Colorado State University

ONR Tropical Cyclone Rapid Intensification (TCRI)

Heating, Cooling, and Rapid Intensity Change in Tropical Cyclones

The central objectives of this research are to improve our understanding of diabatic heating and cooling during TC rapid intensification (RI). The proposed research will accomplish these objectives through an analysis of field observations in conjunction with high-resolution numerical modeling. The central hypothesis of this research is that RI is caused by high efficiency diabatic heating and associated potential vorticity generation that requires cooperation across multiple spatial and temporal scales.

Acknowledgement: ONR N000142012069

How does CD influence TC intensification?

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Group Members: Eleanor Casas , Michael M. Bell

We developed a new, simplified conceptual model that relates TC boundary layer structural quantities---the maximum tangential wind, its radius, its height, its underlying vertical gradient, and its underlying drag coefficient---to the TC’s potential for further intensification. In addition, the new conceptual model is also able to be inverted and be used to retrieve values of interest, namely the drag coefficient under the maximum tangential wind.

Are the asymmetric dynamics of Hurricane Michael (2018) polygonal eyewall consistent with vortex Rossby wave (VRW) theory?

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Group Members: Ting-Yu Cha , Michael M. Bell , Alex DesRosiers

While polygonal eyewall shapes have been seen in previous hurricanes, the corresponding evolution of wind asymmetries has never been quantitatively deduced due to limitations from previous observations. Here we show the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the winds at 5-minute intervals from single-Doppler Next Generation Weather Radar (NEXRAD) observations. The single Doppler radar analysis shows that the propagation speeds of different VRWs are consistent with linear wave theory.

On the contributions of incipient vortex circulation and environmental moisture to tropical cyclone expansion

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Group Members: Jonathan Martinez , C. Chelsea Nam , Michael M. Bell

Idealized numerical simulations of tropical cyclones are created to investigate the relative contributions of incipient vortex circulation and environmental moisture to tropical cyclone expansion. The principal findings demonstrate that an initially large vortex can expand more quickly than its relatively smaller counterpart. Increasing the environmental moisture further promotes expansion but mostly expedites the intensification process. Differences in the amount and scale of outer-core convection are associated with varying the incipient vortex circulation, resulting in variable expansion rates. Note: This project was also funded by the National Science Foundation Bridge to the Doctorate Fellowship Award 004863-00003.

On the nature and evolution of asymmetric structures during tropical cyclone rapid intensification

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Group Members: Jonathan Martinez , Michael M. Bell

Do asymmetries facilitate or interfere with tropical cyclone (TC) intensification? An idealized, high-resolution simulation of a rapidly intensifying TC is examined to assess asymmetric contributions to the intensification process. Scale-dependent contributions to the azimuthal-mean PV tendency reveal positive contributions from asymmetries that vary throughout the intensification period. Note: This project was also funded by the National Science Foundation Bridge to the Doctorate Fellowship Award 004863-00003.