Create a directory in your root directory, called pvor, by using the following
command: mkdir pvor . As a general rule, it is best to run gempak from a
specific directory where you know that the defaults are set up properly.
Move to that directory, by using the command: cd pvor
Copy 1 file from your old assignment directory (baro)
to your new directory by using the following commands:
(1) cp ../baro/gemglb.nts gemglb.nts
Use the command `more' to look at this file to see that it contains
a long list of parameters that will be set when any gempak routine is
called. Look at the Gempak manual to see what a few of these parameters are.
This file is updated after each time a gempak routine successfully
ends; if you abort a gempak program, then this file will not be updated
and you may have to reenter any modifications to parameters that you had made.
The first part of this exercise uses scripts developed originally
by Ron Miller, UCAR,
and Brad Coleman, Seattle WSFO. They are located in $GEMPAKHOME/scripts/ezscripts. You should look in that directory and `more' each of the scripts that
you use to understand what they are doing.
Type `ezset_nov0994'.
Enter the time as 94110900; enter the model as eta (80 km resolution and
38 vertical levels); enter the
device as xw.
This script sets the file based on the time and model type and sets the
device to be used as the interactive xwindow device.
To keep from having the hassles of the last exercise, all of the files
should remain until the due date; don't plan on them being
around after that time, however.
Type `ezarea'. This script defines the plotting area. Be patient and wait
until the pointer changes to a cross-hair. Then, click near the upper left
corner of the figure and drag over the western half of the domain.
Type `ezpvor'. This script plots the potential vorticity in the 200-300 mb layer
and the wind at the 250 mb layer
for the forecast times from F000 to
F036 at intervals of 6 hours. This will take a while.
Winds are in m/s and potential vorticity is contoured at 1 PVU (see text)
and shaded at 2,4, 6, etc. PVUs.
After the script completes, and the prompt is evident, type `gdcntr'.
Type `;l' to loop the frames and `;s' to step through them one at a time.
Answer the questions below based on the gempak output before plotting the
next fields.
After you finish each session be sure to type `gpend' to terminate the gempak
device driver!!!
Describe (briefly) the evolution of potential vorticity over this
48 hour period. Explain the dynamical processes associated
with the change in position of the
positive potential vorticity anomaly. A more difficult question: explain
the dynamical processes associated
with the change with time
of the maximum magnitude of potential vorticity.
To visualize the vertical extent of the region of high potential voriticity
(positive potential vorticity anomaly), use the `ezcross.ipv' script by typing
ezcross.ipv.
Begin with the 36 hour forecast (36) and then
specify that the cross-section will be determined graphically (2).
Wait until a window pops up and the cursor changes to a cross-hair.
Then carefully click near 50N;140W and drag to 30N;100W.
Wait until the cross-section appears. Potential vorticity is shaded
and contours at 1 PVU are also shown in light blue dashed lines.
Isotachs ( in knots) are shown in yellow for the component of the wind
perpendicular to the cross-section. Potential temperature (K) is shown by
red contours at intervals of 5K.
(1)
Identify regions of high and low static stability in the upper troposphere.
How do these regions relate to the distribution of potential vorticity?
(2) Identify the height of the tropopause at several points along the
cross-section.
(3) Describe the relationship between potential vorticity and the locations
of the jets. How can you explain this relationship, i.e., what principle
relates the distribution of potential vorticity and wind?
(4) Describe how the vertical change in
wind speed and horizontal gradient in potential temperature are related.
How can you explain this relationship?
(5) Identify tropopause fold regions and discuss the relationship between
potential vorticity, potential temperature, and wind speed in the tropopause
folds.
Repeat ezcross.ipv with forecast times 00, 12, 24, and 36.
Use option 0 to specify the same cross-section path.
Identify and discuss the way in which the positive potential vorticity
maximum changes as a function of forecast time.
Discuss the evolution of any tropopause folds.
Repeat ezcross.ipv with forecast time 36 (36 hour forecast) but slice
through the potential vorticity anomaly at an angle parallel to the
jet at 250 mb.
How well related are the wind speeds perpendicular
to this cross-section and potential temperature? Why?
Repeat the steps preceded by yellow and orange balls for the meso-eta model (30 km horizontal resolution and 50 levels in the vertical) for the 36 hour forecast only and for the first cross-section angle only; that is, using the same graphical area (you need not run ezarea or change the cross-section path) and model type `meso', loop through the meso-eta forecast for the same period. (Note: the meso-eta forecasts extends to 36 hours only.)
What features in the potential vorticity, potential temperature, and wind field
show more structure in the meso-eta forecast compared to
the operational eta?
Are there any differences in the forecast timing of the movement and
magnitude of the positive potential vorticity anomaly between the
two model simulations?
