An Investigation of a Bifurcating Pathway to Tropical Cyclogenesis

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

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.