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悬而未决的科学问题。最近的数值模拟研究研究了弱初始中对流层气旋涡旋(MCV)如何转化为热带气旋。这些研究表明，热带气旋沿着明显不同的路径发展。因此，出现了两种相互竞争的理论：分别称为“自下而上”的过程和“自上而下”的过程，在这个过程中，地面风在表面风的初始旋转后形成；“自上而下”，在形成强的表面风后，表面风增强。在这个项目中，我们将调查这些明显相互矛盾的模拟结果是否由于模型初始条件在分叉点附近所致。如果是这样，那么对初始条件或模型物理的相对较小的改变可能会改变热带气旋发生的途径。将使用天气研究和预报模型(WRF；由国家大气研究中心开发)和区域大气建模系统(RAMS，由科罗拉多州立大学开发)进行模拟，以检查在各种环境条件下采取哪种途径。特别是，将探索这样的假设，即表面分叉的原因与深对流塔中产生的冰量有关。将研究海洋表面温度在云冰产生中的作用以及气旋发生的途径。还将分析其对微物理、辐射和垂直风切变的敏感性。这项研究将探索热带气旋发生存在分叉路径的可能性，正如最近的数值模拟研究所表明的那样。它将评估分叉点存在的物理原因，并检查路径对环境条件的敏感性。这项研究的一个独特方面是对两个不同的建模系统(WRF和RAMS)进行比较，以便更好地评估结果的准确性和可能存在的分叉点。如果这项研究的结果为分支路径的存在提供了证据，将为热带气旋发生提供一个新的视角。了解热带气旋发生的分支路径的存在对天气预报是有益的。其中一条路径似乎经常形成非常小的、迅速增强的热带气旋，目前热带预报员尤其难以预测。这项研究将阐明有利于热带气旋发展的环境条件，并提供有助于预测其发展的热带气旋形态和可观测特征的细节。这项研究也将引起研究气候条件变化的研究人员的兴趣，例如海表面温度和垂直风切变对热带气旋活动的影响。对微物理和辐射方案的敏感性测试有助于改进热带气旋的数值预报。使用双矩微物理方案可以为多参数雷达等遥感比较提供更好的能力。此外，分叉点的识别对于研究分叉系统的其他领域的研究人员也是有意义的。这项工作的成果将通过参加各次会议和同行评议的期刊积极传播。此外，该项目将直接促进研究生的教育。

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.