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1. Introduction Extratropical cyclones, and their associated fronts, are responsible for the majority of total rainfall in the extratropics and are particularly important for rainfall extremes and flood risk ( Dowdy and Catto 2017 ; Pepler and Dowdy 2021a ; Pfahl and Wernli 2012 ; Utsumi et al. 2017 ). In a warming climate, the Southern Hemisphere storm track is expected to move poleward, with a corresponding decline in cyclone frequency in the Southern Hemisphere midlatitudes ( Utsumi
1. Introduction Extratropical cyclones, and their associated fronts, are responsible for the majority of total rainfall in the extratropics and are particularly important for rainfall extremes and flood risk ( Dowdy and Catto 2017 ; Pepler and Dowdy 2021a ; Pfahl and Wernli 2012 ; Utsumi et al. 2017 ). In a warming climate, the Southern Hemisphere storm track is expected to move poleward, with a corresponding decline in cyclone frequency in the Southern Hemisphere midlatitudes ( Utsumi
1. Introduction Extratropical transition (ET) is the process by which a tropical cyclone (TC), upon encountering a baroclinic environment and reduced sea surface temperature (SST) at higher latitudes, transforms into an extratropical cyclone ( Jones et al. 2003 ; Evans et al. 2017 ). The transformation of a TC into an extratropical cyclone during ET is characterized by the acquisition of frontal structures as the TC interacts with the midlatitude baroclinic zone (e.g., Klein et al. 2000
1. Introduction Extratropical transition (ET) is the process by which a tropical cyclone (TC), upon encountering a baroclinic environment and reduced sea surface temperature (SST) at higher latitudes, transforms into an extratropical cyclone ( Jones et al. 2003 ; Evans et al. 2017 ). The transformation of a TC into an extratropical cyclone during ET is characterized by the acquisition of frontal structures as the TC interacts with the midlatitude baroclinic zone (e.g., Klein et al. 2000
1. Introduction Extratropical transition (ET) is a complex, multistage physical process during which a tropical cyclone (TC) interacts with the midlatitude environment and evolves from a warm-cored tropical system into a cold-cored midlatitude cyclone. A full review of extratropical transition and recent research is given in Jones et al. (2003) . Although every tropical cyclone undergoes a unique evolution during and after the ET process, transitioning storms are often placed into one of two
1. Introduction Extratropical transition (ET) is a complex, multistage physical process during which a tropical cyclone (TC) interacts with the midlatitude environment and evolves from a warm-cored tropical system into a cold-cored midlatitude cyclone. A full review of extratropical transition and recent research is given in Jones et al. (2003) . Although every tropical cyclone undergoes a unique evolution during and after the ET process, transitioning storms are often placed into one of two
1. Introduction The genesis and evolution of cyclones has received considerable attention for almost a century. In the early twentieth century, Bjerknes (1919) proposed the frontal-cyclone model. Since that time, cyclones have been the subject of ongoing research aimed at furthering our understanding and prediction of cyclones, especially for extratropical marine cyclones. In the early stage of research the simplified quasigeostrophic equations ( Charney 1947 ) were used for studying the
1. Introduction The genesis and evolution of cyclones has received considerable attention for almost a century. In the early twentieth century, Bjerknes (1919) proposed the frontal-cyclone model. Since that time, cyclones have been the subject of ongoing research aimed at furthering our understanding and prediction of cyclones, especially for extratropical marine cyclones. In the early stage of research the simplified quasigeostrophic equations ( Charney 1947 ) were used for studying the
cyclone (Luis, the Queen, and the Buoys). Preprints, 24th Conf. on Hurricanes and Tropical Meteorology, Fort Lauderdale, FL, Amer. Meteor. Soc., CD-ROM, 8B.7 . Bowyer , P. J. , and A. W. MacAfee , 2005 : The theory of trapped-fetch waves in tropical cyclones—An operational perspective. Wea. Forecasting , 20 , 229 – 244 . Colle , B. A. , 2003 : Numerical simulations of the extratropical transition of Floyd (1999): Structural evolution and responsible mechanisms for the heavy rainfall
cyclone (Luis, the Queen, and the Buoys). Preprints, 24th Conf. on Hurricanes and Tropical Meteorology, Fort Lauderdale, FL, Amer. Meteor. Soc., CD-ROM, 8B.7 . Bowyer , P. J. , and A. W. MacAfee , 2005 : The theory of trapped-fetch waves in tropical cyclones—An operational perspective. Wea. Forecasting , 20 , 229 – 244 . Colle , B. A. , 2003 : Numerical simulations of the extratropical transition of Floyd (1999): Structural evolution and responsible mechanisms for the heavy rainfall
