Extratropical Synoptic-Scale Processes and Severe Convection
Severe Convective Storms
Prediction, or NCEP) in the 1960s. However, the real value in the QG system today is not in prediction, but in qualitative understanding of midlatitude, synoptic-scale processes (Durran and Snellman 1987) . Equations (1) and (2) are derived from the primitive equations by making a number of assumptions about the flow they describe: namely, the flow is adiabatic and hydrostatic and the ageostrophic part of the flow 3 Phillips attributes the first use of the term "quasi-geostrophic" to Sutcliffe
... phic" to Sutcliffe (1939). However, Sutcliffe (1938) includes the following passage: "It is suggested that the term 'quasi-geostrophic' would be a better description in that the motion is geostrophic only to a first approximation." 4 makes no contribution to advection, etc. Textbook discussions (e.g., Holton 1992, p 166 ff.; Bluestein 1992, Ch. 5) point out that vertical motion in the QG system is simply a theoretical response to the disruptions of geostrophic and thermal wind balance caused by thermal and differential vorticity advection, with the QG response acting to restore hydrostatic and geostrophic balance. Disturbances in the height field (that are reflected in the relative vorticity) move by vorticity advection, weakening or intensifying as a consequence of differential thermal advection. Generally speaking, QG theory predicts rising motion ahead of cyclonic disturbances and descending motion behind. In QG theory, however, s is assumed to be, at most, a function of pressure, whereas in reality, s varies in space and time. Rising motion favors a decrease in stability below the level of peak ascent, usually somewhere in mid-troposphere, whereas sinking motion increases the stability below the level of maximum descent. 4 Therefore, ETCs should exhibit some asymmetry in their vertical motion patterns beyond that derived from QG theory: ascent should be more intense than descent. Further, Emanuel et al. (1987) have shown (in a non-QG context) that if ascent is saturated and descent is unsaturated, the difference between moist ascent and dry descent can be parameterized by using a dry static stability for descent and a weaker, moist static stability for ascent. Doing so results in ascent being localized and relatively intense, whereas descent is weaker and more widespread (Whitaker and Barcilon 1992; Fantini 1995). These concepts generally are consistent with the observed behavior in mid-latitude cyclones (e.g., Whitaker et al. 1988) which certainly include non-QG processes; upward motion is typically stronger than downward motion and more localized (at times, ascent is concentrated in narrow bands associated with fronts, of course). ETC development, (e.g., as described in Palmén and Newton 1969; Ch. 11; Uccellini 1990; Bosart 1999) is strongly dependent on the vertical motions. Vertical motion, in turn, depends on s (even in the QG system considered above), so the environmental static stability is an important factor in cyclogenesis and its associated frontogenesis (Roebber 1993). Moreover, convection acts 4 This also can be inferred from a static stability tendency equation; e.g., Panfosky (1964; p. 105 ff.).