Colorado State University Tropical Weather and Climate Research



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


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.

A Comparison of the Polarimetric Radar Characteristics of Heavy Rainfall From Hurricanes Harvey (2017) and Florence (2018)


Group Members: Jennifer C. DeHart , Michael M. Bell

Polarimetric coastal radar data are used to compare the rainfall characteristics of Hurricanes Harvey (2017) and Florence (2018). Intense rainfall was an infrequent yet important contributor to the total rainfall in Harvey, but its relative contribution varied spatially. The total rainfall over land maximized near the coast over Beaumont, TX due to intense convection resulting from prolonged onshore flow downshear from the circulation center. Overall, polarimetric radar observations in Harvey show a dominance of high concentrations of small-to-medium drops, consistent with prior tropical cyclone studies. The microphysical characteristics were spatially and temporally inhomogeneous however, with larger drops more frequent on 27 August and higher number concentrations more frequent on 28 and 30 August. The polarimetric variables and raindrop characteristics observed during Florence share broad similarities to Harvey, but had reduced variability, fewer observations of stronger reflectivity and differential reflectivity, and a lower frequency of high number concentrations and medium-sized drops. The radar data indicate Florence had reduced coverage of stronger convection compared to Harvey. We hypothesize that differences in storm motion, intensity decay rates, and vertical wind shear produce the distinct precipitation structures and microphysical differences seen in Harvey and Florence.

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


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.

The Unconventional Eyewall Replacement Cycle of Hurricane Ophelia (2005)


Group Members: Naufal Razin , Michael M. Bell

Using flight-level and airborne radar data, Hurricane Ophelia was shown to have undergone an unconventional eyewall replacement cycle (ERC). Ophelia\'s ERC was unconventional because it occurred while the storm was at Category \1 intensity and located over anomalously cold sea surface temperatures. Airborne radar analyses showed that the expansion of Ophelia\'s wind field associated with the ERC occurred in the dominantly stratiform rainbands, indicating that the stratiform kinematics in Ophelia's rainbands played a dominant role in Ophelia's ERC.

How do the precursor waves develop prior to Tropical Cyclogenesis?


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.

What are the pros and cons of using single Doppler radar technique (GVTD) and pseudo-dual Doppler airborne retrievals for studying advantages and disadvantages of each technique for studying tropical cyclone structure?


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

In this study, a comparison between the two techniques shows that the axisymmetric tangential winds are generally comparable between the two techniques after the improvements to Generalized Velocity Track Display (GVTD) retrievals. Fourier decomposition of asymmetric kinematic and convective structure shows more discrepancies due to spatial and temporal aliasing in the retrievals. Complementary information can be retrieved from both single and multiple Doppler retrievals.