1. Introduction A subset of tropical cyclones (TCs) interact with the midlatitudes at some point in their life cycle. When this interaction leads to a change in the structure of the TC into a structure more comparable to that of an extratropical cyclone and, on occasion, a reintensification of the cyclone, the process is termed extratropical transition (ET). The extratropical transition of tropical cyclones worldwide has been well documented through climatologies (e.g., Sinclair 1993 , 2002
1. Introduction A subset of tropical cyclones (TCs) interact with the midlatitudes at some point in their life cycle. When this interaction leads to a change in the structure of the TC into a structure more comparable to that of an extratropical cyclone and, on occasion, a reintensification of the cyclone, the process is termed extratropical transition (ET). The extratropical transition of tropical cyclones worldwide has been well documented through climatologies (e.g., Sinclair 1993 , 2002
1. Introduction In the United States, tornado outbreaks often occur in the warm sector of extratropical cyclones, where the atmosphere is thermodynamically unstable and the horizontal winds have strong vertical shear (e.g., Carr 1952 ; Fujita et al. 1970 ; Galway 1975 , 1977 ; Grazulis 1993 ). In the warm sector, strong low-level southerly winds advect warm and moist air, and upper-level strong westerly winds bring cold and dry air above (e.g., Newton 1967 ; Johns and Doswell 1992
1. Introduction In the United States, tornado outbreaks often occur in the warm sector of extratropical cyclones, where the atmosphere is thermodynamically unstable and the horizontal winds have strong vertical shear (e.g., Carr 1952 ; Fujita et al. 1970 ; Galway 1975 , 1977 ; Grazulis 1993 ). In the warm sector, strong low-level southerly winds advect warm and moist air, and upper-level strong westerly winds bring cold and dry air above (e.g., Newton 1967 ; Johns and Doswell 1992
1. Introduction Extratropical transition (ET) is a process in which a tropical cyclone (TC) loses its radially symmetric warm-core structure and becomes an extratropical cyclone with frontal features and a cold core ( Jones et al. 2003 ; Evans et al. 2017 ). To identify the ET of individual storms, forecasters in TC warning centers analyze a wide range of satellite images, model output, and observations. In the TC best track archives, a storm that is determined to have completed ET based on
1. Introduction Extratropical transition (ET) is a process in which a tropical cyclone (TC) loses its radially symmetric warm-core structure and becomes an extratropical cyclone with frontal features and a cold core ( Jones et al. 2003 ; Evans et al. 2017 ). To identify the ET of individual storms, forecasters in TC warning centers analyze a wide range of satellite images, model output, and observations. In the TC best track archives, a storm that is determined to have completed ET based on
by including the contributions from forcing mechanisms at each pressure level. The composite budgets of near-surface cyclonic geostrophic vorticity (CGV) and the variability of these values for intense extratropical cyclones (ETCs) over the North Atlantic are the subject of this paper and an accompanying paper on the decaying phase ( Azad and Sorteberg 2014 , hereafter Part II ). Zwack and Okossi (1986) presented a methodology [the Zwack–Okossi (Z–O) equation] to diagnose the surface
by including the contributions from forcing mechanisms at each pressure level. The composite budgets of near-surface cyclonic geostrophic vorticity (CGV) and the variability of these values for intense extratropical cyclones (ETCs) over the North Atlantic are the subject of this paper and an accompanying paper on the decaying phase ( Azad and Sorteberg 2014 , hereafter Part II ). Zwack and Okossi (1986) presented a methodology [the Zwack–Okossi (Z–O) equation] to diagnose the surface
1. Introduction Satellite imagery and weather maps show various types of synoptic-scale cyclones developing over the global ocean. In particular, two representative types of cyclones are tropical cyclones (TCs) in the low latitudes and extratropical cyclones (ECs) in the midlatitudes. A typical TC has spiral-shaped convective clouds and a warm-core structure, whereas a typical EC is accompanied by a comma-shaped cloud and a cold-core structure with warm and cold fronts. There are also
1. Introduction Satellite imagery and weather maps show various types of synoptic-scale cyclones developing over the global ocean. In particular, two representative types of cyclones are tropical cyclones (TCs) in the low latitudes and extratropical cyclones (ECs) in the midlatitudes. A typical TC has spiral-shaped convective clouds and a warm-core structure, whereas a typical EC is accompanied by a comma-shaped cloud and a cold-core structure with warm and cold fronts. There are also
