Role of Western Boundary Currents in the Extratropical Transition Process
This work involves considering how the ocean surface modulates the evolution of the extratropical transition (ET) process that can occur for TCs that move poleward, where they develop cold and warm fronts and change their fundamental structure from a warm-core to a cold-core system. This is considered in the context of the Gulf Stream, a western boundary current (WBC) off the East Coast of North America that has a large gradient in sea surface temperatures in a relatively narrow region.
The Gulf Stream and other WBC’s are regions of baroclinicity, or horizontal temperature differences, and prior work has demonstrated that they can modulate the development of fronts at the surface, but this has not been looked at in the context of ET before. Since the exact mechanisms by which fronts develop during extratropical transition is still a debated question, I believe that considering the role of how the Gulf Stream may or may not influence the extratropical transition process could give key insights into frontogenetical processes and structural changes.
I am currently investigating statistical relationships between various conditions of the sea surface temperature (SST) distribution in the North Atlantic and how this impacts the movement of tropical cyclones through the cyclone phase space (CPS) as they transition into extratropical cyclones. Fundamentally, I am looking to see if there is a detectable difference in the CPS path based on different SST gradient strengths, patterns, etc.
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One way to look at this is by analyzing frontogenesis, as seen in the gif on the right for Hurricane Teddy (2020) in ERA5. There is a quasi-stationary, semi-permanent front located within the vicinity of the Gulf Stream (as seen north of Hurricane Teddy) that is removed once Teddy passes through. The presence of quasi-stationary fronts over regions of enhanced baroclinicity such as the WBCs has been documented in prior work. I am also analyzing how its presence aids/modifies the ET process.
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Stay tuned for more work in the future where I plan to model the extratropical transition process using WRF!
Tropical Cyclone Sandy (2012) precipitation accumulation snapshots from the ERA-5 reanalysisd ataset during the tropical cyclone phase and extratropical transition phase and track alignment across the Gulf Stream. SSTs are contoured in °C and the official track is shown in solid blue.
Frontogenesis (shaded) in the North Atlantic for the duration of Hurricane Teddy (2020) as represented in the ERA5 reanalysis. Each panel shows a different atmospheric pressure level.
Tropical Cyclone Precipitation Representation in Reanalyses
Hurricane Michael (2018) 3-hourly accumulated rainfall in the TRMM 3B42 satellite dataset, where precipitation is assigned falling inside a 500-km radius from the center of the TC.
Tropical cyclones (TCs) inflict great damage annually around the globe through flooding rainfall. Reanalyses, numerical weather prediction models constrained by observations, are used in research to study various aspects of Earth’s climate, including precipitation and TCs. Since TC rainfall can comprise a large fraction of total rainfall in some regions, knowing how much TC precipitation varies in reanalyses is critical information.
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While numerous studies have used reanalyses to study precipitation and TC representation, this is the first study to consider TC precipitation representation in reanalyses. TC precipitation is assigned using manual tracking via a fixed 500-km radius from each TC center. The reanalyses capture similar general spatial patterns of TC precipitation and TC precipitation fraction, defined as the fraction of annual precipitation assigned to TCs, and the spread in TC precipitation is larger than the spread in total precipitation across reanalyses. Compared to satellite observations, reanalyses show lower climatological mean annual TC precipitation over most areas. Testing the sensitivity of TC precipitation assignment to tracking method shows that climatological mean annual TC precipitation is systematically larger when assigned via manual tracking versus objective tracking. However, this tendency is greatly minimized when TC precipitation is normalized by TC density. These results highlight the need for continued improvement in the representation of tropical cyclones and their precipitation in reanalyses to improve overall precipitation.
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This work was recently accepted for publication in the Journal of Climate. Take a look over at the "Publications" tab for more information!
Climatological mean annual TC precipitation fraction in each reanalysis at their native spatial output resolutions (1980-2001).