retrogression of long waves. These rules are discussed in the following text. PROGRESSION   OF   LONG   WAVES.— Progression  (eastward  movement)  of  long  waves  is usually found in association with relatively short wave lengths and well defined major troughs and ridges. At the surface, there are usually only one or two prominent cyclones  associated  with  each  major  trough  aloft. Beneath the forward portion of each major ridge there is usually a well developed surface anticyclone moving toward  the  east  or  southeast.  The  24-hour  height changes  at  upper  levels  usually  have  a  one-to-one association  with  major  troughs  and  ridges;  that  is, motion of maximum height fall and rise areas associated with major trough and ridge motion. The tracks of the height change centers depend on the movement and changes in intensity of the long waves. STATIONARY  LONG  WAVE  PATTERNS.— Once established, stationary long wave patterns usually persist  for  a  number  of  days.  The  upper  airflow associated with the long wave pattern constitutes a steering pattern for the smaller scale disturbances  (short waves).  These  short  waves,  with  their  associated  height change  patterns  and  weak  surface  systems,  move  along in the flow of the large scale, long wave pattern. Short wave troughs deepen as they move through the troughs of the long waves, and fill as they move through the ridges of the long waves. The same changes in intensity occur in sea level troughs or pressure centers that are associated with minor troughs aloft. Partly as a result of the presence of these smaller scale systems, the troughs and ridges of the stationary long waves are often spread out and hard to locate exactly. RETROGRESSION  OF  LONG  WAVES.—  A continuous retrogression of long wave troughs is a rare event. The usual type of retrogression takes place in a discontinuous   manner;   a   major   trough   weakens, accelerates eastward, and becomes a minor trough, while a major wave trough forms to the west of the former position of the old long wave trough. New major troughs are generally formed by the deepening of minor troughs into deep, cold troughs. Retrogression  is  seldom  a  localized  phenomenon, but appears to occur as a series of retrogressions in several  long  waves.  Retrogression  generally  begins  in a  quasi-stationary  long  wave  train  when  the  stationary wavelength  shows  a  significant  decrease.  This  can happen as a result of a decrease in zonal wind speed, or of  a  southward  shift  in  the  zonal  westerlies.  Some characteristics of retrogression are as follows: l Trajectories of 24-hour height change patterns at 500-hPa deviate from the band of maximum wind. l  New  centers  appear,  or  existing  ones  rapidly increase in intensity. .  Rapid  intensification  of  surface  cyclones  occurs to the west of existing major trough positions. Location Of The Jet Stream The  AG2  TRAMAN,   volume   1,   discusses   the migration  of  the  jet  stream  both  northward  and southward.  Some  general  considerations  can  be  made concerning this migration and the movement of waves in  the  troposphere: . In a northward migrating jet stream, a west wind maximum  emerges  from  the  tropics  and  gradually moves   through   the   lower   midlatitudes.   Another maximum,  initially  located  in  the  upper  midlatitudes, advances toward the Arctic Circle while weakening. Open  progressive  wave  patterns  with  pronounced amplitude and a decrease in the number of waves due to cutoff  centers  exist.  The  jet  is  well  organized  and troughs  extend  into  low  latitudes. . As the jet progresses northward, the amplitude of the long waves decrease and the cutoff lows south of the westerlies dissipate. By the time the jet reaches the midlatitudes,   a   classical   high   zonal   index   (AG2 TRAMAN, volume 1) situation exists. Too, we have weak,  long  waves  of  large  wavelength  and  small amplitude,  slowly  progressive  or  stationary.  Few extensions of troughs into the low latitudes are present, and  in  this  situation,  the  jet  stream  is  weak  and disorganized. . As the jet proceeds farther northward, there will often be a sharp break of high zonal index with rapidly increasing   wave   amplitudes   aloft.   Long   waves retrograde. As the jet reaches the upper midlatitudes and into the sub-Arctic region, it is still the dominant feature, while a new jet of the westerlies gradually begins to form in the subtropical regions. Long waves now begin to increase in number, and there is a reappearance of troughs in the tropics. The cycle then begins again. With a southward migrating jet, the processes are reversed  from  that  of  the  northward  moving  jet.  It should be noted that shortwaves are associated with the jet maximum and move with about the same speed as these  jet  maximums. 2-3


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