Abstract
Contributions of different physical processes to the development of a super explosive cyclone (SEC) migrating over the Gulf Stream with the maximum deepening rate of 3.45 Bergeron were investigated using the ERA5 atmospheric reanalysis from European Centre for Medium-Range Weather Forecasts (ECMWF). The evolution of the SEC resembled the Shapiro-Keyser model. The moisture transported to the bent-back front by easterlies from Gulf Stream favored precipitation and enhanced the latent heat release. The bent-back front and warm front were dominated by the water vapor convergence in the mid-low troposphere, the cyclonic-vorticity advection in the mid-upper troposphere and the divergence in the upper troposphere. These factors favored the rapid development of the SEC, but their contributions showed significant differences during the explosive-developing stage. The diagnostic results based on the Zwack-Okossi equation suggested that the early explosive development of the SEC was mainly forced by the diabatic heating in the mid-low troposphere. From the early explosive-developing moment to maximum-deepening-rate moment, the diabatic heating, warm-air advection and cyclonic-vorticity advection were all enhanced significantly, their combination forced the most explosive development, and the diabatic heating had the biggest contribution, followed by the warm-air advection and cyclonic-vorticity advection, which is different from the previous studies of ECs over the Northwestern Atlantic. The cross section of these factors suggested that during the rapid development, the cyclonic-vorticity advection was distributed and enhanced significantly in the mid-low troposphere, the warm-air advection was strengthened significantly in the mid-low and upper troposphere, and the diabatic heating was distributed in the middle troposphere.
Highlights
The results indicated that the total tendency of the Z-O equation had similar evolution with the deepening rate of the super explosive cyclone (SEC), showing the reasonableness of the result calculated by the Z-O equation
The precipitation was significantly enhanced during the rapid development
The bent-back front and the warm front were dominated by the water vapor convergence in the mid-low troposphere, the cyclonic-vorticity advection in the mid-upper troposphere and the divergence in the upper troposphere (Figures 3C2–E2)
Summary
It has been observed that the extratropical cyclones can develop explosively by a rapid drop of the sea level pressure (SLP) at the center and a sharp enhancement of the intensity within a very short time. Rice (1979) called them Meteorological bombs and they are one of the most dangerous weather systems in winter over the mid-latitude oceans, with 2–5 days lifespan and 2,000–3,000 km horizontal scales, leading to extremely severe winds and heavy precipitation and potentially causing serious losses of life and property (Liberato et al, 2011; Liberato et al, 2013). Sanders and Gyakum (1980) defined the explosive cyclone (EC) as the one with the central SLP drops more than 24 hPa within 24 h when adjusted geostrophically to 60°N. Sanders and Gyakum (1980) defined the explosive cyclone (EC) as the one with the central SLP drops more than 24 hPa within 24 h when adjusted geostrophically to 60°N. Given the facts that ECs usually occur in the mid-latitude and that the high-resolution reanalysis data are widely used, Zhang et al (2017) adjusted the latitude to 45°N and set the pressure fall to 12 hPa/12 h. The one “Bergeron” of a 24 hPa/24 h at 60°N in Sanders and Gyakum (1980) was modified into a 12 hPa/12 h at 45°N These ECs were classified into four categories using K-means clustering algorithm based on the maximum deepening rate: Super (≥2.30 Bergeron), strong (1.70–2.29 Bergeron), moderate (1.30–1.69 Bergeron) and weak (1.00–1.29 Bergeron)
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