EXTRATROPICAL SYNOPTIC-SCALE PROCESSESAND SEVERE CONVECTIONChapter 2 inSevere Convective StormsA Meteorological MonographPublished byThe American Meteorological SocietyCHARLES A. DOSWELL IIINational Severe Storms LaboratoryNorman, Oklahoma 73069LANCE F. BOSARTDepartment of Earth and Atmospheric SciencesThe University at Albany/SUNYAlbany, New York 12222Review Panel:HOWARD B. BLUESTEIN (Chair), JOHN M. BROWN, BRADLEY COLMAN, CHRISTOPHER DAVISSubmitted: May 2000____________________Corresponding author: Dr. Charles A. Doswell III, NOAA/National Severe Storms Laboratory, 1313 Halley Circle,Norman, Oklahoma 73069 e-mail: [email protected]. Introduction:Within this chapter, we intend to give a broad perspective of the interaction between severeconvection and extratropical synoptic-scale processes. A traditional view of this interaction is thatthe synoptic-scale processes simply provide a setting in which severe convection develops (see, e.g.,Newton 1963; Barnes and Newton 1983; Johns and Doswell 1992). This view could beinterpreted as implying that convection has little or no direct impact on synoptic scales. However,there have been many recent developments in mesoscale meteorology as it relates to severeconvection (as described in Ch. 3 of this volume), wherein upscale effects of convection are seenmost clearly. Mesoscale processes often act as a sort of "intermediary" between convective andsynoptic scales. We take the view that, in spite of the intermediation by mesoscale processes, it stillis possible to take a synoptic-scale view of the impacts of deep, moist convection, especially in itsmost severe manifestations.The subject of scale separation is always a thorny one. Orlanski's (1975) scale divisions areessentially arbitrary, based on powers of ten in space and time. There are also dynamicallymotivated ways to divide scales [Emanuel 1986; Doswell 1987], but there is no universallyaccepted way to separate scales of motion. For the purposes of this review, we are concerned withthe processes associated with extratropical weather systems in midlatitudes; tropical synoptic-scaleprocesses are considered in Ch. 7 of this volume.Quasigeostrophic (QG) theory is arguably the simplest statement of what it means to be"synoptic-scale" (Doswell 1987), at least outside of the Tropics. In section II, we provide briefoverviews of QG principles, potential vorticity thinking and basic jet streak-related processes.Section III presents a discussion of planetary boundary layer processes, focusing on how theserelate both to synoptic scales and to severe convection. Section IV provides some basicclimatological distributions of convection, both in space and in time. These observed climatologicaldistributions provide important clues as to the interaction between synoptic-scale processes andconvection. The climatology of severe forms of deep, moist convection is the topic of Section V.In section VI, brief overviews of a number of cases are presented, in part to illustrate the principles2developed, but also to show the variety of synoptic-scale structures in which severe convection candevelop. Section VII presents some perspectives on the synoptic contributions to severe convection,and section VIII provides a discussion of the reverse feedbacks of convection to the synoptic scale.Finally, section IX provides some discussion and conclusions.II. Brief overviewsDeep, moist convection (DMC)1 is associated with a triad of necessary and sufficientingredients: moisture, low static stability, and ascent of parcels to their level of free convection(LFC) by some lifting mechanism (Doswell 1987).2 Synoptic-scale processes, notably theextratropical cyclone (ETC), play a reasonably well-understood role in moistening anddestabilization, but the relatively weak vertical motions (on the order of a few cm s-1) of synoptic-scale systems usually are too slow to lift potentially buoyant parcels to their LFCs in less thanabout a day. On the other hand, ETCs provide an environment that favors processes operating onsmaller scales, such as drylines (see Schaefer 1986; Ziegler and Rasmussen 1998) and fronts, andvertical motions substantially larger than those of the synoptic scale can be created by thosesubsynoptic processes. Thus, there is a strong association between the development of DMC andETCs, even though DMC is not confined exclusively to the ETC environment. Severe convectionalso exhibits this association, perhaps even more strongly than ordinary convection. To understandthis connection, we begin with consideration of the simplest model of synoptic-scale processes inmidlatitudes, quasigeostropic theory. Then, we move to consider potential vorticity, a modernperspective on synoptic-scale processes. Jet streaks are reviewed briefly in this same context.1. QG principles 1 As in Ch. 1 of this volume, we use the term "deep moist convection" rather than "thunderstorm"since not all severe forms of deep, moist convection produce lightning and, hence, thunder.2 We assume that the presence of the LFC implies that the convection will, indeed, be “deep”although rare exceptions to this might be found.3Quasigeostrophic (QG) theory is not a concept that springs from a single, brilliantexposition; instead, it is a child of many parents. This is manifest in the excellent historicaltreatments of QG theory's development by Phillips (1990) or Bosart (1999).3 Many respectedscientists in the history of modern meteorology have made contributions to QG theory. The quasi-geostrophic system is contained in the two equations (following Holton 1992, p. 158 ff.)—2+∂∂pfo2s∂∂pÊ Ë Á ˆ ¯ È Î Í ˘ ˚ ˙ c= - foVg• —1fo—2F + fÊ Ë Á ˆ ¯ ˜ -∂∂p-fo2sVg• — -∂F∂pÊ Ë Á ˆ ¯ È Î Í ˘ ˚ ˙ , (1)and—2+fo2s∂2∂p2Ê Ë Á ˆ ¯ w=fos∂∂pVg• —1fo—2F + fÊ Ë Á ˆ ¯ ˜ È Î Í ˘ ˚ ˙ +1s—2Vg• — -∂F∂pÊ Ë Á ˆ ¯ È Î Í ˘ ˚ ˙ , (2)where — is the horizontal gradient operator on a p-surface, F is geopotential height, the heighttendency (c) is defined by c≡∂F /∂t, w≡ dp dt, the static stability (s) is definedbys≡ -aoqo( )dqodp( ). Static stability and the basic-state variables ao (specific volume, orinverse density) and qo (potential temperature) are assumed to be functions of p alone, and fodenotes a constant reference value of the Coriolis parameter. Equations (1) and (2) exhibit a near-symmetry that is apparent in the