Abstract:
Near-surface atmospheric Vapor Pressure Deficit (VPD) is a key environmental variable controlling vegetation water stress and evapotranspiration. VPD is readily derived from in situ standard weather station measurements, global reanalysis and optical-IR remote sensing. Complementary to the available approaches, satellite passive microwave radiometers are sensitive to air temperature and atmospheric water vapor, enabling global estimation of VPD with one- to three-day fidelity. In this study, we document a new method to estimate daily (both a.m. and p.m.) global land surface VPD at a 25-km resolution using a satellite passive microwave remotely sensed Land Parameter Data Record (LPDR) derived from the Advanced Microwave Scanning Radiometer (AMSR) sensors. The AMSR-derived VPD record shows strong correspondence (correlation coefficient ≥ 0.80, p-value < 0.001) and overall good performance (0.48 kPa ≤ Root Mean Square Error ≤ 0.69 kPa) against independent VPD observations from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) data. The estimated AMSR VPD retrieval uncertainties vary with land cover type, satellite observation time, and underlying LPDR data quality. The changing trend of global VPD from 2002 to 2017 was analyzed using the resulting data record. The study provides new satellite capabilities for global mapping and monitoring of land surface VPD dynamics from AMSR-E and ongoing AMSR2 operations. Overall good accuracy and similar observations from both AMSR2 and AMSR-E allow for near real-time monitoring of mesoscale vegetation stress and the development of climate data records documenting recent (from 2002) VPD trends and potential impacts on vegetation, land surface evaporation, and energy budgets.