Taking 6-h IR brightness temperature differences, Dunion et al. (2014) found that in major hurricanes, an area of cold cloud tops routinely propagated radially outward from the storm core at around 5–10 m/s-1 over the course of each day. They defined this feature as a “diurnal pulse” and created a 24-h conceptual clock that identified at which radius the coldest cloud tops would be located based on local time (LT). Due to the inherent predictability of these pulses, this dissertation was undertaken to gain a deeper understanding of their frequency, structure, and characteristics.
George Bryan's Cloud Model 1 in axisymmetric mode was used in an attempt to replicate and investigate the cause of observed diurnal pulses. While the diurnal cycle timing of inner-core deep convection, rainfall, cirrus canopy oscillation, and storm size found in the literature was able to be replicated, diurnal pulses were not reproduced.
Turning to observations, an objective metric was created using 6-h IR brightness temperature differences of GridSat-B1 satellite imagery to generate a 36-y climatology of Atlantic basin diurnal pulses that had similar temporal phasing as the Dunion et al. (2014) clock. The metric identified cooling pulses, similar to those found in Dunion et al. (2014), and warming pulses, a previously unidentified pulse type where warm cloud tops propagated radially outward from the storm core following the Dunion et al. (2014) clock. Diurnal cooling and warming pulses were found to be near-ubiquitous features of tropical cyclones, present 88% of the time. Additionally, the environment prior to outward propagation of cooling pulses differed from warming pulse days by having more favorable conditions between 00–03 LT for enhanced inner-core convection: higher SST and ocean heat content, more moisture throughout the troposphere, and stronger low-level vorticity and upper-level divergence.
To stratify the climatology into pulses that were and were not associated with deep convection, WWLLN lightning data were used to identify electrically-active (ACT) and electrically-inactive (INACT) pulses based on an objective metric that incorporated lightning flash density, areal coverage, and longevity within a pulse. It was found that ACT pulses occurred around 61% of the time, primarily when pulses propagated outward on the right-of-shear side of the storm, the dominant quadrant for outer-rainband lighting activity. Interestingly, ACT warming pulses were associated with off-the-clock cooling pulses that propagated outward ahead of the warming pulse. The propagation speed of ACT cooling pulses, ACT warming pulses, and INACT cooling pulses offered support to the gravity wave interpretation of diurnal pulses proposed in the literature and to the Dunion et al. (2019) result that pulses take on tropical squall line characteristics after propagating away from the inner core. Since INACT warming pulses were not associated with convection, however, the tropical-squall-line interpretation and, possibly, the gravity wave interpretation of diurnal pulses are incomplete.
In order to gain further insight into pulse characteristics, including their initiation and propagation mechanisms, a case study of an ACT cooling pulse and an off-the-clock cooling pulse associated with an ACT warming pulse that occurred in Hurricane Harvey (2017) was conducted. The ACT and off-the-clock cooling pulses shared many similar characteristics: 1) column-deep total condensate, 2) mid-level warming that was perhaps associated with latent heat release, 3) a surface cold pool, 4) a front-to-rear radial inflow jet and a rear-to-front outflow jet with an overturning updraft, and 5) enhanced tangential wind. These characteristics are similar to those found in tropical squall lines, thus supporting the tropical squall line interpretation of diurnal pulses put forth by Dunion et al. (2019). A hypothesis for ACT pulse initiation was then introduced, tested, and confirmed: inner rainbands that propagated outward into a more favorable environment for deep convection reinvigorated into ACT pulses that had tropical squall line characteristics.
|Commitee:||Corbosiero, Kristen L., Fovell, Robert G., Tang, Brian H.|
|School:||State University of New York at Albany|
|Department:||Atmospheric and Environmental Sciences|
|School Location:||United States -- New York|
|Source:||DAI-B 81/2(E), Dissertation Abstracts International|
|Keywords:||Major hurricanes, Diurnal pulse, Cooling pulses|
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