Publication: Subtropical Cyclone Formation via Warm Seclusion Development: The Importance of Surface Fluxes
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2020-04-27
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Quitián Hernández, L.
Santos Muñoz, D.
Fernández González, S.
Martín, M.L.
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American Geophysical Union
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Abstract
Subtropical cyclones (STCs) are characterized by a thermal hybrid structure with tropical and extratropical features. STCs are considered a numerical modeling challenge because of their rapid intensification. A fundamental part of their strength is derived from diabatic processes associated with convection and heat fluxes from the ocean. This study evaluates the importance of surface turbulent heat fluxes during the transition of an extratropical precursor into a STC. This cyclone evolved embedded within a strong meridional flow, having a Shapiro‐Keyser structure and undergoing a warm seclusion process. To assess the importance of those heat fluxes, two Weather Research and Forecasting simulations were defined considering the presence and absence of those fluxes. Results of both simulations reveal a warm seclusion process, which weakened in absence of the heat fluxes. During the system genesis and in absence of heat fluxes, the wind and rainfall values were increased due to the remarkably intense area of frontogenesis to the northwest. Given these results and the lack of transition in the absence of heat fluxes, the frontal nature of the system was verified. Considering the heat fluxes, the obtained potential vorticity values diminished, reducing wind shear and intensifying convection in the system, which favored its transition into an STC. This study is groundbreaking in that no STC has been linked to a warm seclusion process in the Eastern North Atlantic. Additionally, simulated wind field shows an underestimation in comparison with Atmospheric Motion Vectors, used as observational data so as to give a weight to the wind analysis.
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© 2020 The Authors. This work was partially supported by research projects PCIN‐2014‐013‐C07‐04, PCIN2016‐080 (UE ERA‐NET Plus NEWA Project), CGL2016‐78702‐C2‐1‐R, CGL2016‐78702‐C2‐2‐R, and CGL2016‐81828‐REDT and the ECMWF special projects (SPESMART and SPESVALE). The second author was funded through PhD Grant BES‐2014‐067905 by the Spanish Ministry of Science, Innovation and University, and cofunded by the European Social Fund. We would like to express our gratitude to the anonymous reviewers for their rigorous review and helpful suggestions which improved this manuscript. Authors also thank the ECMWF for providing the analysis database (available at https:// apps.ecmwf.int/mars‐catalogue/? stream=oper&expver=1&month= oct&year=2014&type=an&class=od) and the NWC SAF of AEMET for providing the AMV images (http:// www.nwcsaf.org/). We would also like to thank the NCEP GFS for supplying the initial and boundary conditions (available at https://rda.ucar.edu/ datasets/ds083.2/index.html?hash= sfol‐wl‐/data/ds083.2#sfol‐wl‐/data/ ds083.2?g=22014). The postprocessing software used for plotting the WRF model data has been the NCAR Command Language (NCL) software Version 6.4.0, and it is available at https://www.ncl.ucar.edu/index.shtml website. Regarding the data used to support the main conclusions of this work, they are mostly model outputs. The WRF model configuration scripts and postprocessing scripts are available at the following repository (DOI: https://doi.org/10.17632/t83g8jj4mb.1). Because this work is part of an ongoing research, the facilitated data set will be under embargo until 1 year from publication.