One-third of the Earth’s land surface experiences seasonal freezing and thawing.Freezing-thawing transitions strongly impact land-atmosphere interactions and,thus,also the lower atmosphere above such areas.Observatio...One-third of the Earth’s land surface experiences seasonal freezing and thawing.Freezing-thawing transitions strongly impact land-atmosphere interactions and,thus,also the lower atmosphere above such areas.Observations of two L-band satellites,the Soil Moisture Active Passive(SMAP)and Soil Moisture and Ocean Salinity(SMOS)missions,provide flags that characterize surfaces as either frozen or not frozen.However,both state transitions—freezing and thawing(FT)—are continuous and complex processes in space and time.Especially in the L-band,which has penetration depths of up to tens of centimeters,the brightness temperature(T_(B))may be generated by a vertically-mixed profile of different FT states,which cannot be described by the current version of the Community Microwave Emission Model(CMEM).To model such complex state transitions,we extended CMEM in Fresnel mode with an FT component by allowing for(1)a varying fraction of an open water surface on top of the soil,and(2)by implementing a temporal FT phase transition delay based on the difference between the soil surface temperature and the soil temperature at 2.5 cm depth.The extended CMEM(CMEM-FT)can capture the T_(B)progression from a completely frozen to a thawed state of the contributing layer as observed by the L-band microwave radiometer ELBARA-III installed at the Maqu station at the northeastern margin of the Tibetan Plateau.The extended model improves the correlation between the observations and CMEM simulations from 0.53/0.45 to 0.85/0.85 and its root-mean-square-error from 32/25 K to 20/15 K for H/V-polarization during thawing conditions.Yet,CMEM-FT does still not simulate the freezing transition sufficiently.展开更多
基金This research was funded by the Deutsche Forschungsgemeinschaft(DFG)via the research group FOR2131 on“Data Assimilation for Improved Characterization of Fluxes across Compartmental Interfaces”,subproject P2,the National Natural Science Foundation of China(Grant 42075150)the Natural Science Foundation of Shanghai(No.21ZR1405500).
文摘One-third of the Earth’s land surface experiences seasonal freezing and thawing.Freezing-thawing transitions strongly impact land-atmosphere interactions and,thus,also the lower atmosphere above such areas.Observations of two L-band satellites,the Soil Moisture Active Passive(SMAP)and Soil Moisture and Ocean Salinity(SMOS)missions,provide flags that characterize surfaces as either frozen or not frozen.However,both state transitions—freezing and thawing(FT)—are continuous and complex processes in space and time.Especially in the L-band,which has penetration depths of up to tens of centimeters,the brightness temperature(T_(B))may be generated by a vertically-mixed profile of different FT states,which cannot be described by the current version of the Community Microwave Emission Model(CMEM).To model such complex state transitions,we extended CMEM in Fresnel mode with an FT component by allowing for(1)a varying fraction of an open water surface on top of the soil,and(2)by implementing a temporal FT phase transition delay based on the difference between the soil surface temperature and the soil temperature at 2.5 cm depth.The extended CMEM(CMEM-FT)can capture the T_(B)progression from a completely frozen to a thawed state of the contributing layer as observed by the L-band microwave radiometer ELBARA-III installed at the Maqu station at the northeastern margin of the Tibetan Plateau.The extended model improves the correlation between the observations and CMEM simulations from 0.53/0.45 to 0.85/0.85 and its root-mean-square-error from 32/25 K to 20/15 K for H/V-polarization during thawing conditions.Yet,CMEM-FT does still not simulate the freezing transition sufficiently.
