The Dynamical Tropopause Location as a Predictor for North Atlantic Tropical Cyclone Activity
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Author
Date
2023Type
- Doctoral Thesis
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Abstract
Tropical cyclones are violent weather systems that can cause great damage to coastlines. Their successful prediction, on timescales ranging from a few days to the climate scale, has thus been a subject of scientific research for several decades. For the North Atlantic basin, seasonal and subseasonal forecasts are operationally produced by various agencies to aid in estimating the potential for damage caused throughout the season. The 2013 hurricane season was predicted to be above average in activity. In reality, only two category 1 hurricanes, and no major hurricane, occurred, which is far below average. Predictions of the total number of tropical cyclones were generally correct, which suggests that the intensification of tropical cyclones was impeded. It has since been argued that an abundance of Rossby wave breaking events in the North Atlantic basin caused an increase in vertical wind shear and a reduction in mid-tropospheric humidity, which would militate against intensification. Rossby wave breaking event frequency, along with other related metrics, has therefore been proposed as a predictor for tropical cyclone activity. This thesis explores the latitudinal position of the tropopause in the western North Atlantic region, which is intrinsically linked to Rossby wave breaking events, as a simplified and more powerful predictor for tropical cyclone activity.
The implementation of the tropopause latitude in predictions requires the use of numerical models. To assess whether the relation between the tropopause latitude and tropical cyclone activity is represented properly, tropical cyclones must be tracked. A tracking algorithm was thus developed for use with the ICON model. The tracking algorithm uses varying parameter thresholds to detect weaker systems and the comparatively weak tail ends of tropical cyclone tracks. It is shown that the algorithm can detect tropical cyclones during their formation stage, and can terminate them as they dissipate or transition into extratropical systems. False positives and other tracking issues are shown to not significantly impact the representation of the accumulated cyclone energy throughout a season. The tracking algorithm is thus capable of extracting full tropical cyclone tracks from the data, and adequately reflects tropical cyclone activity.
1980 to 2017 reanalysis data have been used in conjunction with observed tropical cyclone data to show strong correlation between the tropopause latitude on the 360 K isentropic surface in the western North Atlantic region and the accumulated cyclone energy throughout a hurricane season. The strong correlation is argued to be due to reduced vertical wind shear, and thus reduced ventilation of tropical cyclone inner cores, for more poleward tropopause positions. The tropopause location is also shown to be linked to changes in the genesis location, landfalling probability, and lifetime of tropical cyclones. The tropopause location thus shows strong potential to be of use as a predictor of tropical cyclone activity on a seasonal timescale.
The regional variant of the ICON atmosphere model has been used to produce ensemble simulations for the month of September in the years 1980 to 2021, with the aim of reproducing the results found in reanalysis and observational data. The simulations underestimate North Atlantic tropical cyclone activity and place the mean tropopause latitude on the 360 K isentropic surface slightly too far equatorwards. However, the simulations succeed in reproducing the link between September mean tropopause latitude in the western North Atlantic region and September accumulated cyclone energy in the North Atlantic basin.
This thesis demonstrates that the September mean tropopause latitude in the western North Atlantic region is linked to tropical cyclone activity on a seasonal and subseasonal scale, both in observational and reanalysis data as well as in numerical simulation data. The tropopause latitude shows potential for use as a predictor in seasonal tropical cyclone forecasting. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000642452Publication status
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Publisher
ETH ZurichSubject
Tropical Cyclone, Seasonal Prediction, Atmospheric DynamicsOrganisational unit
03690 - Lohmann, Ulrike / Lohmann, Ulrike
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ETH Bibliography
yes
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