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  1. Abstract

    Most fog detection from space cannot differentiate fog and low stratus clouds, and cannot estimate fog deposition. This study assessed the feasibility of using spaceborne lidar observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) in fog detection and estimation. We tested the method in the central Namib Desert, Namibia, where frequent fog events occur and fog observations are available. Results showed the CALIPSO backscatter signal at 532 nm can differentiate low clouds and fog due to its high‐resolution vertical profiles. Backscatter signals during fog events were significantly higher than those during non‐fog periods. TheR2between backscatter signals and fog observations was 0.85. Moreover, the backscatter signal was also sensitive to relative humidity variation (R2 = 0.66). These results indicate that the CALIPSO data are feasible to estimate fog occurrence and deposition, providing a new perspective for space‐based fog studies.

     
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  2. Abstract

    As a supplementary or the only water source in dry regions, dew plays a critical role in the survival of organisms. The new hydrological tracer17O-excess, with almost sole dependence on relative humidity, provides a new way to distinguish the evaporation processes and reconstruct the paleoclimate. Up to now, there is no published daily dew isotope record on δ2H, δ18O, δ17O, d-excess, and17O-excess. Here, we collected daily dew between July 2014 and April 2018 from three distinct climatic regions (i.e., Gobabeb in the central Namib Desert with desert climate, Nice in France with Mediterranean climate, and Indianapolis in the central United States with humid continental climate). The δ2H, δ18O, and δ17O of dew were simultaneously analyzed using a Triple Water Vapor Isotope Analyzer based on Off-Axis Integrated Cavity Output Spectroscopy technique, and then d-excess and17O-excess were calculated. This report presents daily dew isotope dataset under three climatic regions. It is useful for researchers to use it as a reference when studying global dew dynamics and dew formation mechanisms.

     
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  3. Abstract

    Monitoring dryland vegetation trends and examining the drivers are of great importance to understand the dryland vegetation response to future climate changes. Recent findings through satellite data indicate that vegetation greenness has increased in several regions worldwide. These greening patterns are driven by human activities or combined human activities and environmental factors. However, the analyses of greenness trend for regions without direct human activities and shrub expansion in drylands are still lacking. To this end, this study investigates the vegetation trend across the Namib sand sea over March 2000 to December 2018 using monthly Normalized Difference Vegetation Index (NDVI) and examines several potential drivers including precipitation, temperature and atmospheric CO2concentration. For the NDVI time series across the whole study region, a significant greening trend was found over March 2000 to September 2012 based on Mann–Kendall test but not over the whole study period. Structural equation modelling results indicated that precipitation and CO2were the dominant drivers of greening. Temperature showed negative effects on vegetation greenness, indicating warming would reduce plant growth in the study region. Spatially, 75% of the region showed statistically significant greening over March 2000 to September 2012 and 39.30% for March 2000 to December 2018. The different vegetation trend results between the entire region and the pixel scale implied that location‐specific greening could be masked by an overall trend. Our study suggested that precipitation (especially the large episodic precipitation events) and CO2are dominant drivers of the observed greening in the Namib. Our findings fill an important knowledge gap of vegetation dynamics in regions without direct human activities.

     
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  5. Abstract

    Despite the growing interest in predicting global and regional trends in vegetation productivity in response to a changing climate, changes in water constraint on vegetation productivity (i.e., water limitations on vegetation growth) remain poorly understood. Here we conduct a comprehensive evaluation of changes in water constraint on vegetation growth in the extratropical Northern Hemisphere between 1982 and 2015. We document a significant increase in vegetation water constraint over this period. Remarkably divergent trends were found with vegetation water deficit areas significantly expanding, and water surplus areas significantly shrinking. The increase in water constraints associated with water deficit was also consistent with a decreasing response time to water scarcity, suggesting a stronger susceptibility of vegetation to drought. We also observed shortened water surplus period for water surplus areas, suggesting a shortened exposure to water surplus associated with humid conditions. These observed changes were found to be attributable to trends in temperature, solar radiation, precipitation, and atmospheric CO2. Our findings highlight the need for a more explicit consideration of the influence of water constraints on regional and global vegetation under a warming climate.

     
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  6. Abstract

    The oak (Quercus) species of eastern North America are declining in abundance, threatening the many socioecological benefits they provide. We discuss the mechanisms responsible for their loss, many of which are rooted in the prevailing view that oaks are drought tolerant. We then synthesize previously published data to comprehensively review the drought response strategies of eastern US oaks, concluding that whether or not eastern oaks are drought tolerant depends firmly on the metric of success. Although the anisohydric strategy of oaks sometimes confers a gas exchange and growth advantage, it exposes oaks to damaging hydraulic failure, such that oaks are just as or more likely to perish during drought than neighboring species. Consequently, drought frequency is not a strong predictor of historic patterns of oak abundance, although long-term climate and fire frequency are strongly correlated with declines in oak dominance. The oaks’ ability to survive drought may become increasingly difficult in a drier future.

     
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  7. Abstract

    Tap water isotopic compositions could potentially record information on local climate and water management practices. A new water isotope tracer17O-excess became available in recent years providing additional information of the various hydrological processes. Detailed data records of tap water17O-excess have not been reported. In this report, monthly tap water samples (n = 652) were collected from December 2014 to November 2015 from 92 collection sites across China. The isotopic composition (δ2H, δ18O, and δ17O) of tap water was analyzed by a Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique and two second-order isotopic variables (d-excess and17O-excess) were calculated. The geographic location information of the 92 collection sites including latitude, longitude, and elevation were also provided in this dataset. This report presents national-scale tap water isotope dataset at monthly time scale. Researchers and water resource managers who focus on the tap water issues could use them to probe the water source and water management strategies at large spatial scales.

     
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  8. Abstract

    Meteorological drought indices like the Standardized Precipitation Evaporation Index (SPEI) are frequently used to diagnose “ecological drought,” despite the fact that they were not explicitly designed for this purpose. More recently developed indices like the Evaporative Stress Index (ESI), which is based on the degree of coupling between actual to potential evapotranspiration, may better capture dynamic plant response to moisture limitations. However, the skill of these indices at describing plant water stress is rarely evaluated at sub‐seasonal timescales over which drought evolves. Moreover, it remains unclear how variability in phenological timing impacts and complicates early drought detection. Here, we compared the ability of ESI and SPEI to reflect the dynamics of ecological drought in forests and grasslands, based on anomalies of Gross Primary Productivity (GPP), surface conductance (Gs, a proxy for stomatal conductance), soil moisture, and vapor pressure deficit. ESI performed better than SPEI in capturing the dynamics of GPP andGs, but still missed early ecological drought signals due to biases linked to earlier onset of spring leaf development. Thus, we developed a modified variant of the ESI () that accounts for the complicating effects of phenological shifts in leaf area index (LAI). Thedetected drought onset up to 7–10 weeks earlier than SPEI and ESI. Additionally, drought onset dates determined fromare close to (±2 weeks) the dates determined from LAI‐corrected anomalies ofGs, and GPP, as well as the onset dates of soil water deficit and atmospheric aridity.

     
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