Abstract
Near-surface atmospheric Vapor Pressure Deficit (VPD) is a key environmental variable affecting vegetation water stress, evapotranspiration, and atmospheric moisture demand. Although VPD is readily derived from in situ standard weather station measurements, more spatially continuous global observations for regional monitoring of VPD are lacking. Here, 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. These results provide new satellite capabilities for global mapping and monitoring of land surface VPD dynamics from ongoing AMSR2 operations. Overall good accuracy and similar observations from both AMSR2 and AMSR-E allow for the development of climate data records documenting recent (from 2002) VPD trends and potential impacts on vegetation, land surface evaporation, and energy budgets.
Highlights
Vapor Pressure Deficit (VPD), defined as the difference between saturated and actual atmospheric vapor pressures at a particular temperature [1], is an important environmental variable that quantifies the atmospheric moisture demand influencing evapotranspiration, latent heat exchange, and the surface energy budget [2,3]
For the Advanced Microwave Scanning Radiometer (AMSR) Land Parameter Data Record (LPDR) Ts, Γ, and precipitable water vapor (PWV) parameters, a low-quality flag was assigned to the 25-km grid cells having larger expected retrieval uncertainties due to the following conditions: high vegetation biomass levels (e.g., X-band vegetation optical depth (VOD) > 2.3), saturated emission signals indicated by Tb polarization differences at 18 GHz or 23 GHz less than 1.0 K, or large water bodies occupying more than 20% of a grid cell the 25-km grid cells having larger expected retrieval uncertainties due to the following conditions: high vegetation biomass levels (e.g., X-band VOD > 2.3), saturated emission signals indicated by Tb RpemoloaterSizenast.i2o0n18d, 1if0f,e1r1e7n5 ces at 18 GHz or 23 GHz less than 1.0 K, or large water bodies occupying15mofo2r0e than 20% of a grid cell [32]
A new method was proposed for satellite-based global assessment and monitoring of land surface VPD dynamics at 25 km spatial resolution using AMSR passive microwave remote sensing observations
Summary
Vapor Pressure Deficit (VPD), defined as the difference between saturated and actual atmospheric vapor pressures at a particular temperature [1], is an important environmental variable that quantifies the atmospheric moisture demand influencing evapotranspiration, latent heat exchange, and the surface energy budget [2,3]. Near-surface VPD defines the atmospheric moisture deficit in the foundational Penman-Monteith equation for estimating latent energy exchange and evapotranspiration [6,7], and provides a critical environmental input for operational satellite-based methods for predicting global evapotranspiration and ecosystem productivity [8,9,10]. Knowledge of global VPD dynamics is required to characterize carbon-water coupling in ecosystems, analyze drought and climate variability and impacts, and improve understanding of terrestrial water, carbon, energy budgets, and linkages. Multi-decadal records from such systems are readily available from reanalysis data products, such as the Modern-Era Retrospective Analysis for Research and Applications (MERRA) [16,17], the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim product [18], and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis [19,20,21]
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