• Background:
  • Can divide total flow into geostrophic & ageostrophic parts
  • Ageostrophic motions can be created to maintain even geostrophic circulations. For example:
    1. quasi-geostrophic system: so called "secondary circulation" for a developing wave. e.g. p. 178 in Holton's book.
    2. geostrophic advections of temperature and differential vorticity advection from equation lead to vertical motion (which must be "ageostrophic" motion).
  • This chapter focusses on ageostrophic motions in the formation of fronts and in maintaining isolated maxima in the wind fields.
  • Here, we discuss an ageostrophic circulation set up around a jet streak (defined next).

• Definitions:
  • A jet streak is a core of relative maximum winds,
    1. usually centered near tropopause level.
    2. A typical jet streak is roughly 500 - 1000 km long and
    3. a few hundred km wide.
    4. term does NOT refer to the climatological maxima in the subtropical jet. Jet streaks referred to here move.
  • Four quadrants: divide horizontal area enclosing a jet streak into four regions based upon: upstream or downstream from the max winds PLUS left or right of the max winds as one looks downstream.
    • Upstream is called "jet entrance" region;
    • Downstream is called "jet exit" region;

• General Notes:
  • from thermal wind balance, expect the jet streak max to be forced from a stronger area of horizontal T gradient below.
  • Since wind increases with height, that T gradient may not be moving with the same speed of the air above.
  • So, one expects air parcels to blow through a jet streak.
  • Thus, air parcels must accelerate then deccelerate as they approach then pass through the jet streak
  • Hence the names: the upstream side is often called the "jet entrance" quadrant, the downstream side the "jet exit" quadrant.
  • Using the horizontal momentum eqns, one deduces that accelerations are proportional to the ageostrophic wind directed across the flow:
    du/dt = f v_ag         and        
    dv/dt = - f u_ag

• Straight jet streak: fig. 14.1
  • fig is applicable at the level of the jet streak.
    In practice, this is often near the tropopause, say 200 mb.
  • du/dt >0 in the orientation shown, hence v_ag >0 as indicated in the entrance regions.
  • Note jet streak-relative trajectory shown by dot-dashed line: it crosses the height contours revealing the ageostrophic wind.
    FYI: This is a subtle point that is not always clear in discussions of jet streaks. The confluent nature of the height contours would lead to acceleration which need not have any divergent circulation associated with it. The reason why we do have a divergent circulation here is because the jet streak winds vary with elevation. It is forced from below, the jet streak winds also increase with increasing elevation. If the height contours had the same gradient at all levels, then the vertical dimenstion is removed from the problem.
  • v_ag has largest magnitude along the jet streak axis.
  • v_ag is directed towards the cold air in the entrance region; towards the warm air in the exit region.
  • Associated with the v_ag winds will be a pattern of divergence and convergence:
    • divergence in the right entrance and left exit regions
    • convergence in the left entrance and right exit regions
  • From Dines compensation, expect convergence below these areas of divergence, etc.
  • From QG vorticity eqn, expect PVA where there is divergence and NVA where there is convergence at upper levels.
  • Reasoning from Dines compensation or from PVA/NVA one expects rising motion in the right entrance and left exit regions.
  • Thus, one expects clouds and possibly precipitation to be enhanced in the right entrance and left exit quadrants.

• Application of this information:
  • Fig. 14.2 attempts to illustrate this. Note that the fit is not perfect, in part because other factors come into play. For example, there may be topographically forced rising motion in the right exit region, so that some precipitation still falls there. Also, there may be warm advection occuring below. The interesting feature is the difference on the left side between entrance and exit regions.
  • Jet streaks are sometimes identifiable on satellite imagery. (Quite useful for interpreting imagery for the North Pacific.) Where there is a sharp cloud edge which has a broadening downstream may well indicate a jet streak right at that edge and just upstream from the broadening. Fig. 12.7a is a reasonable illustration.
  • Curving jet streaks are a different story: this is probably why fig. 12.7b seems to have 2 jet streak maxima: the left one (rather near the "90") plus a stronger one further downstream.
    Curving jet streaks are somewhat hard to diagnose. The simplest rule might be that the left or right side becomes enhanced. e.g. A cyclonically curved jet streak tends to have larger regions where the signs match the "left" side of the symmetric jet streak of fig. 14.1.

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