ATSC 5160
HW#2
Hiroshi Takagi
4.5/6 – nice graphics, good interpretation
1.5/2
1. Discuss the effect of varying static stability on vertical motion in a quasigeostrophic
atmosphere.
Most synoptic-scale systems in midlatitudes appear to develop as the result of an
instability of the jet stream flow. This instability, which is called baroclinic
instability, depends on the horizontal temperature gradient, particularly at the surface.
Therefore, through the thermal wind relationship, baroclinic instability depends on
vertical shear of geostrophic wind and tends to occur in the region of the polar frontal
zone.
The vertical motion or omega equation provides a method of estimating omega
(proportional to (-)vertical velocity dp/dt) by observing of geopotential at a single
time thus helps analyze the variation of the vertical motion in a quasigeostrophic
atmosphere. The omega equation is expressed as follows:
 2 f 02  2 
 1 2
f0  
  



V g     
2 
 p 
 p 
 f0

Term A
Term B

 1
  
f    2 V g   

 p 
 

Term C
The static stability parameter  in the equation above is an indicator that tells the
severity of expansion or compression of atmosphere due to temperature advection
(temperature change). It is given as the following:
 
RT  ln 
p p
The  parameter is a positive quantity and greater value of the parameter implies
unstable atmospheric situations. Increasing the parameter value in each term of the
omega equation causes decrease in the magnitude of  value (more negative quantity)
which corresponds to positive w (dz/dt). That is the indication of less upward motion
in a more stable atmosphere (larger ).
(Reference: An introduction to dynamic meteorology by J. R. Holton)
2.
Questions
1.5/2
Locate regions of cold and/or warm advection using the profiles of the geostrophic wind
and locate any fronts.
Searching a location of strong wind shift with increasing height pinpoints where thermal
advection is occurring. The section from the left end of the map (North Platte station,
NE) to the center of the map is where strong wind shift with height is observed. Near the
surface (around 900mb) north to northwesterly wind is observed but the wind changes to
west to south westerly near 600mb surface. This is a backing of the wind with height and
is indicative of cold-air advection toward southeast. Location of a surface cold front is
determined by carefully looking at the wind direction change near the surface. A strong
surface wind shift is observed near the center of the map and that coincides with the
region of densely packed potential temperature contour lines. The surface cold front can
be put there.
Locate any regions of moderate to strong horizontal temperature gradients in the lower
troposphere (below 500mb or so).
The regions should coincide with the location of the highly packed, skewed potential
temperature contours. Also, that region is where strong cold-air advection and the surface
cold front are located.
Relate the structure of the jet (i.e. strength and location of the jet and the location of the
strongest shear) to the horizontal temperature gradients. Using the thermal wind
relationship as your guide, is the strength and location of the jet similar to, or different
from your expectations?
The jet stream is located above where the strongest horizontal temperature gradients and
wind shear are. The location of the jet stream seems to be the upper end of positive ??
wind shear with height. So finding the location of backing why backing ?? of geostrophic
wind that is associated with cold-air advection help identify where the upper air jet flow
may be. This is not clear
3.
LBF
ILN
1.5/2
Questions
Interpret the ageostrophic flow pattern on the chart.
The flow of total wind has two components (geostrophic and ageostrophic component)
and they are correlated one another. One of the general trends in ageostrophic flow
observed on this chart is that it flows across the isobars in the entrance and exit regions of
jet streaks. In the entrance regions of jet streaks, the component of isobar-crossing
ageostrophic flow that is parallel to the isobars contribute to accelerating the total flow
and the other component of the flow that is perpendicular to isobars causes circulations.
In the exit regions of jet streaks, the component of isobar-crossing ageostrophic flow that
is parallel to isobars contributes to decelerating the total flow ??? and the other
component of the flow that is perpendicular to isobars causes circulations in that region.
These contributions by the components of ageostrophic flow account for the total flow
speed maxima near the entrance regions, relatively slower flow near the exit regions, and
divergence and convergence in both entrance and exit regions of jet streak. How about
the component of the ageostrophic wind along the height contours (sub vs
supergeostrophic wind depending on curvature) ??
Interpret the ageostrophic flow pattern in the cross section. Is there evidence of any of
the following? Cold front aloft; dry line; CAPE and possibility for deep convection.
The vectors on this chart indicate the ageostrophic flow in X-Z plane, showing the
vertical circulation in the cross section. A large counterclockwise circulation is seen on
the cross section. The circulation seems to originate near station ILN with increasing
upward flow and the flow tends to subside toward station LBF. These upward and
downward flow patterns seem to be sustained by the upper air convergence and
divergence near the exit region of the jet streak shown in the previous chart (the
indication of that divergence and convergence are also on the chart above). A dry line
may be put where equivalent potential temperature lines and mixing ratio lines are
packed it is a cold front because a dry line is characterized by ONLY moisture, no
temperature gradient. The region of densely packed lines almost coincides with the
leading edge of the cold-air advection. The cold, dry air on the left side of the chart
displaces the moist, (high theta-e value and mixing ratio) warm air on the right side of the
chart. There seems to be a cold pool with lower theta-e in the middle of the seemingly
convective system where specifically is de/dz negative (potential instability) ??. This is
indicative of unstable atmosphere thus possibly inducing deep convective motion. GOOD