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1. Introduction
A conceptual “four quadrant” model (4QM hereafter; e.g., Bluestein 1993; Rose et al. 2004) of upper-tropospheric linear jet streak circulations was first hypothesized by Namias and Clapp (1949) and later inferred through observations by Murray and Daniels (1953). Using the quasigeostrophic momentum equation, it easily can be shown that the ageostrophic wind is directed perpendicular and to the left (in the Northern Hemisphere) of the acceleration of the wind. The ageostrophic flow results in upper-level horizontal divergence (convergence) in the right-entrance and left-exit (left entrance and right exit) regions, resulting in a vertical circulation with upward (downward) motion occurring in quadrants with upper-level divergence (convergence), which satisfies mass conservation. The vertical circulation in the entrance quadrants is “thermally direct” with warm air rising and cool air sinking, while that in the exit region is “thermally indirect” (e.g., Bluestein 1993; Holton 2004).
The jet-induced secondary circulation was first recognized by Beebe and Bates (1955) to be a potential contributing mechanism for severe weather, which later case studies (e.g., Uccellini and Johnson 1979; Bluestein and Thomas 1984; Sanders and Blanchard 1993) and a few climatological studies (e.g., Kloth and Davies-Jones 1980; Harnack and Quinlan 1989; Rose et al. 2004) also evinced. The possible mechanisms by which jet-induced vertical circulations contribute to severe weather include decreasing the static stability, cooling a layer of air to saturation and thus releasing convective available potential energy (CAPE), enhancing moisture convergence at low levels, and increasing CAPE in the exit region through differential temperature and moisture advection with height (Bluestein and Thomas 1984 and references therein). In addition, Bluestein and Thomas (1984) recognize that jet streaks are intrinsically linked to severe weather because they enhance vertical wind shear, which is important in sustaining long-lived convection (e.g., Weisman and Klemp 1982).
In the atmosphere, jet streaks often do not fit the four-quadrant conceptual model. Beebe and Bates (1955) recognized that centripetal acceleration caused by curvature in the flow changes the orientation of the ageostrophic wind, modifying the jet-induced vertical circulation. Later, studies using numerical models of idealized jet streaks (e.g., Newton and Trevisan 1984; Moore and VanKnowe 1992) quantified the expected changes in jet-induced circulations caused by curved flow, and a conceptual model for curved jet streaks was described by Keyser...