A soil moisture rainfall feedback mechanism 1. Theory and observations
A soil moisture rainfall feedback mechanism 1. Theory and observations
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DOI：
10.1029/97wr03499
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发表时间：
1998-04-01
影响因子：
5.4
通讯作者：
Eltahir, EAB
中科院分区：
文献类型：
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作者：
Eltahir, EAB
本文提出了一个关于土壤水分条件在陆气相互作用中的基本作用的假设。我们认为，任何大区域的湿润土壤水分条件都应与相对较大的边界层湿静力能量有关，这有利于发生更多的降雨。由于土壤水分条件本身反映了过去发生过的降雨，该假说暗示了土壤水分与降雨之间的正反馈机制。这一机制是基于对陆地-大气边界能量平衡的考虑，而不是过去提出的、基于水平衡和降水再循环概念的类似机制。土壤水分对地表反照率和波文比的控制是提出土壤水分-降雨反馈机制的基本依据。表层土壤中的水分会影响地表的这两个重要性质，因此，随着表层土壤中水分的增加，这两个变量都会减小。土壤水分对地表反照率的直接影响表明，湿润的土壤水分条件增加了净太阳辐射。土壤湿度对波文比的直接影响表明，湿润的土壤水分条件将通过降低地表温度，减少地面辐射的向上排放，同时增加大气水汽含量和地面辐射的向下通量，从而增加地表净地面辐射。因此，在潮湿的土壤水分条件下，净辐射的两个分量都被增强，导致从地表进入边界层的总热通量更大。这个总通量表示相应的感热和潜热通量的总和。同时，地表温度的冷却应与较小的感热通量和较小的边界层深度相联系。当这些过程发生在足够大的区域上时，从表面进入较小的边界层空气储存库的热通量增加，应该有利于单位质量的边界层空气有相对较大的湿静态能量。前人的一些研究表明，局地对流风暴的动力学和大尺度大气环流的动力学对边界层湿静力能量的分布是敏感的。这些理论概念通过来自堪萨斯州的实地观测进行了测试，并在一篇附带的论文[郑Eltahir，这个问题]中使用一个简单的数值模型进行了进一步的探索。
This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper [Zheng and Eltahir, this issue] using a simple numerical model.