An axisymmetric numerical model is used to evaluate the maximum possible intensity of tropical cyclones. As compared with traditionally formulated nonhydrostatic models, this new model has improved mass and energy conservation in saturated conditions. In comparison with the axisymmetric model developed by Rotunno and Emanuel, the new model produces weaker cyclones (by ∼10%, in terms of maximum azimuthal velocity); the difference is attributable to several approximations in the Rotunno–Emanuel model. Then, using a single specification for initial conditions (with a sea surface temperature of 26°C), the authors conduct model sensitivity tests to determine the sensitivity of maximum azimuthal velocity (υmax) to uncertain aspects of the modeling system. For fixed mixing lengths in the turbulence parameterization, a converged value of υmax is achieved for radial grid spacing of order 1 km and vertical grid spacing of order 250 m. The fall velocity of condensate (Vt) changes υmax by up to 60%, and the ...

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The Maximum Intensity of Tropical Cyclones in Axisymmetric Numerical Model Simulations

Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a tropical cyclone’s intensity. An idealized framework based on steadiness, axisymmetry, and slantwise neutrality is developed to assess how ventilation affects tropical cyclone intensity via two possible pathways: the first through downdrafts outside the eyewall and the second through eddy fluxes directly into the eyewall. For both pathways, ventilation has a detrimental effect on tropical cyclone intensity by decreasing the maximum steady-state intensity significantly below the potential intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper-ventilation bound beyond which no steady tropical cyclone can exist. Ventilation also decreases the thermodynamic efficiency as the eyewall becomes less buoyant relative to the environment, which compounds the effects of v...

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Midlevel Ventilation's Constraint on Tropical Cyclone Intensity

Two phytoplankton blooms in the South China Sea (SCS), triggered by 2 typhoons with different intensities and translation speeds, were compared using remotely sensed chlorophyll a (chl a), sea surface temperature (SST), vector wind field, and best-track typhoon data. Typhoon Ling-Ling in 2001 was strong, with a maximum sustained surface wind speed of 59 m s(-1), and fast-moving with a mean translation speed of 4.52 m s(-1). Typhoon Kai-Tak in 2005 was weak with a maximum sustained surface wind speed of 46 m s(-1), and slow-moving with a mean translation speed of 2.87 m s(-1). The weak, slow-moving typhoon Kai-Tak induced phytoplankton blooms with higher chl a concentrations, while the strong, fast-moving typhoon Ling-Ling induced blooms over a larger area. On average, about 7 typhoons per year affect the SCS, among which 41% are strong (> 50 m s(-1)) and 59% are weak, while 64% are fast-moving (>4.4 m s(-1)) and 36% are slow-moving. We conservatively estimate that typhoon periods may account for 3.5% of the annual primary production in the oligotrophic SCS.

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

The effect of vertical wind shear (VWS) between different pressure levels on TC intensity change is statistically analyzed based on the best track data of tropical cyclones (TCs) in the western North Pacific (WNP) from the Joint Typhoon Warning Center (JTWC) and the ECMWF interim reanalysis (ERA-Interim) data during 1981–2013. Results show that the commonly used VWS measure between 200 and 850 hPa is less representative of the attenuating deep-layer shear effect than that between 300 and 1000 hPa. Moreover, the authors find that the low-level shear between 850 (or 700) and 1000 hPa is more negatively correlated with TC intensity change than any deep-layer shear during the active typhoon season, whereas deep-layer shear turns out to be more influential than low-level shear during the remaining less active seasons. Further analysis covering all seasons exhibits that a TC has a better chance to intensify than to decay when the deep-layer shear is lower than 7–9 m s−1 and the low-level shear is below ...

A Statistical Analysis of the Effects of Vertical Wind Shear on Tropical Cyclone Intensity Change over the Western North Pacific

Although infrequent, tropical cyclones (TCs) can intensify under moderate vertical wind shear (VWS). A potential hypothesis is that other factors—associated with both the TC and its environment—can help offset the effects of VWS and aid intensification. This hypothesis was tested with a large dataset of 6-hourly best tracks and environmental diagnostics for global TCs between 1982 and 2014. Moderate VWS was objectively defined as 4.5–11.0 m s−1, which represents the 25th–75th percentiles of the global distribution of 200–850-hPa VWS magnitude around TCs. Intensifying events (i.e., unique 6-hourly data points) were compared against steady-state events to determine which TC and environmental characteristics favored intensification under moderate VWS. This comparison showed that intensifying events were significantly stronger, closer to the equator, larger, and moving with a more westward motion than steady-state events. Furthermore, intensifying events moved within environments characterized by warm...

Climatological Analysis of Tropical Cyclone Intensity Changes under Moderate Vertical Wind Shear

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 ...

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Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

[1] Based on the best track data of tropical cyclones (TCs) in the North Atlantic and the NCEP/NCAR reanalysis during 1981–2008,the effect of vertical wind shear (VWS) between different vertical levels and with different directions of shear on TC intensity change for TCs of different intensities, translational speeds, and latitudes were statistically analyzed. The results show that TC intensity change and VWS are generally negatively correlated, indicating the overall negative effect of VWS on TC intensity. The strong, slow moving, and low latitude TCs are strongly affected by VWS in a deep layer, while the weak, fast moving, and high latitude TCs are subject to strong effect by VWS in the mid-lower troposphere. Furthermore, overall easterly shear, especially in the mid-lower troposphere, has considerably weaker effects on TC intensity change than westerly shear because part of the easterly shear could be offset by the beta-induced northwesterly shear.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GL041788

A statistical analysis of vertical shear effect on tropical cyclone intensity change in the North Atlantic

A new parameterization scheme of sea surface momentum roughness length for all wind regimes, including high winds, under tropical cyclone (TC) conditions is constructed based on measurements from Global Positioning System (GPS) dropsonde. It reproduces the observed regime transition, namely, an increase of the drag coefficient with an increase in wind speed up to 40 m s−1, followed by a decrease with a further increase in wind speed.

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Zhihua Zeng

Papers

Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

A statistical analysis of vertical shear effect on tropical cyclone intensity change in the North Atlantic

On Sea Surface Roughness Parameterization and Its Effect on Tropical Cyclone Structure and Intensity