美国西南部内陆地区春季大气湿度意外下降及其对森林火灾的影响

Tess W. P. Jacobson, R. Seager, A. P. Williams, I. Simpson, Karen A. McKinnon, Haibo Liu
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摘要

在季节时间尺度上,水汽压差(VPD)是预测美国西南部("西南部")焚烧面积的已知指标。由于温度与饱和蒸汽压之间的指数关系,VPD 会随着大气变暖而增加。比湿度是 VPD 的另一个控制因素,比湿度的增加可以抵消温度导致的 VPD 增加。出乎意料的是,尽管大气变暖使水汽容纳能力增强,但从 1970 年到 2019 年,西南部大部分地区的近地面比湿度却在下降,尤其是在春季、夏季和秋季。在此,我们利用再分析和原位站数据确定了 1970-2019 年美国西南部近地面湿度的下降趋势。我们以夏季森林火灾季节前几个月的西南内陆地区为重点,从大气环流和地表与大气之间湿度通量变化的角度解释了湿度下降的原因。我们发现,内陆地区早春降水量的减少导致了土壤水分和蒸散量的下降,使夏季对流层下部变得干燥。季节前降水量的减少又与北半球静止波模式的趋势有关。最后,利用固定的湿度方案和观测到的 VPD 与森林烧毁面积之间的指数关系,我们估计在温度完全不增加的情况下,湿度下降本身仍将导致 1984-2019 年期间观测到的 VPD 引起的烧毁面积增加的近四分之一。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
An unexpected decline in spring atmospheric humidity in the interior Southwestern United States and implications for forest fires
On seasonal timescales, vapor pressure deficit (VPD) is a known predictor of burned area in the Southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970-2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970-2019 in the Southwestern U.S. with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend towards a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984-2019.
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