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Shun-Nan Wu and Brian J. Soden


This study examines how the structure and amount of cloud water content are associated with tropical cyclone (TC) intensity change using the CloudSat Tropical Cyclone (CSTC) dataset. Theoretical and modeling studies have demonstrated the importance of both the magnitude and vertical structure of latent heating in regulating TC intensity. However, the direct observations of the latent heat release and its vertical profile are scarce. The CSTC dataset provides the opportunity to infer the vertical profile of the latent heating from CloudSat retrievals of cloud ice water content (IWC) and liquid water content (LWC). It is found that strengthening storms have ~20% higher IWC than weakening storms, especially in the midtroposphere near the eyewall. These differences in IWC exist up to 24 h prior to an intensity change and are observed for all storm categories except major TCs. A similar analysis of satellite-observed rainfall rates indicates that strengthening storms have slightly higher rainfall rates 6 h prior to intensification. However, the rainfall signal is less robust than what is observed for IWC, and disappears for lead times greater than 6 h. Such precursors of TC intensity change provide observationally based metrics that may be useful in constraining model simulations of TC genesis and intensification.

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Chun-Chieh Wu, Shun-Nan Wu, Ho-Hsuan Wei, and Sergio F. Abarca


The purpose of this study is to analyze the role of diabatic heating in tropical cyclone ring structure evolution. A full-physics three-dimensional modeling framework is used to compare the results with two-dimensional modeling approaches and to point to limitations of the barotropic instability theory in predicting the storm vorticity structure configuration. A potential vorticity budget analysis reveals that diabatic heating is a leading-order term and that it is largely offset by potential vorticity advection. Sawyer–Eliassen integrations are used to diagnose the secondary circulation (and corresponding vorticity tendency) forced by prescribed heating. These integrations suggest that diabatic heating forces a secondary circulation (and associated vorticity tendency) that helps maintain the original ring structure in a feedback process. Sensitivity experiments of the Sawyer–Eliassen model reveal that the magnitude of the vorticity tendency is proportional to that of the prescribed heating, indicating that diabatic heating plays a critical role in adjusting and maintaining the eyewall ring.

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David S. Nolan, Yoshiaki Miyamoto, Shun-nan Wu, and Brian J. Soden


While the transfer of moist enthalpy from the ocean to the atmosphere is the fundamental energy source for tropical cyclones, the release of latent heat in moist convection is the mechanism by which this energy is converted into the kinetic and potential energy of these storms. Most observational estimates of this heat release rely on satellite estimates of rain rate. Here, examination of five high-resolution numerical simulations of tropical cyclones reveals that there is a close correlation between the total condensate and the total heating, even though the former quantity is an amount and the latter is a rate. This relationship is due to the fact that for condensate to be sustained at large values, it must be rapidly replaced by new condensate and associated latent heating. Total condensate and total rain rate within fixed radial distances such as 111 km also show good correlations with the current intensity of the storm, but surprisingly, high values of condensate at high altitudes and close to the storm center are not good predictors of imminent intensification. These relationships are confirmed with an additional ensemble of 270 idealized simulations of tropical cyclones with varying sizes and intensities. Finally, simulated measurements of total condensate are computed from narrow swaths modeled after the cloud profiling radar on the CloudSat satellite. Despite their narrow footprint and the fact that they rarely cut through the exact center of the cyclone, these estimates of total condensate also show a useful correlation with current intensity.

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Shun-Nan Wu, Brian J. Soden, Yoshiaki Miyamoto, David S. Nolan, and Stefan A. Buehler


This study examines the relationship between frozen hydrometeors and latent heating in model simulations and evaluates the capability of the Weather Research and Forecasting (WRF) Model to reproduce the observed frozen hydrometeors and their relationship to tropical cyclone (TC) intensification. Previous modeling studies have emphasized the importance of both the amount and location of latent heating in modulating the evolution of TC intensity. However, the lack of observations limits a full understanding of its importance in the real atmosphere. Idealized simulations using WRF indicate that latent heating is strongly correlated to the amount of ice water content, suggesting that ice water content can serve as an observable proxy for latent heat release in the mid- to upper troposphere. Based on this result, satellite observations are used to create storm-centered composites of ice water path as a function of TC intensity. The model reasonably captures the vertical and horizontal distribution of ice water content and its dependence upon TC intensity, with differences typically less than 20%. The model also captures the signature of increased ice water content for intensifying TCs, suggesting that observations of ice water content provide a useful diagnostic for understanding and evaluating model simulations of TC intensification.

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