Formation and Maintenance of Thin Tropical Cirrus and Its Role in Troposphere-Stratosphere Exchange

位于热带对流层顶层（海拔15至17公里）的薄卷云超过30%的时间出现在热带西太平洋的大片地区。这些云的形成和维持以及它们在对流层-平流层交换中所起的作用是不确定的。关于热带卷云的起源和维持的问题在二十多年前就提出了。虽然从平流层到对流层的输送相对来说已经很清楚了，但是在热带地区，空气从对流层进入平流层的过程仍然是个谜。薄卷云很可能在这一过程中发挥作用，但其发生的确切性质尚不清楚。以前的研究已经确定动力学、微物理和辐射传递都起着一定的作用，但迄今为止还没有研究将这三者结合到一个单一的耦合模型中。该项目将完成薄卷云高分辨率过程模型的开发，该模型结合了动力学、微物理和辐射传输。该模型将用于研究这三个过程在维持薄卷云层中所起的作用，以及开尔文波活动在层形成中可能起的作用。此外，模式结果将阐明小冰晶的运输和空气经过薄卷云层从对流层进入平流层的潜在脱水。智力价值：这项研究的结果将是对这三个过程在维持薄卷云中的相对重要性的评估，以及它们可能与开尔文波动力学耦合以产生薄卷云的方式。主要的假设是开尔文波活动产生薄卷云，然后卷云通过辐射加热驱动的动态运动生长并维持自身。云冰水的含量是由垂直抬升和沉降过程中晶体生长的平衡维持的。耦合模型将提供关于空气如何通过辐射加热和动力学处理这些薄层的见解，以及循环是否会产生脱水的空气，这些空气随后可以注入平流层。更广泛的影响：热带大气的大尺度模型难以捕捉对流层上层和平流层下层的结构。这个问题的部分原因是由于缺乏足够的垂直分辨率，但缺乏对物理过程的了解也起着很大的作用。这项研究的结果将允许对热带对流层顶层非绝热加热的大小和重要性以及薄卷云在加热中所起的作用进行评估。通过将模式结果与新的卫星观测相结合，可以评估加热的面积程度。同样，可以对诱导垂直输送的大小作出评估。这些结果将约束现有的模型结果，并用于参数化薄卷云的特性和下一代模型的影响。

Thin cirrus clouds located in the tropical tropopause layer (15 to 17 km altitude) occur more than 30% of the time over large areas of the tropical western Pacific Ocean. The formation and maintenance of these clouds and the role they play in troposphere-stratosphere exchange is uncertain. Questions regarding the origin and maintenance of tropical cirrus were posed more than twenty years ago. While transport from stratosphere to troposphere is relatively well understood, the processes that move air from the troposphere into the stratosphere in the tropics remain enigmatic. Thin cirrus may well play a role in this process but the exact nature of how that occurs is not known. Previous investigations have identified dynamics, microphysics and radiative transfer as all playing some role, but no studies to date have combined all three into a single coupled model. This project will finalize development of a high-resolution process model for thin cirrus that combines dynamics, microphysics and radiative transfer. The model will be used to investigate the role that these three processes play in maintaining thin cirrus layers and the role that Kelvin wave activity may play in forming the layers. In addition, model results will shed light on the transport of small ice crystals and the potential dehydration of air processed through the thin cirrus layers from the troposphere into the stratosphere.Intellectual merit: The result of this research will be an assessment of the relative importance of these three processes in maintaining thin cirrus and the way in which they may couple to Kelvin wave dynamics to generate thin cirrus. The main hypothesis is that Kelvin wave activity generates thin cirrus, which then grows and maintains itself through dynamical motions driven by radiative heating. Cloud ice water content is maintained by a balance between crystal growth during vertical lifting and sedimentation. The coupled model will provide insight on how air is processed through these thin layers by radiative heating and dynamics and whether the circulation can produce dehydrated air that can subsequently be injected into the stratosphere.Broader impact: Large scale models of the tropical atmosphere struggle to capture the structure of the upper troposphere and lower stratosphere. Part of this problem is due to a lack of adequate vertical resolution but lack of knowledge about the physical processes also plays a large role. The results of this research will permit an assessment of the magnitude and importance of diabatic heating in the tropical tropopause layer and the role that thin cirrus play in the heating. By combining model results with new satellite observations, the areal extent of the heating can be evaluated. Similarly an assessment can be made of the magnitude of induced vertical transport. These results will both constrain existing model results and be used to parameterize thin cirrus properties and impacts in the next generation of models.