Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

Abstract

The effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone (TC) intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981–2003. In general, both the fast translation and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3 and 8 m s−1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature where their environmental steering flow and vertical shear are both weak. The results show that few TCs intensified when they moved faster than 15 m s−1, or when their large-scale environmental vertical shear is larger than 20 m s−1. The intensification rate of TCs is found to increase with decreasing vertical shear while the majority of the weakening storms experience relatively strong vertical shear. Overall, strong vertical shear prohibits rapid intensification and most likely results in the weakening of TCs, similar to the fast storm translation. Based on the statistical analysis, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST and the outflow temperature as the thermodynamic control. The new empirical MPI can not only provide more accurate estimation of TC maximum intensity but also better explain the observed behavior of the TC maximum intensity and help explain the thermodynamic and environmental dynamical controls of TC intensity. Implications of the new empirical MPI are discussed.

Enhanced western North Pacific tropical cyclone activity in May in recent years

“…Stippled regions are for values significant at or above 95 % confidence level thus the averaged TC intensity dramatically increases during the recent decade. Many studies suggest that both the internal dynamics and external forcing from environmental flow have the largest impact on TC intensification (Wang and Wu 2004;Zeng et al 2007), thus TC intensity can be mainly explained by the MPI and the TC-ambient VWS. To find out why the TC intensity has an upward trend, we examined the MPI and TC ambient VWS over the TC intensification region.…”

Comparison of phytoplankton blooms triggered by two typhoons with different intensities and translation speeds in the South China Sea

“…Lowering of the sea surface temperature (SST) can in turn limit the intensity of the typhoon (Emanuel 1999, Wang & Wu 2004. As a result, slow-moving typhoons cannot generally be very strong, especially when their translation speeds are less than 3 m s -1 (Zeng et al 2007). The translation speed and intensity of a typhoon may have different effects on phytoplankton biomass and primary production.…”

Upper-tropospheric environment–tropical cyclone interactions over the western North Pacific: A statistical study

“…That is, there is no room for the TC to develop. We imposed the final constraint that TCs should be at least 30 m s −1 away from their MPI, which was estimated using the empirical relation given by Zeng et al (2007) for the western North Pacific as: Table 3 shows that when the "no-landing" condition is added, the number of weakening events is greatly reduced from 111 to 67, and the rate of SLP change is also reduced from 5 hPa to 4.5 hPa per six hours. After the "havepotential" condition is applied, the number of weakening events drops to 26, roughly half of that of intensifying events.…”