Modeling of Cold Fronts

自极锋理论出现以来，锋面的性质及其与气旋形成和降水的关系一直是气象研究的一个活跃领域。对锋面的研究很重要，因为它们往往是对中纬度地区人类活动构成危害的重大天气事件（强烈的温度变化、强风、云和降水的形成）的发生地。虽然在过去的一个世纪里，通过观测、理论和数值模拟工作，人们对锋面有了很多了解，但某些类型的锋面结构的发展仍然没有得到清楚的了解。在本次研究中，主要研究中纬度冷锋的两个方面：在一定的气象条件下，在一定的地理区域内，对流层下层冷锋所采用的前倾结构；以及许多海洋冷锋前缘采用的波纹状窄雨带结构。具体来说，他们将测试静态稳定性（和位势涡度）的水平对比对前倾冷锋形成的影响；水平切变不稳定性、降水微物理和被困重力波在窄冷锋雨带波纹胞状结构组织中的相对重要性。解决这个问题的一种自然方法是用数值模型进行模拟，因为它可以用来分离所涉及的基本物理和动力学过程，并且可以将输出与所讨论的现象的许多观察案例进行比较。“理想化”和“真实天气”模拟的结合提供了最大的成功机会，这也是本研究将采用的方法。通过理想化的模拟，指定初始和边界条件，简化物理，以便隔离和理解特定的机制和控制参数。在真实天气模拟中，基于三维观测分析的初始条件和边界条件用于驱动具有完整模型物理的模拟，以测试所提出的控制参数和机制是否在完全复杂的大气中表现如预期的那样。天气研究及预报模式非常适合进行这项研究。这种中尺度模式是专门为解决1-10公里尺度的过程而设计的，具有独特的配置能力，可以进行理想和真实的天气实验。将设计各种模拟来测试上述假设的过程。这些将包括锋面的控制模拟，以及对被认为对控制假设机制很重要的参数的敏感性测试，例如切变和稳定性的垂直剖面、水平风切变模式、微物理过程和绝热加热等。学术价值：一些对流层低空冷锋的前倾结构和波纹状窄雨带结构都有很好的文献记载，但尚未得到很好的理解。已经提出了几个涉及动力和云微物理过程的可行假设来解释这些现象，但任务仍然是对它们进行测试，以便了解有助于预测上升运动、云和降水发生的位置以及它们是如何组织的参数。这项研究将探讨这些控制机制，从而促进我们对这些重要的额叶现象的理解。更广泛的影响：所研究的现象对天气预报具有直接的重要性，因为它们决定了相对于地面冷锋边界的降水和危险天气发生的位置，这一特征往往会引起天气预报员的极大关注。努力更好地了解和预测破坏性天气发生的地点和时间，显然对社会有直接的潜在好处。

Since the inception of the polar front theory, the nature of fronts and their relationship to cyclogenesis and precipitation has been an active area of meteorological research. The study of fronts is important because they are often the locus of significant weather occurrences (strong temperature changes, strong winds, formation of clouds and precipitation) that present hazards to human activities in midlatitudes. While much has been learned about fronts through observational, theoretical, and numerical modeling work in the past century, the development of certain types of frontal structures are still not clearly understood. In this research, the Principal Investigators will study two aspects of mid-latitude cold fronts: the forward-tilting structure that some lower-tropospheric cold fronts adopt under certain meteorological conditions and in certain geographical areas; and the corrugated narrow rainband structure that many oceanic cold fronts adopt at their leading edge. Specifically, they will test the effects of horizontal contrasts in static stability (and potential vorticity) on the formation of forward-tilting cold fronts; and the relative importance of horizontal shear instability, precipitation microphysics, and trapped gravity waves in organizing the corrugated cellular structure of narrow cold-frontal rainbands.A natural approach to this problem is to carry out simulations with a numerical model, because it can be used to isolate the essential physical and dynamical processes involved, and outputs can be compared to the many observed cases of the phenomena in question. A combination of both "idealized" and "real-weather" simulations offers the greatest chance of success, and that is the approach that will be taken in this study. With idealized simulations, initial and boundary conditions are specified and physics are simplified in order to isolate and understand particular mechanisms and controlling parameters. With real-weather simulations, initial and boundary conditions based on 3 D observational analyses are used to drive simulations with full model physics, to test if the proposed controlling parameters and mechanisms behave as expected in the full complexity of the atmosphere. The Weather Research and Forecast Model is ideally suited to carry out the research. This mesoscale model is specifically designed to resolve processes on the scale of 1-10 km, and has the unique capability of being configured to perform both idealized and real weather experiments. A variety of simulations will be designed to test the hypothesized processes mentioned above. These will include control simulations of fronts, and tests of sensitivity to parameters that are considered important in controlling the hypothesized mechanisms, such as vertical profiles of shear and stability, horizontal wind shear patterns, microphysical processes and diabatic heating, etc. Intellectual Merit: Both the forward-tilting structure and the corrugated narrow rainband structure of some lower-tropospheric cold fronts are well documented, but not well understood, phenomena. Several viable hypotheses involving dynamical and cloud microphysical processes have been proposed to explain these phenomena, but the task remains to test them, in order to understand the parameters that will help predict where upward motion, clouds, and precipitation occur and how they are organized. This study will investigate these controlling mechanisms and thereby advance our understanding of these important frontal phenomena. Broader Impacts: The phenomena to be studied are of direct importance to weather prediction because they determine where precipitation and hazardous weather occur relative to the surface cold frontal boundary, a feature that tends to draw much attention from weather forecasters. Working toward better understanding and prediction of where and when disruptive weather occurs clearly has direct potential benefits to society.