Colorado State University

ONR Presidential Early Career Award for Scientists and Engineers (PECASE)

Multiscale Interactions in Tropical Cyclone Structure and Intensity Change

The proposed research will improve our understanding of the multiscale interactions that result in TC genesis, rapid intensification, and weakening through an analysis of field observations collected during the Tropical Cyclone Intensity 2015 (TCI-2015) and Propagation of Intra-Seasonal Oscillations (PISTON) field projects in conjunction with numerical modeling. A better understanding of multiscale processes will enable improved environmental predictive capabilities for tactical planning and decision making.

Acknowledgement: ONR N000141613033

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.

How the wave pouch and vertical wind shear interactions play important roles in cyclongenesis in multi-scales?

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Group Members: C. Chelsea Nam , Michael Bell

Tropical cyclogenesis of pre-depression Hagupit was delayed while it interacted with upper-level trough, experiencing strong VWS. For the downscale cascade from the synoptic to meso-alpha scale, our analysis showed that pre-depression Hagupit was significantly affected by the strong northwesterly vertical wind shear. However, Hagupit survived through the hostile, strongly sheared environment, and eventually developed into a tropical cyclone after it escaped from the influence of upper-level trough. The upscale cascade from the persistent deep convection and its vorticity amplification through vortex tube stretching was a key process that enabled the pouch to persist even under 20 m/s VWS. We highlight the roles of localized low-level shear and cumulus congestus clouds inside the wave pouch as the localized vertical vorticity generated from the convective cells is aggregated through a vortex merger process inside the marsupial pouch.

What separates developing and nondeveloping disturbances?

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Group Members: C. Chelsea Nam , Dandan Tao , Michael M. Bell

To be, or not to be, that is the question of tropical cyclogenesis. Only about 15-20 % of African Easterly Waves develop into tropical cyclones (TCs). A WRF ensemble was created with multiple TC simulations spanning the relevant parameter space for three variables; 1) the VWS magnitude, 2) the environmental humidity, and 3) the initial vortex intensity. Unmeasurable random perturbations result in widely diverging scenarios in TC genesis in moderately sheared and dry environments. Here we hypothesize that the combination of moderate shear and dry air makes an unstable condition for a vortex to intensify or decay, which implifies that TC genesis in such environments may be intrinsically unpredictable in deterministic sense. We are currently looking at the link between the deep convection and the realignment of mid-level and low-level vortices comparing the developing and non-developing ensemble members.

How do the precursor waves develop prior to Tropical Cyclogenesis?

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

PISTON 2018 saw enhanced seasonal vorticity anomaly over western North Pacific that supports easterly wave propagation. Invest98 was an interesting cast that the disturbance was intensifying after spinning up from Super Typhoon Jebi's rainband. Invest98 produced more than 200mm rain over our shipborne Sea-Pol radar. Convective systems ranging from isolated to linearly organized MCS. With this case study, we aim to learn more about the multi-scale TC genesis problem, bridging the gap between large-scale wave mode and meso-scale convection.