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首页> 外文期刊>Journal of the Atmospheric Sciences >The advection-diffusion problem for stratospheric flow. Part II: Probability distribution function of tracer gradients
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The advection-diffusion problem for stratospheric flow. Part II: Probability distribution function of tracer gradients

机译：平流层流动的对流扩散问题。第二部分：示踪剂梯度的概率分布函数

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This paper is a continuation of the study of the advection-diffusion problem for stratospheric flow, and deals with the probability distribution function (PDF) of gradients of a freely decaying passive tracer. Theoretical arguments are reviewed and extended showing that mixing of a weakly diffused tracer by random large-scale flows produces a tracer gradient field whose probability distribution function has "stretched exponential'' tails P( deltheta u ) proportional to exp(-b deltheta(gamma)) with gamma < 1. This contrasts with the lognormal distribution expected for advective mixing in the absence of diffusion. The non-Gaussian distribution of tracer gradients can be derived in terms of the statistics of strain rates of the random driving flow. It is shown that the tails of the gradient PDF provide information about the dissipation scale, the scale selectivity of the dissipation law, and the fluctuations of short-term strain. The gradient PDF is shown to contain information about tracer variability that is not present at all in the power spectrum of the tracer field. To show that the predictions remain valid for the gradient statistics of passive tracers driven by the well-organized lower-stratospheric flow with mixing barriers, a series of advection-diffusion simulations of a decaying passive tracer are presented. The mixing is driven by ECMWF winds on the 420-K isentropic surface using the high-resolution finite-volume model employed in Part I of this paper. It is found that the probability distribution function of the simulated tracer gradients is indeed stretched exponential, with the stretching parameter γ &AP; 0.55. The largest gradients are not found in the regions of highest Lyapunov exponents, but rather in the surf-zone regions adjacent to the reservoirs of high tracer fluctuation amplitude. [References: 30]
机译：本文是对平流层流动对流扩散问题研究的延续，涉及自由衰减的无源示踪剂梯度的概率分布函数（PDF）。对理论论据进行了回顾和扩展，表明通过随机大规模流混合弱扩散示踪剂会产生示踪剂梯度场，该示踪剂梯度场的概率分布函数具有与exp（-b deltheta （）成比例的“拉伸指数”尾巴P（deltheta u）。 gamma <1），这与在不存在扩散的情况下对流混合的预期对数正态分布形成对比，示踪剂梯度的非高斯分布可以根据随机驱动流的应变率统计得出。可以看出，梯度PDF的尾部提供了有关耗散尺度，耗散定律的尺度选择性和短期应变波动的信息，其中梯度PDF包含了根本不存在的示踪剂变异性信息。为了证明预测对于由组织良好的较低层驱动的无源示踪剂的梯度统计仍然有效平流层流动与混合障碍，提出了一系列衰减衰减的无源示踪剂的对流扩散模拟。使用本文第一部分中使用的高分辨率有限体积模型，由ECMWF风在420-K等熵表面上驱动混合。可以发现，模拟示踪剂梯度的概率分布函数的确是指数扩展的，其拉伸参数为γ＆AP; 0.55。最大的梯度未在最高Lyapunov指数的区域中找到，而是在与示踪剂波动幅度大的储层相邻的海浪区域中发现。 [参考：30]

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