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Holton_Fig_6-1 - 140 6 SYNOPTIC~SCALE MOTIONS I briefly...

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Unformatted text preview: 140 6 SYNOPTIC~SCALE MOTIONS I briefly summarize the observed structure of midlatitude synoptic systems and the mean circulations in which they are embedded. We then develop the quasi- geostrophic momentum and thermodynamic energy equations and show how these can be manipulated to form the quasi-geostrophic potential vorticity equation and the omega equation. The former equation provides a method for predicting the evolution of the geopotential field, given its initial three—dimensional distribution; the latter provides a method for diagnosing the vertical motion from the known distribution of geopotential. In both cases, alternative versions of the equations are discussed to help elucidate the dynamical processes responsible for the develop- ment and evolution of synoptic-scale systems. 6.1 THE OBSERVED STRUCTURE OF EXTRATROPICAL CIRCULATIONS Atmospheric circulation systems depicted on a synoptic chart rarely resemble the simple circular vortices discussed in Chapter 3. Rather, they are generally highly asymmetric in form, with the strongest winds and largest temperature gradients concentrated along narrow bands called fronts. Also, such systems generally are highlyfbaroclinic; the amplitudes and phases of the geopotential and velocity per- turbationfb‘bzth‘change substantially» QfififiagWFmeity is due to the facfihaTtTt/he‘semfcys’ystems are not'sup/erposed on a uniform mean flow, but are embedded in a slowly varying planetary scale flow that is itself highly baroclinic. Furthermore, this planetary scale flow is influenced by orography (i.e., by large-scale terrain variations) and continent-ocean heating contrasts so that it is highly longitude dependent. Therefore, it is not accurate to View synoptic sys- tems as disturbances superposed on a zonal flow that varies only with latitude and height. As shown in Chapter 8, however, such a point of View can be useful as a first approximation in theoretical analyses of synoptic—scale wave disturbances. Zonally averaged cross sections do provide sfiome useful information on the gross structure of the planetary scale circulation in which synoptic-scale eddies are embedded. Figure 6.1 shows meridional cross sections of the longitudinally averaged zonal wind and temperature for the solstice seasons of (a) December, January and February (DJF) and (b) June, July and August (IJA). These sections extend from approximately sea level (1000 hPa) to about 32 km altitude (10 hPa). Thus the troposphere and lower stratosphere are shown. This chapter is concerned with the structure of the wind and temperature fields in the troposphere. The strato- sphere is discussed in Chapter 12. The average pole to equator temperature gradient in the Northern Hemisphere troposphere is much larger in winter than in summer. In the Southern Hemisphere the difference between summer and winter temperature distributions is smaller, due mainly to the large thermal inertia of the oceans, together with the greater ‘- fraction of the surface that is covered by oceans in the Southern Hemisphere. Since ' 6.1 THE OBSERVED STRUCTURE OF EXTRATROPICAL CIRCULATIONS 141 (a) 1 O — 30 ..... r25 r" r a“ '8 Height (km) Pressure (hPa) (A) O O ——.- 1O 90$ SOS SOS EQ 30N SON SON 30 25 .s O O I, N 0 Height (km) .4 01 Pressure (hPa) 10 908 608 SOS EQ 30N SON 90N Fig. 6.1 Meridional cross sections of longitudinally and time-averaged zonal wind (solid contours interval of m sh 1) and temperature (dashed contours, interval of 5 K) for December—February (a) and June—August (b). Easterly winds are shaded and 0° C isotherm is darkened. Wind max- ima shown in m s“1 temperature minima shown in ”C (B , , ased on NCEP/NC ' after Wallace, 2003.) AR ”analyses, (3.30) to a high degree of accuracy, the seasonal cycle in zonal wind speeds is Similar to that of the meridional temperature gradient. In the Northern Hemisphere the maxrmum zonal Wind speed in the winter is twice as large as in the, enmmm ...
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