Synoptic Meteorology Lab 6
26 March, 2014 – due Fri 4 April
Isentropic maps, isentropic vertical velocity, and IPV
First, we run gdvint on the gempak model output files to redistribute the data from isobaric to isentropic levels.
GDFILE
your gempak grid file
 something like yyyymmdd_hh _f00_f36.gem
GDOUTF
your choice of output file name
 eg. yyyymmdd_hh _f12_isentropic.gem
GDATTIM
Grid date/time: 18 Z on 19 Dec 2012
GVCORD
PRES/THTA
GLEVEL
285;290;295;300;305;310;315;320;325
MAXGRD
2000
 number of 2D or 3D variables to be transferred
GAREA
leave blank
 same as your model domain
VCOORD
none;mslv;esfc
 this transfers all 2D grids as well, such as surface precipitation
Please repeat this for 6 times in total, from 18Z on 19 Dec to 00Z on 21 Dec 2012, at 6 hour intervals (your forecast hour 06
to 36)
Notes:
1. Experience testifies that the isentropic maps will not look good and may have strange discontinuities if your input
isobaric data is coarse in the vertical, e.g. 100 mb. A 25 mb vertical interval is better. You may have to rerun
wrf2gem, and set num=41 or higher. (The default has been num=41)
2. Your daytime WRF output may have slightly superadiabatic lapse rates in the southern tier of your domain, esp. at
18-00Z. I doubt it, because it is mid-winter, anyway, if it happens then this leads to an ambiguity (two theta levels
at a given location x,y). gdvint may be able to handle this: the algorithm chooses the highest level in this case. But
this too may lead to a discontinuity in the mapped height of an isentropic surface. If you see such discontinuity, I
suggest you take a time 3 hours earlier or later (15Z or 03Z), by which time the sun is lower in the sky and the
surface sensible heat flux in the model has reduced to near-zero.
Next, you open gdplot2. Make sure your new input file is the isentropic one you just created. Plot winds (barbs), pressure
(contours) and relative humidity (color fill) on the 290K and 310K surfaces at 12Z on 20 Dec 2012 – similar to the realtime maps found on the College of Dupage site (http://weather.cod.edu/analysis/) (2 maps, 10% of this lab’s grade).
Questions (7 in total, 7% each)
1.
analyze vertical velocity from the flow crossing isobars, and pencil-mark regions of ascent and descent on the 2
plots
2.
estimate the motion (cx,cy) (units: m/s) of the main trof centered at 12Z on 20 Dec 2012, from an animation of 500
mb heights (generated in a previous lab), or an animation of the 310K isentropic map over the 36 hours of your
WRF run.
Generate a new wind field, (u-cx,v-cy), the storm-relative wind:
VSUB(WND,VECN(cx,cy)) where WND=(u,v) is the horizontal wind field
Plot the storm-relative winds (barbs), pressure (contours) and mixing ratio (color fill) on the 290K and 310K surfaces at the
same time (2 maps, 10%).
3.
Again analyze vertical velocity from the storm-relative flow crossing isobars, and pencil-mark regions of ascent
and descent on the 2 plots
Plot the storm-relative winds (barbs), pressure (contours) and vertical velocity w (color fill) on the 290K and 310K surfaces
at the same time (2 maps, 10%).
4.
Compare the estimated updraft (& downdraft) regions, from your isentropic analysis using actual and stormrelative motion, to the actual large-scale updrafts/downdrafts in the last two maps. Discuss differences. Jim Moore,
in his COMET module “isentropic analysis” (http://www.meted.ucar.edu/isen_ana/index.htm), argues that in
principle, storm-relative motion on isentropic surfaces is the better diagnostic of large-scale vertical motion. Is that
true in this case?
Generate isentropic potential vorticity (IPV): PVOR (PRES, WND). Multiply by 106 to obtain standard PV units:
MUL(PVOR(PRES,WND),1000000)
2
Plot the storm-relative winds (barbs), pressure (contours), and IPV (color fill) on the 290K and 310K surfaces at 6 hour
intervals between 18Z on 19 Dec to 00Z on 21 Dec 2012 (12 maps, 21% of this lab’s grade). PVOR is computed over a
layer (the stability term), so set GLEVEL=285:295 (low levels) and GLEVEL=305:315 (upper levels). For other variables,
GLEVEL is simply 290K or 310K.
Suggestion: try to put the earliest 6 maps in one six-panel figure, 290 K on the left and 310 K on the right, and the
following 6 maps in a second six-panel figure, to facilitate analysis and improve presentation. This makes it easier to
compare low-level to upper-level maps, and to see evolution.
Note: if the 290K level is too low (no data in the warmer south), you could try 295K, and the GLEVEL range would have
to be lifted to 290:300
1.
How large are the main PV anomalies, relative to stratospheric PV values starting at 2 PV units?
2.
Examine the conservation of IPV at low levels and upper levels over time: You will see transport and deformation,
but do you also see generation/decay of area-integrated IPV? If so, is there a possible relationship with latent
heating, or proximity to the terrain (friction)?
3.
Discuss the relative position of the main low-level & upper-level positive PV anomalies in the context of the
storm’s evolution. Is this consistent with theory? (transition from a tilted open-wave system to a vertically stacked
occluded system)