VORTICITY LEARNING OBJECTIVES: Recognize the two components of relative vorticity. Define the term absolute vorticity.  Determine  vorticity impacts on weather processes. Vorticity measures the rotation of very small air parcels. A parcel has vorticity when it spins on its axis as it moves along its path. A parcel that does not spin on its axis is said to have zero vorticity. The axis of spinning or rotation can extend in any direction, but for our  purposes,  we  are  mainly  concerned  with  the rotational motion about an axis that is perpendicular to the surface of the Earth. For example, we could drop a chip of wood into a creek and watch its progress. The chip will move downstream with the flow of water, but it may or may not spin as it moves downstream. If it does spin, the chip has vorticity. When we try to isolate the cause of the spin, we find that two properties of the flow of water cause the chip to spin: (1) If the flow of water is moving faster on one side of the chip than the other, this is shear of the current; (2) if the creek bed curves, the path has curvature. Vorticity always applies to extremely small air parcels; thus, a point on one of our upper air charts may represent such a parcel. We can examine this point and say that the parcel dots or does  not  have  vorticity.  However,  for  this  discussion, larger  parcels  will  have  to  be  used  to  more  easily visualize   the   effects. Actually,  a  parcel  in  the atmosphere  has  three rotational  motions  at  the  same time:  (1)  rotation  of  the  parcel  about  its  own  axis (shear), (2) rotation of the parcel about the axis of a pressure system  (curvature),  and (3) rotation of the parcel due to the atmospheric rotation. The sum of the first two components is known as  relative  vorticity,  and the sum total of all three is known as  absolute  vorticity. RELATIVE  VORTICITY Relative vorticity is the sum of the rotation of the parcel about the axis of the pressure system (curvature) and the rotation of the parcel about its own axis (shear). Figure 1-4.-Illustration of vorticity due to the shear effect. The vorticity of a horizontal current can be broken down into  two  components,  one  due  to  curvature  of  the streamlines and the other due to shear in the current. Shear First, let us examine the shear effect by looking at small air parcels in an upper air pattern of straight contours. Here the wind shear results in each of the three  parcels  having  different  rotations  (fig.  1-4). Refer to figure 1-4. Parcel No. 1 has stronger wind speeds to its right. As the parcel moves along, it will be rotated in a counterclockwise direction. Parcel No. 2 has the stronger wind speeds to its left; therefore, it will rotate in a clockwise direction as it moves along. Parcel No. 3 has equal wind speeds to the right and left. It will move, but it will not rotate. It is said to have zero vorticity. Therefore, to briefly review the effect of shear-a parcel of the atmosphere has vorticity (rotation) when the wind speed is stronger on one side of the parcel than on the other. Now  let’s  define  positive  and  negative  vorticity  in terms  of  clockwise  and  counterclockwise  rotation  of  a parcel. The vorticity is positive when the parcel has a counterclockwise   rotation   (cyclonic,   Northern Hemisphere) and the vorticity is negative when the parcel  has  clockwise  rotation  (anticyclonic,  Northern Hemisphere). Thus,  in  figure  1-4,  parcel  No.  1  has  positive vorticity, and parcel No. 2 has negative vorticity. Curvature Vorticity can also result due to curvature of the airflow or path. In the case of the wood chip flowing with the stream, the chip will spin or rotate as it moves along if the creek curves. To  demonstrate  the  effect  of  curvature,  let  us consider a pattern of contours having curvature but no shear (fig. 1-5). Figure  1-5.-Illustration  of  vorticity  due  to  curvature  effect. 1-8