An Investigation of a Bifurcating Pathway to Tropical Cyclogenesis

热带气旋的形成在很大程度上仍然是一个尚未解决的科学问题。数值模拟研究最近研究了一个弱的早期对流层中气旋涡旋（MCV）如何转变为热带气旋。这些研究表明，热带气旋的发展路径截然不同。因此，出现了两种相互竞争的理论：分别被称为“自下而上”的过程，在此过程中，MCV随着地面风的初始旋转而发展；以及“自上而下”，在强中压形成后，地面风加强。在这个项目中，我们将研究这些明显冲突的模拟结果是否由于模型初始条件接近分岔点。如果是这样的话，那么对初始条件或模式物理的相对较小的变化就可以改变热带气旋形成的途径。模拟将使用天气研究和预报模型（WRF；由国家大气研究中心开发）和区域大气模拟系统（RAMS；由科罗拉多州立大学开发）进行，以检查在各种环境条件下采取的途径。特别地，我们将探讨一个假设，即造成明显分岔的原因与深层对流塔中产生的冰的数量有关。海面温度在云冰产生中的作用和形成气旋的途径将被研究。对微物理、辐射和垂直风切变的敏感性也将进行分析。知识价值。根据最近的数值模拟研究，本研究将探讨热带气旋形成存在分岔路径的可能性。它将评价分叉点存在的物理原因，并检查路径对环境条件的敏感性。本研究的一个独特方面将是两种不同建模系统（WRF和RAMS）之间的比较，这将有助于更好地评估结果的准确性和可能存在的分岔点。如果本研究结果为热带气旋分岔路径的存在提供证据，将为热带气旋形成提供一个新的视角。更广泛的影响。了解热带气旋形成的分岔路径的存在将有利于天气预报。其中一条路径似乎经常形成非常小，迅速增强的热带气旋，目前热带预报员特别难以预测。这项研究将阐明有利于热带气旋发展的环境条件，并提供其形态和可观测特征的细节，这将有助于预测其发展。这项研究也会引起研究气候条件变化（如海面温度和垂直风切变）对热带气旋活动影响的研究人员的兴趣。对微物理方案和辐射方案的敏感性试验有助于改进热带气旋的数值预报。双矩微物理方案的使用可以提高遥感比较的能力，例如多参数雷达。此外，分岔点的识别将引起其他领域研究分岔系统的研究人员的兴趣。这项工作的结果将通过参加会议和在同行评议的期刊上积极传播。此外，该项目将直接有助于研究生的教育。

Tropical cyclone formation is still a largely unsolved scientific problem. Numerical modeling studies have recently examined how a weak incipient mid-tropospheric cyclonic vortex (MCV) transforms into a tropical cyclone. These studies show tropical cyclone development along distinctly different pathways. Consequently, two competing theories have emerged: respectively referred to as processes of "bottom-up," during which an MCV is developed following an initial spinning up of surface winds; and "top-down," during which surface winds are strengthened after a formation of a strong MCV. In this project we will investigate whether these apparently conflicting simulation results are due to the model initial condition is near a bifurcation point. If this is the case, then relatively small changes to the initial conditions, or model physics, could alter the pathway for tropical cyclogenesis.Simulations will be conducted with the Weather Research and Forecasting model (WRF; developed at National Center for Atmospheric Research) and Regional Atmospheric Modeling System (RAMS; developed at Colorado State University) to examine which pathway is taken for a wide variety of environmental conditions. In particular, the hypothesis will be explored that the cause of the apparent bifurcation is related to the quantity of ice produced in deep convective towers. The role of sea surface temperature in cloud ice production and the pathway taken to cyclogenesis will be investigated. The sensitivity to microphysics, radiation, and vertical wind shear will also be analyzed.Intellectual merit. This research will explore the possibility that there exist bifurcating pathways to tropical cyclogenesis, as suggested by recent numerical modeling studies. It will evaluate the physical causes for the existence of a bifurcation point and examine the sensitivity of the pathways to environmental conditions. A unique aspect of this study will be a comparison between two different modeling systems (WRF and RAMS) that will allow a better assessment of the veracity of the results and the possible existence of a bifurcation point. If the results of this study provide evidence for the existence of bifurcating pathways, it will provide a new perspective of tropical cyclogenesis.Broader impacts. Knowledge of the existence of a bifurcating pathway to tropical cyclogenesis would be beneficial to weather forecasting. One of the pathways appears to often form very small, rapidly intensifying tropical cyclones, which are currently particularly difficult for tropical forecasters to predict. This study would elucidate the environmental conditions favoring tropical cyclone development and also provide details of their morphology and observable features that would be useful for predicting their development. This study would also be of interest to researchers who study the effect of changing climatic conditions, such as sea surface temperature and vertical wind shear, on tropical cyclone activity. The sensitivity tests to the microphysical and radiation schemes could help lead to improved numerical prediction of tropical cyclones. The use of a double-moment microphysical scheme may provide improved capability for remote sensing comparisons, such as multiparameter radar. Additionally, the identification of a bifurcation point would be of interest to researchers in other fields who study bifurcating systems. The results of this work will be actively disseminated through participation at conferences and in peer-reviewed journals. Furthermore, the project will directly contribute towards the education of a graduate student.