How Vertical Wind Shear Affects Tropical Cyclone Intensity Change: An Overview

Abstract

Over the last 30 years tropical cyclone (TC) intensity forecasts, for various (yet somewhat puzzling) reasons, have not achieved near the level of improvement of the TC track forecast. Although TCs have been studied intensively throughout the twentieth century, the community has surprisingly little quantitative knowledge as to how these storms interact with their environments, particularly with respect to changes in core structure (Frank & Ritchie, 1999). Rogers et al. (2006) stated that the lack of skill in numerical forecast of TC intensity can be partly attributed to inadequate understanding of the physics of TCs and the way they interact with their environment. In fact, TC structure and intensity changes are affected by a large and complex array of physical processes that govern the inner core structure and the interaction between the storm and both the underlying ocean and its atmospheric environment (Wang & Wu, 2004). Among other issues cited, crude parameterizations, difficulties in treating multiscale interactions, and the uncertainties involved with initializing the model over areas with sparse data coverage have received substantial attention. In order to predict TC intensity, one of the important questions has been how to first accurately predict a TC’s maximum potential intensity (MPI). Despite the fact that various methods for predicting a storm’s MPI have been put forth, the failure of the NWP community to realistically forecast TC intensity largely lies in the fact that there are various unexplained processes keeping TCs from reaching their theoretical MPI. While the mechanisms involved are myriad, there are essentially two kinds that have been identified as having the largest impact on TC intensification: 1) internal dynamics and 2) external forcing from environmental flow. Below these two headings fall most TC intensity-related topics: vertical wind shear-induced asymmetries in the core region, the cooling of the sea surface due to oceanic upwelling under the eyewall region, the role of inner and outer rainbands, vortex Rossby waves (VRWs), embedded mesovortices, and eyewall cycles. Tropical cyclones often fail to reach their theoretical MPI because prominent MPI calculations use the basic assumption of TC axisymmetry (Camp & Montgomery, 2001), whereas TC structure is rarely symmetric, even in mature storms. While the tangential wind field and other TC features are axisymmetric, many significant features, such as VRWs, eyewall cycles, rainfall, convection, radial winds, and outer rainbands are often highly asymmetric attributes that impact TC intensity change; it is thus no surprise that nearly all TCs fail to reach their MPI.