constant during wintertime.However,due to biophysical protection mechanisms,below freezing air temperatures freeze the water content of northern tree species only gradually.As a consequence,the permittivity of many northern tree species decreases with the decrease of air temperature under sub-zero temperature conditions.This results in a monotonic increase of the tree vegetation transmissivity,as the permittivity contrast to the surrounding air decreases.The influence of this tree temperature-transmissivity relationship on the performance of the frequency difference passive microwave snow retrieval algorithms has not been considered.Using ground-based observations and an analytical model simulation based on Mätzler’s approach(1994),the influence of the temperaturetransmissivity relationship on the snow retrieval algorithms,based on the spectral difference of two microwave channels,is characterized.A simple approximation approach is then developed to successfully characterize this influence(the RMSE between the analytical model simulation and the approximation approach estimation is below 0.3 K).The approximation is applied to spaceborne observations,and demonstrates the capacity to reduce the influence of the forest temperature-transmissivity relationship on passive microwave frequency difference brightness temperature.
基金supported by NASA Terrestrial Hydrology Program[grant number NNX09AM10G]Strategic Priority Research Program of Chinese Academy of Sciences[grant number XDA20100300].
文摘Reliablemicrostructuremeasurement of snow is a requirement for microwave radiative transfer model validation.Snow specific surface area(SSA)can be measured using stereological methods,in which snow samples are cast in the field and photographed in the laboratory.Processing stereology photographs manually by counting intersections of test cycloids with air-ice boundaries reduces the problems in binary segmentation.This paper is a case study to evaluate the repeatability of the manually stereology interpretation by two independent research groups.We further assessed how uncertainty in snow SSA influences simulated brightness temperature(TB)driven by the Microwave Emission Model of Layered Snowpacks(MEMLS),and how stereology compares to Near Infrared(NIR)camera and hand lens.Data was obtained from two alpine snow profiles from Steamboat Springs,Colorado.Results showed that stereological SSA values measured by two groups are highly consistent,and the ground radiometer measured T_(B)at 19 and 37 GHz was successfully predicted(RMSE<3.8 K);simulations using NIR SSA and hand-lens geometric grain size(Dg)measurements have larger errors.This conclusion was not sensitive to uncertainty in the free parameters of TB modeling.
