Subtropical Cyclone Formation via Warm Seclusion Development: The Importance of Surface Fluxes

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.

Wind kinetic energy climatology and effective resolution for the ERA5 reanalysis

“…7c) presents the characteristical asymmetry of subtropical cyclones with a predominance of values exceeding 80 km h −1 in the north-west quadrant, sustained winds around 60 km h −1 in the centre, and lower values in the south-east quadrant of the system. This is in line with the results by Quitián-Hernández et al (2020, 2021, who already noted this asymmetry in similar systems. The winds at 500 hPa (Fig.…”

Kinetic Energy Climatology and Effective Resolution for the ERA5 Reanalysis

“…The surface wind speed field on 22 NOV (Figure 5c) presents the characteristical asymmetry of subtropical cyclones with a predominance of values exceeding 80 km h -1 in the north-west quadrant, sustained winds around 60 km h -1 in the centre, and lower values in the south-east quadrant of the system. This is in line with the results by Quitián-Hernández et al (2020, who already noted this asymmetry in similar systems. The winds at 500 hPa (Figures 5e,5f) show notably larger values than surface winds, clearly reinforced by a jet stream on the south-west quadrant.…”

Modelling a tropical-like cyclone in the Mediterranean Sea under present and warmer climate

“…2, we explore how Rolf is affected by future climate change (middle of the 21st century), which corresponds to global warming of 1.5 • C using the pseudo global warming (PGW; e.g. Schär et al, 1996;Rasmussen et al, 2011;Parker et al, 2018) technique. In addition to the effects of climate change, the relative roles of the atmosphere and the ocean in the modulations of medicane Rolf are also investigated separately in this section.…”

“…Alternatively, a pseudo global warming method (PGW; e.g. Schär et al, 1996;Rasmussen et al, 2011;Parker et al, 2018;Mooney et al, 2020) can be used to more explicitly assess the impacts of future climate change on medicanes. PGW is an advantageous method for characterizing the response of a given medicane to climate change by imposing the future changes in atmospheric and ocean variables estimated by CGCMs to boundary conditions of a high-resolution RCM (see details in Sect.…”