. Compendium of meteorology. Meteorology. 772 WEATHER FORECASTING maps, Petterssen has reported how the percentage fre- quency of cyclones and cyclogenesis is distributed over the whole Northern Hemisphere [62, Figs. 14-21]. He has also given the rate of alternation between cyclones and anticyclones, indicating the distribution of travel- ing disturbances [62, Figs. 22-23]. The frontogenesis builds up cyclonic shear vorticity which is transferred into vorticity of cyclonically curved flow at the apex of the frontal wave. Large initial frontal shear is therefore a sign of "stored" kin
. Compendium of meteorology. Meteorology. 772 WEATHER FORECASTING maps, Petterssen has reported how the percentage fre- quency of cyclones and cyclogenesis is distributed over the whole Northern Hemisphere [62, Figs. 14-21]. He has also given the rate of alternation between cyclones and anticyclones, indicating the distribution of travel- ing disturbances [62, Figs. 22-23]. The frontogenesis builds up cyclonic shear vorticity which is transferred into vorticity of cyclonically curved flow at the apex of the frontal wave. Large initial frontal shear is therefore a sign of "stored" kinetic energy which, in being transformed to curvature vor- ticity, favors the evolution of the wave into a cyclone. The intensity of the frontal cyclogenesis may partly be conjectured by the forecaster from the observed horizontal velocity differences between the air masses. In particular, under the assumption of a normal air- mass stratification, sufficiently long waves are usually unstable provided that—according to an old rule—the wind shear along the front in knots is greater than four times the temperature discontinuity in centigrade degrees. Qualitative examples of such reasoning concerning cyclogenesis due to the individual change of vorticity of traveling particles are demonstrated in Fig. 2. The FRICTIONAL EFFECT TILT INCREASING TILT DECREASING SHARP FRONT T—r DIFFUSE FRONT I-KM ISOHYPSE OF FRONT 2-KM ISOHYPSE OF FRONT. CYCLOGENETIC FRONT ACTIVITY ACTIVE MEDIUM PASSIVE KATAFRONT ANAFRONT Fig. 2.—Frontal activitj' and passivitj' according to Bergeron, and vorticity d-j'namics of cyclones according to Bjerknes [33]. cold air sample a moves equatorward and partakes in the general convergence typical for the forward half of a cyclone. For both reasons cyclonic relative vorticity is acquired, which at first shows up on the surface map mainly as a horizontal cyclonic shear inside the cold air along the warm front. In passing the wave apex, this air maintains its cyclo
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