Figure 1-9.-Contour-isotachch pattern for shear analysis.lines are streamlines or contours; dashed lines areisotachs.Figure 1-7 represents a symmetrical sinusoidalstreamline pattern with isotachs parallel to contours.Therefore, there is no gradient of shear along thecontours.In region I, the curvature becomes moreanticyclonic downstream, reaching a maximum at theaxis of the downstream ridge; that is, relative vorticitydecreases from the trough to a minimum at thedownstream ridge. The region from the trough to thedownstream ridge axis is favorable for deepening.The reverse is true west of the trough, region II.This region is unfavorable for deepening.In figure 1-8 there is no curvature of streamlines;therefore, the shear alone determines the relativevorticity. The shear downstream in regions I and IVbecomes less cyclonic; in regions II and III, it becomesmore cyclonic. Regions I and IV are therefore favorablefor deepening downstream.In region I of figure 1-9 both cyclonic shear andcurvature decrease downstream and this region is highlyfavorable for deepening. In region III both cyclonicshear and curvature increase downstream and thisregion is unfavorable for deepening. In region II thecyclonic curvature decreases downstream, but thecyclonic shear increases. This situation is indeterminatewithout calculation unless one term predominates. Ifthe curvature gradient is large and the shear gradientsmall, the region is likely to be favorable for deepening.Figure 1-10.-Contour-isotach pattern for shear analysis.In region IV, the cyclonic curvature increasesdownstream, but the cyclonic shear decreases, so thatthis region is also indeterminate unless one of the twoterms predominates.In region I of figure 1-10 the cyclonic sheardecreases downstream and the cyclonic curvatureincreases. The region is indeterminate; however, if theshear gradient is larger than the curvature gradient,deepening is favored. Region II has increasing cyclonicshear and curvature downstream and is quiteunfavorable. In region III, the shear becomes morecyclonic downstream and the curvature becomes lesscyclonic. This region is also indeterminate unless thecurvature term predominates. In region IV, the shearand curvature become less cyclonic downstream and theregion is favorable for deepening.RELATION OF VORTICITY TO WEATHERPROCESSESVorticity not only affects the formation of cyclonesand anticyclones, but it also has a direct bearing oncloudiness, precipitation, pressure, and height changes.Vorticity is used primarily in forecasting cloudiness andprecipitation over an extensive area. One rule states thatwhen relative vorticity decreases downstream in theupper troposphere, convergence is taking place in thelower levels. When convergence takes place,cloudiness and possibly precipitation will prevail ifsufficient moisture is present.One rule using vorticity in relation to cyclonedevelopment stems from the observation that whencyclone development occurs, the location, almostwithout exception, is in advance of art upper trough.Thus, when an upper level trough with positive vorticityadvection in advance of it overtakes a frontal system inthe lower troposphere, there is a distinct possibility ofcyclone development at the surface. This is usuallyaccompanied by deepening of the surface system. Also,the development of cyclones at sea level takes placewhen and where an area of positive vorticity advectionsituated in the upper troposphere overlies a slow movingor quasi-stationary front at the surface.The relationship between convergence anddivergence can best be illustrated by the term shear. Ifwe consider a flow where the cyclonic shear isdecreasing downstream (stronger wind to the right thanto the left of the current), more air is being removed fromthe area than is being fed into it, hence a net depletionof mass aloft, or divergence. Divergence aloft isassociated with surface pressure falls, and since this is1-10

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